Year 4: Reading a ruler: centimetres and millimetres
Part of the Year 4 perimeter, area and measures chapter. Chapter overview · whole chapter on one page · Download worksheet (PDF) · Answers (PDF)
Reading a ruler: centimetres and millimetres
We measure length with a ruler. A ruler has a scale: a line of marks with numbers. The big numbered marks are centimetres (cm). Between two numbers there are 10 tiny marks. Each tiny mark is a millimetre (mm). 10 mm = 1 cm. The middle mark between two numbers is longer. It is 5 mm.
How to read a ruler. Put the 0 mark at the start of the thing you measure. Then look at where the other end is.
- Read the last whole centimetre before the end.
- Count the small marks after it. Each one is 1 mm.
- Say the length: centimetres and millimetres.
Different ways to write a length. The pencil is 7 cm 4 mm. There are 7 × 10 = 70 mm in 7 cm, so it is also 74 mm. Each millimetre is one tenth of a centimetre (0.1 cm), so we can write it as 7.4 cm.
To the nearest centimetre. Look at the millimetres. If there are 5 or more, round up to the next centimetre. If there are fewer than 5, stay on the same centimetre. 7 cm 4 mm is about 7 cm. And 7 cm 5 mm is about 8 cm.
When the object does not start at 0. Sometimes a strip starts at another mark. Read both ends, then take the smaller reading away from the bigger one.
Key words. The length is how long something is. A millimetre is a very small unit (about the thickness of a coin). A centimetre is about the width of your little fingernail.
Key ideas
- 10 mm = 1 cm. Big marks are cm. Tiny marks are mm. The long middle mark is 5 mm.
- Start at 0. Read the whole cm first, then count the mm.
- 7 cm 4 mm = 74 mm = 7.4 cm.
- To the nearest cm: 5 mm or more rounds up. Less than 5 mm stays the same.
- If a strip does not start at 0, read both ends and subtract.
- Always write the unit: cm or mm.
Key words. A ruler measures length. Centimetres (cm) are the big numbered marks. Millimetres (mm) are the tiny marks: 10 mm make 1 cm. The scale is the line of numbers and marks.
Method. Read the ruler at the end of the object. The big marks are centimetres and the small marks are millimetres.
- Find where the end of the ribbon is on the ruler. The left end of the ribbon is at 0, so the number at its right end is its length.the end of the ribbon is just after the 7 cm mark, with 4 small marks to go.
- Count the small marks after the big mark. The tiny marks are millimetres. There are 10 of them in every centimetre.4 small marks = 4 mm
- Write the length. Say the centimetres, then the millimetres.7 cm and 4 mm = 7 cm 4 mm
Check. 7 cm = 70 mm and 70 mm + 4 mm = 74 mm. ✓
Watch out. Start counting from the 0 mark, not from the end of the ruler. Ten small marks make one centimetre, not five.
Key words. A ruler measures length. Centimetres (cm) are the big numbered marks. Millimetres (mm) are the tiny marks: 10 mm make 1 cm. The scale is the line of numbers and marks.
Method. Read the ruler at the end of the object. The big marks are centimetres and the small marks are millimetres.
- Find where the end of the pencil is on the ruler. The left end of the pencil is at 0, so the number at its right end is its length.the end of the pencil is just after the 12 cm mark, with 6 small marks to go.
- Count the small marks after the big mark. The tiny marks are millimetres. There are 10 of them in every centimetre. The long middle mark is 5 mm.6 small marks = 6 mm
- Change the centimetres to millimetres. 1 cm = 10 mm, so multiply the centimetres by 10.12 × 10 = 120 mm
- Add the extra millimetres. 120 + 6 = 126 mm
Check. 126 ÷ 10 = 12 remainder 6, which is 12 cm 6 mm. ✓
Watch out. Start counting from the 0 mark, not from the end of the ruler. Ten small marks make one centimetre, not five.
Key words. A ruler measures length. Centimetres (cm) are the big numbered marks. Millimetres (mm) are the tiny marks: 10 mm make 1 cm. The scale is the line of numbers and marks.
Method. Read the ruler at the end of the object. The big marks are centimetres and the small marks are millimetres.
- Find where the end of the line is on the ruler. The left end of the line is at 0, so the number at its right end is its length.the end of the line is just after the 9 cm mark, with 5 small marks to go.
- Count the small marks after the big mark. The tiny marks are millimetres. There are 10 of them in every centimetre. The long middle mark is 5 mm.5 small marks = 5 mm
- Look at the extra millimetres. To round to the nearest cm, look at the millimetres. 5 mm or more rounds UP.5 mm is 5 or more, so round up from 9 cm to 10 cm
Check. The length is 9 cm 5 mm. It is closer to 10 cm than to the other whole centimetre. ✓
Watch out. Start counting from the 0 mark, not from the end of the ruler. Ten small marks make one centimetre, not five.
Key words. A ruler measures length. Centimetres (cm) are the big numbered marks. Millimetres (mm) are the tiny marks: 10 mm make 1 cm. The scale is the line of numbers and marks.
Method. Read the ruler at the end of the object. The big marks are centimetres and the small marks are millimetres.
- Find where the end of the ribbon is on the ruler. The left end of the ribbon is at 0, so the number at its right end is its length.the end of the ribbon is just after the 8 cm mark, with 3 small marks to go.
- Count the small marks after the big mark. The tiny marks are millimetres. There are 10 of them in every centimetre.3 small marks = 3 mm
- Write the millimetres as a part of a centimetre. 1 mm is one tenth of a cm, so 1 mm = 0.1 cm.3 mm = 3 tenths of a cm = 0.3 cm
- Put the centimetres and the part together. The whole centimetres go before the decimal point.8 + 0.3 = 8.3 cm
Check. 8.3 cm × 10 = 83 mm. ✓
Watch out. Start counting from the 0 mark, not from the end of the ruler. Ten small marks make one centimetre, not five.
Key words. A ruler measures length. Centimetres (cm) are the big numbered marks. Millimetres (mm) are the tiny marks: 10 mm make 1 cm. The scale is the line of numbers and marks.
Method. When an object does not start at 0, read both ends and subtract the smaller reading from the bigger one.
- Read the left end. The strip does not start at 0, so we need both ends.The left end is at 2 cm 3 mm = 23 mm
- Read the right end. Do the same for the other end.The right end is at 9 cm 6 mm = 96 mm
- Take the left end away from the right end. The length is the distance between the two ends.96 − 23 = 73 mm
- Write the length in the unit asked. 10 mm = 1 cm.73 mm = 7 cm 3 mm
Check. Count on from the left end: 23 mm + 73 mm = 96 mm. ✓
Watch out. Do not just read the right end. That would be the distance from 0, not the length of the strip.
Key words. A ruler measures length. Centimetres (cm) are the big numbered marks. Millimetres (mm) are the tiny marks: 10 mm make 1 cm. The scale is the line of numbers and marks.
Method. Read the ruler where the arrow points: whole centimetres first, then the small marks.
- Find where the arrow is on the ruler. Read the number at the big mark just before it.the arrow is just after the 4 cm mark, with 7 small marks to go.
- Count the small marks after the big mark. The tiny marks are millimetres. There are 10 of them in every centimetre. The long middle mark is 5 mm.7 small marks = 7 mm
- Write the length. Say the centimetres, then the millimetres.4 cm and 7 mm = 4 cm 7 mm
Check. 4 cm = 40 mm and 40 mm + 7 mm = 47 mm. ✓
Watch out. Start counting from the 0 mark, not from the end of the ruler. Ten small marks make one centimetre, not five.
Key words. A ruler measures length. Centimetres (cm) are the big numbered marks. Millimetres (mm) are the tiny marks: 10 mm make 1 cm. The scale is the line of numbers and marks.
Method. Read the ruler at every pointer. Pick the pointer that matches the measurement.
- Find the measurement we want on the ruler. 6 cm 5 mm means go to the 6 cm mark, then count 5 small marks.6 cm 5 mm = 65 mm
- Read each letter. Read the big mark just before the pointer, then count the small marks.D is at 4 cm
A is at 5 cm 6 mm
B is at 6 cm 5 mm
C is at 7 cm 1 mm - Choose the matching letter. The letter at 6 cm 5 mm is B.
Check. B is at 6 cm 5 mm. ✓
Watch out. Count the small marks carefully. 6 cm 5 mm is not the same as 5 cm 6 mm.
Watch out
Start at 0, and count the tiny marks. There are 10 tiny marks in each centimetre, so after the 7 cm mark the first tiny mark is 7 cm 1 mm. 6 cm 5 mm is not the same as 5 cm 6 mm. And 4.5 cm is 45 mm, not 4.5 mm.Exercise 7A Reading a ruler
Non-calculatorKey words. A ruler measures length. Centimetres (cm) are the big numbered marks. Millimetres (mm) are the tiny marks: 10 mm make 1 cm. The scale is the line of numbers and marks.
Method. Read the ruler at the end of the object. The big marks are centimetres and the small marks are millimetres.
- Find where the end of the string is on the ruler. The left end of the string is at 0, so the number at its right end is its length.the end of the string is just after the 5 cm mark, with 8 small marks to go.
- Count the small marks after the big mark. The tiny marks are millimetres. There are 10 of them in every centimetre. The long middle mark is 5 mm.8 small marks = 8 mm
- Write the length. Say the centimetres, then the millimetres.5 cm and 8 mm = 5 cm 8 mm
Check. 5 cm = 50 mm and 50 mm + 8 mm = 58 mm. ✓
Watch out. Start counting from the 0 mark, not from the end of the ruler. Ten small marks make one centimetre, not five.
Key words. A ruler measures length. Centimetres (cm) are the big numbered marks. Millimetres (mm) are the tiny marks: 10 mm make 1 cm. The scale is the line of numbers and marks.
Method. Read the ruler at the end of the object. The big marks are centimetres and the small marks are millimetres.
- Find where the end of the pencil is on the ruler. The left end of the pencil is at 0, so the number at its right end is its length.the end of the pencil is just after the 10 cm mark, with 3 small marks to go.
- Count the small marks after the big mark. The tiny marks are millimetres. There are 10 of them in every centimetre.3 small marks = 3 mm
- Write the length. Say the centimetres, then the millimetres.10 cm and 3 mm = 10 cm 3 mm
Check. 10 cm = 100 mm and 100 mm + 3 mm = 103 mm. ✓
Watch out. Start counting from the 0 mark, not from the end of the ruler. Ten small marks make one centimetre, not five.
Key words. A ruler measures length. Centimetres (cm) are the big numbered marks. Millimetres (mm) are the tiny marks: 10 mm make 1 cm. The scale is the line of numbers and marks.
Method. Read the ruler at the end of the object. The big marks are centimetres and the small marks are millimetres.
- Find where the end of the line is on the ruler. The left end of the line is at 0, so the number at its right end is its length.the end of the line is just after the 8 cm mark, with 6 small marks to go.
- Count the small marks after the big mark. The tiny marks are millimetres. There are 10 of them in every centimetre. The long middle mark is 5 mm.6 small marks = 6 mm
- Write the length. Say the centimetres, then the millimetres.8 cm and 6 mm = 8 cm 6 mm
Check. 8 cm = 80 mm and 80 mm + 6 mm = 86 mm. ✓
Watch out. Start counting from the 0 mark, not from the end of the ruler. Ten small marks make one centimetre, not five.
Key words. A ruler measures length. Centimetres (cm) are the big numbered marks. Millimetres (mm) are the tiny marks: 10 mm make 1 cm. The scale is the line of numbers and marks.
Method. Read the ruler at the end of the object. The big marks are centimetres and the small marks are millimetres.
- Find where the end of the strip is on the ruler. The left end of the strip is at 0, so the number at its right end is its length.the end of the strip is just after the 3 cm mark, with 7 small marks to go.
- Count the small marks after the big mark. The tiny marks are millimetres. There are 10 of them in every centimetre. The long middle mark is 5 mm.7 small marks = 7 mm
- Change the centimetres to millimetres. 1 cm = 10 mm, so multiply the centimetres by 10.3 × 10 = 30 mm
- Add the extra millimetres. 30 + 7 = 37 mm
Check. 37 ÷ 10 = 3 remainder 7, which is 3 cm 7 mm. ✓
Watch out. Start counting from the 0 mark, not from the end of the ruler. Ten small marks make one centimetre, not five.
Key words. A ruler measures length. Centimetres (cm) are the big numbered marks. Millimetres (mm) are the tiny marks: 10 mm make 1 cm. The scale is the line of numbers and marks.
Method. Read the ruler at the end of the object. The big marks are centimetres and the small marks are millimetres.
- Find where the end of the strip is on the ruler. The left end of the strip is at 0, so the number at its right end is its length.the end of the strip is just after the 6 cm mark, with 4 small marks to go.
- Count the small marks after the big mark. The tiny marks are millimetres. There are 10 of them in every centimetre.4 small marks = 4 mm
- Change the centimetres to millimetres. 1 cm = 10 mm, so multiply the centimetres by 10.6 × 10 = 60 mm
- Add the extra millimetres. 60 + 4 = 64 mm
Check. 64 ÷ 10 = 6 remainder 4, which is 6 cm 4 mm. ✓
Watch out. Start counting from the 0 mark, not from the end of the ruler. Ten small marks make one centimetre, not five.
Key words. A ruler measures length. Centimetres (cm) are the big numbered marks. Millimetres (mm) are the tiny marks: 10 mm make 1 cm. The scale is the line of numbers and marks.
Method. Read the ruler at the end of the object. The big marks are centimetres and the small marks are millimetres.
- Find where the end of the strip is on the ruler. The left end of the strip is at 0, so the number at its right end is its length.the end of the strip is just after the 10 cm mark, with 8 small marks to go.
- Count the small marks after the big mark. The tiny marks are millimetres. There are 10 of them in every centimetre. The long middle mark is 5 mm.8 small marks = 8 mm
- Change the centimetres to millimetres. 1 cm = 10 mm, so multiply the centimetres by 10.10 × 10 = 100 mm
- Add the extra millimetres. 100 + 8 = 108 mm
Check. 108 ÷ 10 = 10 remainder 8, which is 10 cm 8 mm. ✓
Watch out. Start counting from the 0 mark, not from the end of the ruler. Ten small marks make one centimetre, not five.
Key words. A ruler measures length. Centimetres (cm) are the big numbered marks. Millimetres (mm) are the tiny marks: 10 mm make 1 cm. The scale is the line of numbers and marks.
Method. Read the ruler at the end of the object. The big marks are centimetres and the small marks are millimetres.
- Find where the end of the ribbon is on the ruler. The left end of the ribbon is at 0, so the number at its right end is its length.the end of the ribbon is just after the 5 cm mark, with 2 small marks to go.
- Count the small marks after the big mark. The tiny marks are millimetres. There are 10 of them in every centimetre.2 small marks = 2 mm
- Write the millimetres as a part of a centimetre. 1 mm is one tenth of a cm, so 1 mm = 0.1 cm.2 mm = 2 tenths of a cm = 0.2 cm
- Put the centimetres and the part together. The whole centimetres go before the decimal point.5 + 0.2 = 5.2 cm
Check. 5.2 cm × 10 = 52 mm. ✓
Watch out. Start counting from the 0 mark, not from the end of the ruler. Ten small marks make one centimetre, not five.
Key words. A ruler measures length. Centimetres (cm) are the big numbered marks. Millimetres (mm) are the tiny marks: 10 mm make 1 cm. The scale is the line of numbers and marks.
Method. Read the ruler at the end of the object. The big marks are centimetres and the small marks are millimetres.
- Find where the end of the ribbon is on the ruler. The left end of the ribbon is at 0, so the number at its right end is its length.the end of the ribbon is just after the 7 cm mark, with 9 small marks to go.
- Count the small marks after the big mark. The tiny marks are millimetres. There are 10 of them in every centimetre. The long middle mark is 5 mm.9 small marks = 9 mm
- Write the millimetres as a part of a centimetre. 1 mm is one tenth of a cm, so 1 mm = 0.1 cm.9 mm = 9 tenths of a cm = 0.9 cm
- Put the centimetres and the part together. The whole centimetres go before the decimal point.7 + 0.9 = 7.9 cm
Check. 7.9 cm × 10 = 79 mm. ✓
Watch out. Start counting from the 0 mark, not from the end of the ruler. Ten small marks make one centimetre, not five.
Key words. A ruler measures length. Centimetres (cm) are the big numbered marks. Millimetres (mm) are the tiny marks: 10 mm make 1 cm. The scale is the line of numbers and marks.
Method. Read the ruler at the end of the object. The big marks are centimetres and the small marks are millimetres.
- Find where the end of the ribbon is on the ruler. The left end of the ribbon is at 0, so the number at its right end is its length.the end of the ribbon is just after the 11 cm mark, with 5 small marks to go.
- Count the small marks after the big mark. The tiny marks are millimetres. There are 10 of them in every centimetre. The long middle mark is 5 mm.5 small marks = 5 mm
- Write the millimetres as a part of a centimetre. 1 mm is one tenth of a cm, so 1 mm = 0.1 cm.5 mm = 5 tenths of a cm = 0.5 cm
- Put the centimetres and the part together. The whole centimetres go before the decimal point.11 + 0.5 = 11.5 cm
Check. 11.5 cm × 10 = 115 mm. ✓
Watch out. Start counting from the 0 mark, not from the end of the ruler. Ten small marks make one centimetre, not five.
Key words. A ruler measures length. Centimetres (cm) are the big numbered marks. Millimetres (mm) are the tiny marks: 10 mm make 1 cm. The scale is the line of numbers and marks.
Method. Read the ruler at the end of the object. The big marks are centimetres and the small marks are millimetres.
- Find where the end of the line is on the ruler. The left end of the line is at 0, so the number at its right end is its length.the end of the line is just after the 4 cm mark, with 3 small marks to go.
- Count the small marks after the big mark. The tiny marks are millimetres. There are 10 of them in every centimetre.3 small marks = 3 mm
- Look at the extra millimetres. To round to the nearest cm, look at the millimetres. Less than 5 mm rounds DOWN.3 mm is less than 5, so stay at 4 cm
Check. The length is 4 cm 3 mm. It is closer to 4 cm than to the other whole centimetre. ✓
Watch out. Start counting from the 0 mark, not from the end of the ruler. Ten small marks make one centimetre, not five.
Key words. A ruler measures length. Centimetres (cm) are the big numbered marks. Millimetres (mm) are the tiny marks: 10 mm make 1 cm. The scale is the line of numbers and marks.
Method. Read the ruler at the end of the object. The big marks are centimetres and the small marks are millimetres.
- Find where the end of the line is on the ruler. The left end of the line is at 0, so the number at its right end is its length.the end of the line is just after the 6 cm mark, with 5 small marks to go.
- Count the small marks after the big mark. The tiny marks are millimetres. There are 10 of them in every centimetre. The long middle mark is 5 mm.5 small marks = 5 mm
- Look at the extra millimetres. To round to the nearest cm, look at the millimetres. 5 mm or more rounds UP.5 mm is 5 or more, so round up from 6 cm to 7 cm
Check. The length is 6 cm 5 mm. It is closer to 7 cm than to the other whole centimetre. ✓
Watch out. Start counting from the 0 mark, not from the end of the ruler. Ten small marks make one centimetre, not five.
Key words. A ruler measures length. Centimetres (cm) are the big numbered marks. Millimetres (mm) are the tiny marks: 10 mm make 1 cm. The scale is the line of numbers and marks.
Method. Read the ruler at the end of the object. The big marks are centimetres and the small marks are millimetres.
- Find where the end of the line is on the ruler. The left end of the line is at 0, so the number at its right end is its length.the end of the line is just after the 7 cm mark, with 8 small marks to go.
- Count the small marks after the big mark. The tiny marks are millimetres. There are 10 of them in every centimetre. The long middle mark is 5 mm.8 small marks = 8 mm
- Look at the extra millimetres. To round to the nearest cm, look at the millimetres. 5 mm or more rounds UP.8 mm is 5 or more, so round up from 7 cm to 8 cm
Check. The length is 7 cm 8 mm. It is closer to 8 cm than to the other whole centimetre. ✓
Watch out. Start counting from the 0 mark, not from the end of the ruler. Ten small marks make one centimetre, not five.
Key words. A ruler measures length. Centimetres (cm) are the big numbered marks. Millimetres (mm) are the tiny marks: 10 mm make 1 cm. The scale is the line of numbers and marks.
Method. Read the ruler where the arrow points: whole centimetres first, then the small marks.
- Find where the arrow is on the ruler. Read the number at the big mark just before it.the arrow is just after the 2 cm mark, with 5 small marks to go.
- Count the small marks after the big mark. The tiny marks are millimetres. There are 10 of them in every centimetre. The long middle mark is 5 mm.5 small marks = 5 mm
- Write the length. Say the centimetres, then the millimetres.2 cm and 5 mm = 2 cm 5 mm
Check. 2 cm = 20 mm and 20 mm + 5 mm = 25 mm. ✓
Watch out. Start counting from the 0 mark, not from the end of the ruler. Ten small marks make one centimetre, not five.
Key words. A ruler measures length. Centimetres (cm) are the big numbered marks. Millimetres (mm) are the tiny marks: 10 mm make 1 cm. The scale is the line of numbers and marks.
Method. Read the ruler where the arrow points: whole centimetres first, then the small marks.
- Find where the arrow is on the ruler. Read the number at the big mark just before it.the arrow is just after the 4 cm mark, with 7 small marks to go.
- Count the small marks after the big mark. The tiny marks are millimetres. There are 10 of them in every centimetre. The long middle mark is 5 mm.7 small marks = 7 mm
- Write the length. Say the centimetres, then the millimetres.4 cm and 7 mm = 4 cm 7 mm
Check. 4 cm = 40 mm and 40 mm + 7 mm = 47 mm. ✓
Watch out. Start counting from the 0 mark, not from the end of the ruler. Ten small marks make one centimetre, not five.
Key words. A ruler measures length. Centimetres (cm) are the big numbered marks. Millimetres (mm) are the tiny marks: 10 mm make 1 cm. The scale is the line of numbers and marks.
Method. Read the ruler where the arrow points: whole centimetres first, then the small marks.
- Find where the arrow is on the ruler. Read the number at the big mark just before it.the arrow is just after the 8 cm mark, with 2 small marks to go.
- Count the small marks after the big mark. The tiny marks are millimetres. There are 10 of them in every centimetre.2 small marks = 2 mm
- Write the length. Say the centimetres, then the millimetres.8 cm and 2 mm = 8 cm 2 mm
Check. 8 cm = 80 mm and 80 mm + 2 mm = 82 mm. ✓
Watch out. Start counting from the 0 mark, not from the end of the ruler. Ten small marks make one centimetre, not five.
Key words. A ruler measures length. Centimetres (cm) are the big numbered marks. Millimetres (mm) are the tiny marks: 10 mm make 1 cm. The scale is the line of numbers and marks.
Method. When an object does not start at 0, read both ends and subtract the smaller reading from the bigger one.
- Read the left end. The strip does not start at 0, so we need both ends.The left end is at 1 cm 5 mm = 15 mm
- Read the right end. Do the same for the other end.The right end is at 6 cm 2 mm = 62 mm
- Take the left end away from the right end. The length is the distance between the two ends.62 − 15 = 47 mm
- Write the length in the unit asked. 10 mm = 1 cm.47 mm = 4 cm 7 mm
Check. Count on from the left end: 15 mm + 47 mm = 62 mm. ✓
Watch out. Do not just read the right end. That would be the distance from 0, not the length of the strip.
Key words. A ruler measures length. Centimetres (cm) are the big numbered marks. Millimetres (mm) are the tiny marks: 10 mm make 1 cm. The scale is the line of numbers and marks.
Method. When an object does not start at 0, read both ends and subtract the smaller reading from the bigger one.
- Read the left end. The strip does not start at 0, so we need both ends.The left end is at 3 cm 1 mm = 31 mm
- Read the right end. Do the same for the other end.The right end is at 8 cm 9 mm = 89 mm
- Take the left end away from the right end. The length is the distance between the two ends.89 − 31 = 58 mm
- Write the length in the unit asked. 10 mm = 1 cm.58 mm = 5 cm 8 mm
Check. Count on from the left end: 31 mm + 58 mm = 89 mm. ✓
Watch out. Do not just read the right end. That would be the distance from 0, not the length of the strip.
Key words. A ruler measures length. Centimetres (cm) are the big numbered marks. Millimetres (mm) are the tiny marks: 10 mm make 1 cm. The scale is the line of numbers and marks.
Method. When an object does not start at 0, read both ends and subtract the smaller reading from the bigger one.
- Read the left end. The strip does not start at 0, so we need both ends.The left end is at 2 cm 4 mm = 24 mm
- Read the right end. Do the same for the other end.The right end is at 10 cm 7 mm = 107 mm
- Take the left end away from the right end. The length is the distance between the two ends.107 − 24 = 83 mm
- Write the length in the unit asked. 10 mm = 1 cm.83 mm = 8 cm 3 mm
Check. Count on from the left end: 24 mm + 83 mm = 107 mm. ✓
Watch out. Do not just read the right end. That would be the distance from 0, not the length of the strip.
Key words. A ruler measures length. Centimetres (cm) are the big numbered marks. Millimetres (mm) are the tiny marks: 10 mm make 1 cm. The scale is the line of numbers and marks.
Method. Read the ruler at every pointer. Pick the pointer that matches the measurement.
- Find the measurement we want on the ruler. 3 cm 8 mm means go to the 3 cm mark, then count 8 small marks.3 cm 8 mm = 38 mm
- Read each letter. Read the big mark just before the pointer, then count the small marks.C is at 2 cm 9 mm
A is at 3 cm 8 mm
D is at 4 cm 7 mm
B is at 8 cm 3 mm - Choose the matching letter. The letter at 3 cm 8 mm is A.
Check. A is at 3 cm 8 mm. ✓
Watch out. Count the small marks carefully. 6 cm 5 mm is not the same as 5 cm 6 mm.
Key words. A ruler measures length. Centimetres (cm) are the big numbered marks. Millimetres (mm) are the tiny marks: 10 mm make 1 cm. The scale is the line of numbers and marks.
Method. Read the ruler at every pointer. Pick the pointer that matches the measurement.
- Find the measurement we want on the ruler. 8 cm 3 mm means go to the 8 cm mark, then count 3 small marks.8 cm 3 mm = 83 mm
- Read each letter. Read the big mark just before the pointer, then count the small marks.C is at 2 cm 9 mm
A is at 3 cm 8 mm
D is at 4 cm 7 mm
B is at 8 cm 3 mm - Choose the matching letter. The letter at 8 cm 3 mm is B.
Check. B is at 8 cm 3 mm. ✓
Watch out. Count the small marks carefully. 6 cm 5 mm is not the same as 5 cm 6 mm.
Key words. A ruler measures length. Centimetres (cm) are the big numbered marks. Millimetres (mm) are the tiny marks: 10 mm make 1 cm. The scale is the line of numbers and marks.
Method. Read both lengths, change them to the same unit, then compare or subtract.
- Read the first length. Change it to millimetres so both can be compared easily.Priya’s pencil: 12 cm 6 mm = 126 mm
- Read the second length. Do the same.Tom’s pencil: 11 cm 8 mm = 118 mm
- Find which is longer. The bigger number of millimetres is the longer length.126 is more than 118, so Priya’s pencil is longer.
Check. 118 + 8 = 126. ✓
Watch out. Read each ruler carefully before you compare. Do not compare cm of one with mm of the other.
Key words. A ruler measures length. Centimetres (cm) are the big numbered marks. Millimetres (mm) are the tiny marks: 10 mm make 1 cm. The scale is the line of numbers and marks.
Method. Read both lengths, change them to the same unit, then compare or subtract.
- Read the first length. Change it to millimetres so both can be compared easily.Priya’s pencil: 12 cm 6 mm = 126 mm
- Read the second length. Do the same.Tom’s pencil: 11 cm 8 mm = 118 mm
- Find which is longer. The bigger number of millimetres is the longer length.126 is more than 118, so Priya’s pencil is longer.
- Find the difference. Take the shorter length away from the longer length.126 − 118 = 8 mm = 8 mm
Check. 118 + 8 = 126. ✓
Watch out. Read each ruler carefully before you compare. Do not compare cm of one with mm of the other.
Key words. A ruler measures length. Centimetres (cm) are the big numbered marks. Millimetres (mm) are the tiny marks: 10 mm make 1 cm. The scale is the line of numbers and marks.
Method. Read the ruler at the end of the object. The big marks are centimetres and the small marks are millimetres.
- Find where the end of the ribbon is on the ruler. The left end of the ribbon is at 0, so the number at its right end is its length.the end of the ribbon is just after the 11 cm mark, with 2 small marks to go.
- Count the small marks after the big mark. The tiny marks are millimetres. There are 10 of them in every centimetre.2 small marks = 2 mm
- Write the length. Say the centimetres, then the millimetres.11 cm and 2 mm = 11 cm 2 mm
Check. 11 cm = 110 mm and 110 mm + 2 mm = 112 mm. ✓
Watch out. Start counting from the 0 mark, not from the end of the ruler. Ten small marks make one centimetre, not five.
Key words. To add or subtract measures, they must be in the same unit. The total is all the amounts added together. The difference is how much more one amount is than another.
Method. Change the measures to the same unit. Then add or subtract the numbers, and give the answer in the unit asked.
- Change both measures to millimetres. We can only add or subtract measures that are in the same unit. Use the smaller unit so we have whole numbers.11 cm 2 mm = 11 × 10 + 2 = 110 + 2 = 112 mm
4 cm 7 mm = 4 × 10 + 7 = 40 + 7 = 47 mm - Take the smaller measure away from the bigger. Subtract column by column. Exchange when a top digit is too small.112 mm − 47 mm = 65 mm
H T O 0 110 112 2− 4 7 6 5 - Write the answer in centimetres and millimetres. 10 mm = 1 cm, so divide by 10 and keep the remainder.65 ÷ 10 = 6 remainder 5
65 mm = 6 cm 5 mm
Check. Use the inverse: 65 + 47 = 112. ✓
Watch out. You can only add or subtract measures in the same unit. 1 m + 45 cm is not 46 of anything: change it to 145 cm first.
Key words. A ruler measures length. Centimetres (cm) are the big numbered marks. Millimetres (mm) are the tiny marks: 10 mm make 1 cm. The scale is the line of numbers and marks.
Method. Read both ends of the strip. The strip starts at 2 cm, not at 0.
- Look at what Leo did. Leo read the number at the right end of the strip and said that was the length.Right end = 9 cm 3 mm. Leo says the length is 9 cm 3 mm.
- Find the mistake. The strip does not start at 0. Its left end is at 2 cm. Leo measured from 0 to the right end, which includes the empty part.The left end is at 2 cm = 20 mm. The right end is at 9 cm 3 mm = 93 mm.
- Work out the correct length. Take the left reading away from the right reading.93 − 20 = 73 mm = 7 cm 3 mm
Check. Count on from the left end: 2 cm + 7 cm 3 mm = 9 cm 3 mm. ✓
Watch out. Check where the strip starts. If it does not start at 0, subtract.
Key words. A ruler measures length. Centimetres (cm) are the big numbered marks. Millimetres (mm) are the tiny marks: 10 mm make 1 cm. The scale is the line of numbers and marks.
Method. To the nearest centimetre, look at the millimetres. 5 mm or more rounds up.
- Read the line. The line starts at 0 and ends 5 small marks after 7 cm.7 cm 5 mm
- Look at the millimetres. We round to the nearest cm, so we look at the mm. 5 mm is exactly halfway, and halfway rounds UP.5 mm is 5 or more, so round up from 7 cm to 8 cm
- Is Aisha right? She said 7 cm.No. The length is 8 cm to the nearest cm.
Check. 7 cm 5 mm is the same distance from 7 cm and 8 cm. We always round halves up, so it is 8 cm. ✓
Watch out. Halfway rounds up.
Key words. A ruler measures length. Centimetres (cm) are the big numbered marks. Millimetres (mm) are the tiny marks: 10 mm make 1 cm. The scale is the line of numbers and marks.
Method. A length rounds to 8 cm if it is closer to 8 cm than to 7 cm or 9 cm. Think about the millimetres.
- Find the shortest length. Half way between 7 cm and 8 cm is 7 cm 5 mm. That rounds UP to 8 cm. Anything shorter rounds down to 7 cm.7 cm 4 mm rounds to 7 cm, but 7 cm 5 mm rounds to 8 cm
- Find the longest length. 8 cm 5 mm would round up to 9 cm. So the longest length is 1 mm less.8 cm 5 mm rounds to 9 cm, but 8 cm 4 mm rounds to 8 cm
- Write both answers in mm as a check. Shortest: 75 mm. Longest: 84 mm.
Check. 75 mm: tens and ones 7 cm 5 mm rounds to 8. 84 mm = 8 cm 4 mm rounds to 8. 85 mm rounds to 9. ✓
Watch out. 5 mm rounds up. Only 4 mm or less rounds down.