Reading a Ruler in cm and mm

A ruler is a number line you can hold

A ruler is just a number line printed on a strip you can lay against an object. The numbers you can see are centimetres (cm). Between those numbers are lots of tiny marks. Each tiny mark is one millimetre (mm).

The most important fact about a ruler is this:

There are 10 millimetres in every centimetre.

So between the 4 and the 5 on your ruler, there are 10 small spaces. If you can read both the big marks and the small marks, you can measure anything to the nearest millimetre. You can revise the units themselves in Units of Length, Mass and Capacity.

Step 1: line up the zero

This is the step most people get wrong. Do not start measuring from the metal edge or the very end of the ruler. On most rulers there is a small gap before the 0 mark.

• Put the 0 mark of the ruler right at the start of the object.
• Hold the ruler flat and steady against the object.
• Now look at where the object ends.

If you start from the edge instead of zero, every measurement comes out a few millimetres too long.

Step 2: read the whole centimetres

Look at the far end of the object. Find the last numbered mark it has passed. That is your number of whole centimetres.

For example, if a crayon reaches past the 7 but not yet the 8, it is 7-and-a-bit centimetres. Now we need to find the "bit".

Step 3: count the extra millimetres

After the last whole centimetre, count the small marks up to the end of the object. Each small mark is 1 mm.

If the crayon ends on the 4th small mark past 7 cm, the extra is 4 mm. So the crayon is 7 cm and 4 mm.

Writing it as a decimal

Because 10 mm make 1 cm, each millimetre is one tenth of a centimetre. That lets us write the length as a decimal.

• 7 cm and 4 mm = 7.4 cm
• 5 cm and 7 mm = 5.7 cm
• 12 cm and 0 mm = 12.0 cm, or just 12 cm

The number after the decimal point is simply the number of extra millimetres.

Worked example 1: a leaf

A leaf is laid along a ruler with its tip at the 0 mark. The other end reaches the 6th small mark past 9 cm.

1. Whole centimetres: 9 cm.
2. Extra small marks: 6, so 6 mm.
3. Length = 9 cm 6 mm = 9.6 cm.

Worked example 2: a screw

A screw's flat end is at zero. The pointed end stops exactly on the long mark for 4 cm, with no small marks past it.

1. Whole centimetres: 4 cm.
2. Extra millimetres: 0.
3. Length = 4 cm = 4.0 cm.

Landing exactly on a numbered mark means the millimetre count is zero.

Worked example 3: changing the answer to mm only

Sometimes you need the answer in millimetres alone. To turn centimetres into millimetres, multiply by 10, then add the extra millimetres.

The leaf above was 9 cm 6 mm. How many millimetres is that?

1. 9 cm × 10 = 90 mm.
2. Add the 6 extra: 90 + 6 = 96 mm.

So 9 cm 6 mm = 96 mm. This is the same idea as Multiplying by 10, 100 and 1000.

Quick reference

You see	What it means
A long numbered mark	a whole centimetre
A small mark between numbers	one millimetre
10 small marks	one full centimetre
End on 3rd mark past 6 cm	6 cm 3 mm = 6.3 cm
6 cm 3 mm in mm	63 mm

Why millimetres matter

Why bother with the tiny marks at all? Because the world is full of objects that are not a whole number of centimetres. A key, a button or a Lego brick almost never ends exactly on a numbered mark. Millimetres let you give an honest, accurate measurement instead of just rounding to the nearest centimetre. Engineers, tailors and builders rely on millimetre accuracy every day.

Try it yourself

Grab a ruler and five small objects — a coin, a rubber, a key, a crayon and a paperclip work well.

1. For each object, line up the zero carefully.
2. Read the whole centimetres, then count the extra millimetres.
3. Write each length two ways, like 6 cm 4 mm and 6.4 cm.
4. Then write each one in millimetres only.

Well done!

You can now read a ruler in centimetres and millimetres, line up the zero properly, and write lengths as decimals. Practise more in Measuring Length and Height, or learn to read other scales in Reading Scales and Measuring.