Calcylator
Millimetres to Centimetres

Millimetres to cm:
divide by 10

Convert millimetres to centimetres for rainfall, screens and craft projects, with a chart, worked examples and the mistakes to avoid.

Calcylator Editorial Team

Updated · 5 min read

How many centimetres are in a millimetre?

One millimetre is 0.1 centimetres, because there are exactly 10 mm in 1 cm. So you divide millimetres by 10 to get centimetres.

This is a metric conversion, so it works by moving the decimal point one place to the left. There is no rounding and no memorised factor beyond ten.

The mm to cm formula

Millimetres to centimetres =cm = mm ÷ 10
cm:
length in centimetres
mm:
length in millimetres
10:
millimetres in one centimetre (exact)
To go the other way, multiply by 10: mm = cm × 10.

  • Length

    37 mm

  • Divide

    37 ÷ 10

Length in cm

3.7 cm

Move the decimal one place left: 37 becomes 3.7.

In reverse, a length of 8.5 cm is 8.5 × 10 = 85 mm, found by moving the decimal one place to the right.

  1. Write the length in millimetres.
  2. Move the decimal point one place to the left, or divide by 10.
  3. For cm to mm, move it one place to the right.
  4. Keep the digits the same; only the decimal point moves.

Chart: millimetres to centimetres from 1 to 1000

Use this to check a conversion at a glance.

Millimetres to centimetres
MillimetresCentimetres
10.1
20.2
50.5
101
252.5
505
10010
25025
50050
1000100

Ten millimetres is exactly 1 cm, and 1,000 mm is 100 cm, which is one metre.

Where mm to cm comes up

Rainfall is reported in millimetres, but a gardener measuring a gauge with a ruler in centimetres sees 17.2 cm of water as 172 mm.

Screen and photo sizes often mix the two. A 0.45 mm pen tip is only 0.045 cm, which shows why such small sizes are written in millimetres.

In sewing, a seam allowance of 15 mm is 1.5 cm, so a pattern in cm can be checked against a tape marked in millimetres. A 24 cm ruler is 240 mm long.

Product sizes are a good place to practise. A phone listed as 147 mm tall is 14.7 cm, and a 6 mm screw is 0.6 cm across. Packaging also mixes the two: a box labelled 250 x 180 x 40 mm measures 25 by 18 by 4 cm. Reading the smaller number, 40 mm, as 4 cm makes it easy to picture how slim the box is on a shelf.

Common mistakes converting mm to cm

  • Multiplying by 10 instead of dividing. A cm is bigger than a mm, so the number of centimetres is always smaller.
  • Moving the decimal two or three places. Two places is the step from cm to metres and three is mm to metres, but mm to cm is only one.
  • Reading a ruler wrongly. The small lines are millimetres and the numbered ones are centimetres.
  • Mixing up cm with m. 100 cm is 1 m, but 100 mm is only 10 cm.

Common questions

How many cm is 1 mm?

One millimetre is 0.1 centimetres. There are exactly 10 mm in 1 cm, so you divide the millimetres by 10 to get centimetres. This is an exact metric relationship with no rounding needed.

How do I convert mm to cm?

Divide the millimetres by 10, or move the decimal point one place to the left. For example, 250 mm is 25 cm and 37 mm is 3.7 cm. To go back to mm, multiply the centimetres by 10.

How many mm are in 8.5 cm?

There are 85 mm in 8.5 cm, found by multiplying 8.5 by 10. Move the decimal point one place to the right, so 8.5 becomes 85. The same rule works for any centimetre length.

Is 10 mm the same as 1 cm?

Yes. 10 mm is exactly 1 cm. Because the metric system is based on tens, you can convert between millimetres and centimetres without a calculator by shifting the decimal point one place.

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