Calcylator
Centimetres to Millimetres

Centimetres to millimetres:
multiply by 10

Convert cm to mm for rulers, drawings and screen sizes, with a chart and the slips that cause a tenfold error.

Calcylator Editorial Team

Updated · 5 min read

How many millimetres are in a centimetre?

There are exactly 10 millimetres in one centimetre. A centimetre is a hundredth of a metre, a millimetre is a thousandth of a metre, so each centimetre holds ten millimetres.

On a standard ruler, the numbered lines mark centimetres and the small unnumbered lines between them mark millimetres. Count ten small gaps between any two numbers.

The centimetre and the millimetre both belong to the metric system, which is built on powers of ten, so converting between them never needs more than shifting a decimal point. Even so, mistakes are common because everyday life uses both. Clothing sizes, furniture and body measurements are usually in centimetres, while hardware, screws, drill bits, camera lenses and screen thicknesses are quoted in millimetres. Knowing the factor of 10 lets you move between a shop's measurements and a manufacturer's data sheet with confidence. A ruler is the best teacher here, because the same scale shows the centimetre marks and the millimetre ticks together.

The cm to mm formula

Centimetres to millimetres =mm = cm × 10
mm:
length in millimetres
cm:
length in centimetres
10:
millimetres in one centimetre (exact)
To go back, divide: cm = mm ÷ 10.

  • Phone width

    8.5 cm

  • Multiply

    8.5 × 10

Length in mm

85 mm

Move the decimal point one place to the right.

Going back works the same way in reverse. A 245 mm screw is 245 ÷ 10 = 24.5 cm, and a 12.5 cm shelf bracket is 125 mm.

  1. Write the length in centimetres.
  2. Multiply by 10, or move the decimal point one place right.
  3. Round only if the job allows it.
  4. Check that the mm figure is ten times the cm figure.

Chart: centimetres to millimetres

Centimetres to millimetres
cmmm
1 cm10 mm
2 cm20 mm
5 cm50 mm
10 cm100 mm
25 cm250 mm
50 cm500 mm
100 cm1000 mm
250 cm2500 mm
500 cm5000 mm
1000 cm10000 mm

Because the factor is only 10, you can read any value off the chart by shifting a single digit.

Where cm and mm both appear

  • Building and DIY: a drawing may show 2.5 cm trim, while a saw guide or drill bit is marked in millimetres.
  • Tailoring and crafts, where a pattern in cm needs a seam allowance given in mm.
  • Sports and science charts that record ball diameters, insect lengths or rainfall.
  • Product pages that list screens or parts in both centimetres and millimetres.

Quick sanity checks for centimetres to millimetres

Think of a sheet of A4 paper. It is 21 cm by 29.7 cm, which is 210 mm by 297 mm, and the factor of 10 explains both.

If you can read a ruler in centimetres, you can read it in millimetres: just count the small lines. Seven and a half centimetres is 75 mm.

Remember the tenfold check. A millimetre value that is not exactly ten times the centimetre figure is wrong.

Common mistakes when converting cm to mm

  • Dividing when you should multiply. Millimetre numbers are always ten times bigger than centimetre numbers.
  • Moving the decimal point two places, which is a metre-to-centimetre habit.
  • Mixing up 0.5 cm and 5 cm. Half a centimetre is 5 mm, while 5 cm is 50 mm.
  • Quoting sizes without units. A hole of 8 could mean 8 mm or 8 cm, and the two differ tenfold.

Common questions

How many mm are in a cm?

There are exactly 10 millimetres in one centimetre. So 2 cm is 20 mm and 0.5 cm is 5 mm. Each centimetre on a ruler is divided into ten equal small steps, and each step is one millimetre.

How do I convert cm to mm?

Multiply the centimetres by 10, which shifts the decimal point one place to the right. For example, 12.5 cm is 125 mm. The millimetre figure is always ten times bigger than the centimetre figure.

How do I convert mm to cm?

Divide the millimetres by 10, or move the decimal point one place to the left. For example, 245 mm is 24.5 cm. The result is always one tenth of the millimetre value.

How many cm and mm are in a metre?

A metre is 100 centimetres, which is 1,000 millimetres. So 1 cm is 10 mm, and a 30 cm ruler is 300 mm long. Each step between metre, centimetre and millimetre is a simple shift of the decimal point.

Was this guide helpful?

Continue reading

View all blogs