How many millimetres are in a centimetre?

There are exactly 10 millimetres in one centimetre. Both units are part of the International System of Units (SI). The centimetre is defined as one hundredth of a metre, and the millimetre is one thousandth of a metre, so a centimetre is always exactly 10 millimetres. To convert cm to mm, multiply by 10. To convert mm to cm, divide by 10.

How do I convert cm to mm?

To convert centimetres to millimetres, multiply the number of centimetres by 10. For example: 5 cm = 5 x 10 = 50 mm. 12.5 cm = 12.5 x 10 = 125 mm. 0.8 cm = 0.8 x 10 = 8 mm. The conversion is exact because both units are defined within the SI metric system.

When would I use mm instead of cm?

Millimetres are preferred for small, precise measurements where fractions of a centimetre matter. Engineering drawings, joinery, cabinetmaking, plumbing fittings, and medical measurements typically use millimetres to avoid decimal fractions. For example, a 15 mm pipe fitting is clearer than 1.5 cm. Centimetres are more commonly used for everyday body measurements, clothing sizes, and distances that do not require fine precision.

cm to mm Converter

Enter a length in centimetres or millimetres to convert instantly between the two units, using the exact factor of 1 cm = 10 mm. See the equivalent in cm, mm and metres, plus a full reference table below.

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Exact conversion Based on the SI definition: 1 metre = 100 centimetres = 1,000 millimetres (BIPM, 2019).

1. Centimetres to Millimetres

Centimetres (cm)

Preset values (optional)

2. Millimetres to Centimetres

Millimetres (mm)

Preset values (optional)

Conversion Results

Millimetres
100
mm

Centimetres
10
cm

Metres

0.1

m (for reference)

cm to mm Reference Table

Centimetres (cm)	Millimetres (mm)	Metres (m)

How to Convert cm to mm

The conversion between centimetres and millimetres is one of the simplest in the metric system because both units share the same base (the metre) and differ by a factor of exactly 10.

The formula is:

mm = cm x 10

To go the other way:

cm = mm / 10

Worked Example

Convert 10 cm to mm:

Start with 10 cm.
Multiply by 10: 10 x 10 = 100.
Result: 100 mm.

Reverse: convert 100 mm to cm: 100 / 10 = 10 cm.

Understanding the Metric Length Units

The metric system defines length units as powers of ten relative to the metre:

Unit	Symbol	Relation to metre	Relation to cm
Kilometre	km	1,000 m	100,000 cm
Metre	m	1 m	100 cm
Centimetre	cm	0.01 m	1 cm
Millimetre	mm	0.001 m	0.1 cm

Because the conversion is a simple multiplication or division by 10, you can convert between cm and mm by shifting the decimal point one place: 4.7 cm becomes 47 mm, and 83 mm becomes 8.3 cm.

Common Uses of Millimetres vs Centimetres

Millimetres are the preferred unit in trades and engineering because they eliminate decimal fractions for small measurements. A joiner cutting timber to 47.5 mm is more precise and less ambiguous than saying 4.75 cm. Common uses of millimetres include:

Engineering and architectural drawings (all dimensions typically in mm)
Plumbing and pipe fittings (e.g. 15 mm, 22 mm, 32 mm pipes)
Woodworking and cabinetmaking
Medical measurements (wound sizes, tumour dimensions, catheter sizes)
Rainfall measurements (e.g. 12.5 mm of rain)
Tyre tread depth (legal minimum in NZ is 1.5 mm)

Centimetres are more common in everyday contexts: clothing measurements, height, ruler markings, and casual distance descriptions.

Quick Conversion Tips

To convert cm to mm: move the decimal point one place to the right (or add a zero to a whole number).
To convert mm to cm: move the decimal point one place to the left.
1 cm = 10 mm (exact, by definition).
1 mm = 0.1 cm (exact).
A standard ruler shows both cm and mm markings; the small lines between the centimetre marks are each 1 mm apart.

Related Calculators

Essential Calculators: browse all unit converters and everyday tools.
cm to Inches Converter: convert centimetres to imperial inches.
cm to Feet and Inches Converter: convert a height in centimetres to feet and inches.
km to Miles Converter: convert kilometres to miles.
Area Conversion Calculator: convert between square metres, square feet, and more.

Sources and method: International Bureau of Weights and Measures (BIPM), The International System of Units (SI), 9th edition, 2019. Conversion factor: 1 cm = 10 mm (exact, by definition of SI prefixes). No rounding is applied.

This converter uses the exact SI conversion factor of 1 cm = 10 mm. All results are mathematically exact for the precision shown.

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