CM to MM Converter

Convert centimeters to millimeters for engineering, crafts, and precision work.

Conversion table

Length	Converted value
0.5	5 mm
1	10 mm
1.5	15 mm
2	20 mm
2.5	25 mm
3	30 mm
4	40 mm
5	50 mm
6	60 mm
7	70 mm
8	80 mm
9	90 mm
10	100 mm
12	120 mm
15	150 mm

📊 Quick Reference

1 cm10 mm
1 inch25.4 mm exactly
A4 paper thickness0.1 mm
Standard pencil lead0.5 mm

Working in Millimetres

The 10 is a definition, not a measurement. `Milli-` means one thousandth and `centi-` one hundredth, so the ratio is fixed by the SI prefixes themselves and can never drift.
Technical drawings are dimensioned in millimetres and the unit is usually omitted. A drawing marked 250 means 250 mm — reading it as centimetres is a factor-of-ten error on the shop floor.
Moving the decimal one place right is the whole operation. If you reach for a calculator for cm to mm, you are more likely to typo than to be wrong by hand.
Millimetres are where the tolerance lives. A part specified at 45.0 mm and one at 4.50 cm are the same size, but the first tells the machinist the precision expected.

Why Engineering Chose the Millimetre

A centimetre is one hundredth of a metre and a millimetre one thousandth, which makes the conversion exactly ten — not approximately ten, exactly, because both are defined from the metre by the SI prefixes rather than measured against it. What makes the millimetre interesting is not the arithmetic but its status: it is the default unit of mechanical engineering almost everywhere outside the United States. ISO-style technical drawings are dimensioned in millimetres and, by convention, the unit symbol is left off entirely. That convention is why the conversion matters. A dimension written as 250 on a drawing is 250 mm, and anyone who reads it as 25 cm has understood the number correctly and the part wrongly.

Multiply by Ten, or Move the Point

Multiply centimetres by 10 to get millimetres, or divide millimetres by 10 to go the other way. In practice nobody multiplies: you move the decimal point one place to the right. 4.7 cm becomes 47 mm; 0.35 cm becomes 3.5 mm. The only place this gets slippery is in trailing zeros and significant figures. Writing 5 cm as 50 mm is correct arithmetically, but if the original measurement was only good to the nearest centimetre, the 50 implies a precision the measurement never had. In engineering documentation that difference is the difference between a part that passes inspection and one that does not.

Where It Goes Wrong

Reading an undimensioned technical drawing as centimetres. Drawings default to millimetres, and the mistake is a factor of ten on a finished part.
Confusing millimetres with mils. A mil is a thousandth of an inch — 0.0254 mm — and is common in coatings, plastics and PCB work in the United States.
Inventing precision. 5 cm written as 50.0 mm claims tenth-of-a-millimetre accuracy that a centimetre ruler cannot provide.
Converting to millimetres and then rounding. Do arithmetic at full precision and round once, at the end, or errors accumulate across a chain of dimensions.
Assuming a metric drawing means metric hardware. Plenty of assemblies mix millimetre dimensions with imperial fasteners.
The inch is defined as exactly 25.4 mm since the 1959 international agreement, so imperial-to-metric is also exact — the approximation is in the measuring, not the maths.

Millimetres You Will Recognise

0.1 mm — a sheet of office paper, and roughly the finest layer a consumer 3D printer will lay down.
0.5 mm — the standard mechanical pencil lead, and about the width of the line it draws.
1.75 mm — the thin filament standard for desktop 3D printers; the thick one is 2.85 mm.
1.6 mm — the thickness of a standard PCB, which is why so many enclosures are cut with a 1.6 mm slot.
25.4 mm — one inch, exactly, by international definition since 1959.
300 mm — a 30 cm ruler, and near enough one foot (304.8 mm) to be confusing on a mixed-unit bench.

Worked Examples

Reading a drawing and specifying a tolerance

A dimension of 4.7 cm on a part

4.7 cm × 10 = 47 mm
Move the decimal one place right
On the drawing this is written as 47, with no unit

47 mm

A tolerance of ±0.05 cm

0.05 cm × 10 = 0.5 mm
The part may vary by half a millimetre either way
Written on the drawing as ±0.5

±0.5 mm

Frequently Asked Questions

How many millimetres are in a centimetre?

Exactly 10. Both are defined from the metre by SI prefixes — centi- is one hundredth, milli- one thousandth — so the ratio is fixed by definition and is not a measured approximation.

Why are technical drawings in millimetres and not centimetres?

ISO drafting practice uses millimetres as the default and omits the unit symbol entirely, so a dimension reads as a bare number. Millimetres give whole-number dimensions for most machined parts and keep tolerances readable without decimals.

What is the difference between a millimetre and a mil?

A millimetre is a thousandth of a metre. A mil is a thousandth of an inch, which is 0.0254 mm — about forty times smaller. Mils are common in US coating thickness, plastic film and PCB specifications, and mixing the two is a costly error.

How do I convert centimetres to millimetres in my head?

Move the decimal point one place to the right. 4.7 cm is 47 mm, 0.35 cm is 3.5 mm, 12 cm is 120 mm. Going back to centimetres, move it one place left.

Should I write 5 cm as 50 mm or 50.0 mm?

Write 50 mm. Adding the decimal zero claims your measurement was accurate to a tenth of a millimetre, and if it came off a centimetre scale it was not. Significant figures should follow the measurement, not the conversion.

Is one inch exactly 25.4 millimetres?

Yes. The international yard and pound agreement of 1959 defined the inch as exactly 25.4 mm, so the conversion is exact by definition. Any imprecision comes from the measuring instrument, not the factor.