Centimeters to Millimeters Converter
Convert cm to mm — and how a drawing states its unit once, not per dimension.
Updated
millimeters
10.0mm
1 cm = 10.0 mm
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In short
How many millimeters are in a centimeter?
Exactly 10. Multiply centimetres by 10, so 2.5 cm is 25 mm. It is the shortest step on the metric ladder and the most useful one in practice, because nearly every trade that makes physical things specifies in millimetres rather than centimetres.
On a drawing the unit is normally stated once for the whole sheet, so a dimension reading 25 carries no suffix.
centimeters to millimeters — quick reference
| centimeters (cm) | millimeters (mm) |
|---|---|
| 0.5 cm | 5.0 mm |
| 1 cm | 10.0 mm |
| 2 cm | 20.0 mm |
| 5 cm | 50.0 mm |
| 10 cm | 100.0 mm |
| 25 cm | 250.0 mm |
| 50 cm | 500.0 mm |
| 100 cm | 1,000.0 mm |
The formula, worked line by line
One multiplication by ten, and the shortest step in the whole metric system. Both prefixes are exact powers of ten, so a centimetre is ten millimetres by construction rather than by measurement.
“Millimetres keep the numbers whole. That is the entire argument, and it is enough.”
It is worth stating plainly because unit choice is usually presented as a matter of habit or nationality. It is neither. A trade picks the rung of the ladder that puts its working dimensions in the integer range, and stays there.
millimetres = centimetres x 10
centi- = 10^-2, milli- = 10^-3
2.5 cm x 10 = 25 mm
1 in = 25.4 mm exactly
1 ft = 304.8 mm exactly- The sheet states
- dimensions in millimetres
- A dimension reads
- 2400
- Which means
- 2,400 mm
- Or
- 240 cm, or 2.4 m
- On the drawing
- 2400
The bare number is complete because the unit was declared once. Reading it as centimetres gives a ceiling twenty-four metres high, which is the kind of error that is obvious in a sentence and invisible in a column of figures.
How fine is a millimetre, really
Finer than most people assume and coarser than a sixteenth of an inch, which is 1.5875 mm. A standard metric rule marked in millimetres therefore resolves less finely than an imperial tape marked in sixteenths — the opposite of the usual assumption about the two systems.
Where the centimetre still earns its place
- Clothing and body measurements
- centimetres
- Joinery, metalwork, fasteners
- millimetres
- Construction drawings
- millimetres
- Paper and print sizes
- millimetres
The centimetre survives where the thing being measured is soft, approximate, or a person. Everywhere a tolerance matters, the millimetre took over.
How to use the centimeters to millimeters converter
Multiply by ten. Milli- is a thousandth and centi- a hundredth, so a centimetre is ten millimetres exactly, and the conversion moves the decimal point one place. There is nothing here that can be measured better or disputed by anyone.
The interesting part is why the step is worth taking at all, and the answer is that the millimetre is the unit people actually build in. Machine shops, joinery, sheet metal, fasteners, printing and construction outside the United States all specify in millimetres. The centimetre survives mainly in clothing, schoolwork and describing people.
10
Millimetres in a centimetre
exactly
25.4
Millimetres in an inch
exactly, since 1959
1.5875
Millimetres in a sixteenth of an inch
the finest common tape marking
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The reason trades prefer millimetres is that it removes decimal points from working numbers. A 2.4 metre stud becomes 2400, a 12.5 millimetre board becomes 12.5 rather than 1.25 cm, and a whole drawing can be dimensioned in integers. A decimal point is the easiest mark to lose on a print, a fax or a photocopy.
That convention is also why mixing units on one drawing is treated so seriously. If most dimensions are unmarked millimetres, a stray centimetre figure looks identical to a millimetre one and there is nothing on the face of the drawing to catch it. The rule is not fussiness; it is the only thing keeping the sheet readable.
- 1 cm
- 10 mm
- 2.54 cm
- 25.4 mm, one inch exactly
- 10 cm
- 100 mm
- 30.48 cm
- 304.8 mm, one foot exactly
- 100 cm
- 1,000 mm, one metre
The inch and foot rows are exact rather than rounded, because both units are defined against the metre. There is no approximation anywhere in this table.
Convert millimeters to inches instead
If the figure has to cross into imperial, that page runs the exact 25.4 factor and covers the fractional readings a tape measure actually gives.
Open the mm to inches converter →Centimetres converted to millimetres, with the inch figure alongside for the sizes that cross between systems.
| Centimetres | Millimetres | In inches |
|---|---|---|
| 0.5 cm | 5 mm | 0.197 in |
| 1 cm | 10 mm | 0.394 in |
| 1.27 cm | 12.7 mm | 0.5 in exactly |
| 2 cm | 20 mm | 0.787 in |
| 2.54 cm | 25.4 mm | 1 in exactly |
| 5 cm | 50 mm | 1.969 in |
| 10 cm | 100 mm | 3.937 in |
| 15 cm | 150 mm | 5.906 in |
| 20 cm | 200 mm | 7.874 in |
| 30.48 cm | 304.8 mm | 1 ft exactly |
| 50 cm | 500 mm | 19.685 in |
| 100 cm | 1,000 mm | 39.370 in |
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Why the millimetre wins in workshops
A unit is chosen for the size of the numbers it produces, not for elegance. Millimetres put most manufactured dimensions between one and a few thousand, which is the range where integers are natural and decimals are rare. Centimetres put the same dimensions into awkward one-decimal figures for no benefit.
- Board thickness
- 18 mm, or 1.8 cm
- Screw length
- 45 mm, or 4.5 cm
- Sheet width
- 1220 mm, or 122 cm
- Tolerance
- plus or minus 0.5 mm
- In practice
- always the millimetre column
Note the tolerance row. Expressing half a millimetre in centimetres means 0.05 cm, which puts a significant figure two places after the point — exactly the kind of number that gets misread.
The imperial equivalent is instructive. Machinists in the United States moved to decimal inches rather than fractions for the same reason: thousandths of an inch are integers in the range they work in, where sixty-fourths are not. Both trades solved the same problem by picking the unit that makes their numbers whole.
Millimetres against the inch
The two systems meet at exactly 25.4 millimetres to the inch, fixed by the 1959 international agreement. That means every inch fraction converts to an exact millimetre figure, which is why dual-dimensioned drawings can be exactly right in both columns rather than approximately right in one.
- 1/16 in
- 1.5875 mm
- 1/8 in
- 3.175 mm
- 1/4 in
- 6.35 mm
- 1/2 in
- 12.7 mm
- 1 in
- 25.4 mm
Every value is exact, not rounded. The awkward decimals are a consequence of 25.4 having factors of 2 and 5 but not of 16.
A sixteenth of an inch is 1.5875 mm, which is finer than a millimetre — worth knowing, because the common assumption runs the other way. A tape marked in sixteenths resolves more finely than one marked in whole millimetres, and a tape marked in thirty-seconds resolves twice as finely again.
Full guide
How Many Decimals to Keep When Converting Units
The factor is exact to nine digits. The measurement you typed is good to one. Only one of those two limits the answer, and it is not the factor.
Read the full guide →Every centimeters value, worked out
106 common centimeters figures each get their own page, with the answer at full precision, the arithmetic, what rounding costs, and the nearest real-world reference point on the scale.
Under 1 cm
10 to 99 cm
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Questions people ask
How many millimeters are in a centimeter?
Exactly 10. The prefix centi- means one hundredth and milli- one thousandth, so a centimetre contains ten millimetres by definition. Multiplying by ten moves the decimal point one place and introduces no error of any kind.
Covered in depth in How Many Decimals to Keep When Converting Units →
Why do drawings use millimeters instead of centimeters?
To keep working dimensions as whole numbers. A board is 18 mm rather than 1.8 cm, and a tolerance is 0.5 mm rather than 0.05 cm. Removing decimal points removes the mark most easily lost on a printed or copied drawing, and integers are simply easier to read in a column.
How many millimeters are in an inch?
Exactly 25.4, fixed by the 1959 international agreement rather than measured. That exactness carries down to the fractions: a half inch is exactly 12.7 mm and a quarter exactly 6.35 mm, which is why dual-dimensioned drawings can be exactly right in both columns.
Is a millimeter smaller than a sixteenth of an inch?
Yes, and by a noticeable margin. A sixteenth of an inch is 1.5875 mm, so it is more than half again as large as a millimetre. A tape marked in sixteenths therefore resolves more finely than a rule marked in whole millimetres, which surprises most people.
Why does a dimension on a plan have no unit on it?
Because drawing standards let the predominant linear unit be stated once for the sheet and omitted from each dimension. A bare 2400 is complete provided the sheet declares millimetres. Any dimension in a different unit does have to be identified individually.
Sources
Where the constants and formulas on this page come from. Each line names the figure it backs.
SI prefixes are exact powers of ten, so a centimetre is exactly ten millimetres.
The International System of Units (SI), 9th edition — Bureau International des Poids et Mesures
One inch is exactly 25.4 millimetres, following the 1959 international yard and pound agreement.
SI Units — Length — National Institute of Standards and Technology
The predominant linear unit on a technical drawing may be specified once and omitted from individual dimensions.
ISO 129-1:2018 — Technical product documentation, indication of dimensions and tolerances — International Organization for Standardization
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