CM to MM converter, centimeters to millimeters.

To convert cm to mm, multiply the centimeters by 10. So 3.5 cm is 35 mm, 12.7 cm is 127 mm and 0.5 cm is 5 mm. The cards also show meters, inches and mils (thousandths of an inch).

How to use the converter

1. Type a length in the Centimeters (cm) box. The Millimeters (mm) box and the big result follow as you type.
2. Look at the cards under the result for the same length in other units. Choose Decimals in the side column to fix the number of places.
3. Click a row in Common values to load it. Use Copy result, Copy link, Copy table or Print chart to keep the answer, the ⇄ button to go the other way and Start again to reset.

Where the factor comes from

The NIST list of metric (SI) prefixes gives kilo as a thousand, centi as a hundredth and milli as a thousandth (checked 24 Oct 2026). Ten millimeters make a centimeter.

Two numbers that tie to the inch

Because an inch is exactly 2.54 cm (NIST Special Publication 811, Appendix B.8, checked 24 Oct 2026), the centimeter figure times 10 gives a clean millimeter figure. 2.54 cm is 25.4 mm, which is one inch. 12.7 cm is 127 mm, which is 5 × 25.4 mm, so exactly 5 inches.

Centimeters in millimeters

Worked examples

• A card 8.5 cm wide. 85 mm.
• A pipe 2.2 cm across. 22 mm, which is 0.8661 inches.
• A tolerance of 0.05 cm. 0.5 mm, which is 19.7 mil.
• A board 183 cm long. 1,830 mm, or 1.83 m.

A quick check in your head

Add a zero to a whole number, or move the point one place right. 6 cm becomes 60 mm, and 6.4 cm becomes 64 mm. A check is that 1 cm gives 10 mm and 0.1 cm gives 1 mm, the smallest mark on most rulers.

Where you meet it

Phone cases, camera sensors, drill bits and screws are sold in millimeters, while a home tape often reads in centimeters. A shelf pin hole of 0.5 cm is 5 mm, and a board 1.8 cm thick is 18 mm. Quote the millimeters when you order, since one wrong digit is a tenth of the size.

Write the sum as a unit chain

A unit chain shows why the sum works. Write the length, then a fraction that equals 1 with the unit you want on top: 3.5 cm × 10 mm / 1 cm = 35 mm. The cm on the left cancels the cm under the line, and only mm is left. You can also go through a third unit, meters, in two steps: 3.5 cm × 0.01 m / 1 cm × 1,000 mm / 1 m = 0.035 m × 1,000 mm / 1 m = 35 mm. Both routes give the same answer, which is a handy check when a factor looks odd. If the wrong unit is left over, the fraction is upside down.

Squares and cubes of the same factor

The factor changes when you convert an area or a volume. Square it for an area and cube it for a volume. The reason is that an area has two lengths in it and a volume has three. Take a square with a side of 3.5 cm. Its side is 35 mm. Its area is 3.5 × 3.5 = 12.25 cm², and in the other unit it is 35 × 35 = 1,225 mm². The area factor is the length factor squared, which is 100.

Mixed units in one measurement

Lengths are often given in two or three units, such as 2 cm 5 mm. Convert each part to millimeters, then add. 2 cm = 20 mm; 5 mm stays 5 mm. The total is 25 mm. Rounded parts can drift in the last digit, so when the parts are not exact, convert the whole once instead. In centimeters the same length is 2.5 cm.

The metric ladder

Read along a row: the row is one of that unit, and each column says how many of the column unit it holds. Exact values are shown in full. Long ones are rounded to five decimals.

Check by converting back

Type the number 3.5 in the Centimeters box and note the answer, 35 mm. Now press the ⇄ button, so the answer becomes the input. It should return 3.5 cm. If it does not, one of the boxes was read wrongly. This takes two seconds and catches a slipped decimal point.

Mistakes to avoid

Multiplying by 100 or 1,000 is the usual slip. A 3.5 cm part is 35 mm. Another is reading a number in a drawing as centimeters when the sheet says millimeters. For area, 1 cm² is 100 mm², since the factor is squared. The Inch to MM converter goes from inches directly.