Centimeters to Millimeters
One centimeter is 10 millimeters, so multiply centimeters by 10 to get millimeters: 2.5 cm is 25 mm and 21 cm is 210 mm.
1 centimeter = 10 millimeters
1 cm = 10 mm · multiply by 10
Paper sizes in millimeters
The ISO 216 paper sizes are defined in whole millimeters, though people often quote them in centimeters. A4 is 21 × 29.7 cm, which is 210 × 297 mm; A5 is 14.8 × 21 cm, or 148 × 210 mm; and A3 is 29.7 × 42 cm, or 297 × 420 mm. Each size is half of the one before, with sides in the ratio 1 to the square root of two. Print shops, PDF tools and page-setup dialogs often want millimeters, so a quick multiply by 10 avoids a rejected file. To make an existing document fit, use the PDF to A4 converter.
Page margins follow the same pattern: a 2.5 cm margin is 25 mm, and the 1-inch margin common in US templates is 25.4 mm.
How to convert centimeters to millimeters
mm = cm × 10
1 cm = 10 mm
25 cm × 10 = 250 mm
- Start with the value in centimeters.
- Multiply it by 10.
- Round the result to the precision the job needs: whole numbers for everyday use, more decimals for engineering or recipes.
From a furniture listing to a workshop drawing
Shops list furniture, shelves and appliances in centimeters, but anything you build or fabricate is usually drawn in millimeters. A shelf of 80 × 30 cm becomes 800 × 300 mm on a cut list. 3D-printing slicers and CNC software use millimeters as their default unit, so a part sketched as 4.2 cm wide must be entered as 42 mm, or the model will print at a tenth of its intended size.
Hardware is sized the same way. A hinge cup for kitchen cabinets is 35 mm in diameter, which is 3.5 cm, and a standard wall plug for a 6 mm drill bit needs a 0.6 cm hole.
Centimeters in millimeters, and back
Rounded to six significant digits. Change any value in the converter above for an exact figure.
| Centimeters (cm) | Millimeters (mm) |
|---|---|
| 1 | 10 |
| 2 | 20 |
| 3 | 30 |
| 4 | 40 |
| 5 | 50 |
| 6 | 60 |
| 7 | 70 |
| 8 | 80 |
| 9 | 90 |
| 10 | 100 |
| 12 | 120 |
| 15 | 150 |
| 20 | 200 |
| 25 | 250 |
| 30 | 300 |
| 40 | 400 |
| 50 | 500 |
| 60 | 600 |
| 70 | 700 |
| 75 | 750 |
| 80 | 800 |
| 90 | 900 |
| 100 | 1,000 |
| Millimeters (mm) | Centimeters (cm) |
|---|---|
| 1 | 0.1 |
| 2 | 0.2 |
| 3 | 0.3 |
| 4 | 0.4 |
| 5 | 0.5 |
| 6 | 0.6 |
| 7 | 0.7 |
| 8 | 0.8 |
| 9 | 0.9 |
| 10 | 1 |
| 12 | 1.2 |
| 15 | 1.5 |
| 20 | 2 |
| 25 | 2.5 |
| 30 | 3 |
| 40 | 4 |
| 50 | 5 |
| 60 | 6 |
| 70 | 7 |
| 75 | 7.5 |
| 80 | 8 |
| 90 | 9 |
| 100 | 10 |
A quick sense check
Millimeters are the smaller unit, so the number always grows tenfold. If 2.5 cm comes out as 250 mm you multiplied by 100, and if it comes out as 0.25 you divided. For the reverse direction, see millimeters to centimeters.
Frequently asked questions
- How many millimeters is 1 cm?
- 1 cm is 10 mm. The prefix milli means a thousandth and centi a hundredth of a meter.
- What is 29.7 cm in mm?
- 29.7 cm is 297 mm, the long side of an A4 sheet.
- How do I enter 4.5 cm in software that uses millimeters?
- Type 45. Most CAD, slicing and layout programs read a bare number as millimeters.
- Is 0.3 cm the same as 3 mm?
- Yes. Multiply 0.3 by 10 and you get 3 mm, roughly the thickness of a stack of thirty sheets of office paper.