- Millimeter
- Also called mm · one thousandth of a meter
- The smallest mark on a metric ruler. Ten millimeters make one centimeter, so multiplying centimeters by 10 gives millimeters: 4.5 cm is 45 mm.
Centimeters to millimeters table
| Centimeters (cm) | Millimeters (mm) |
|---|---|
| 1 | 10 |
| 2 | 20 |
| 3 | 30 |
| 4 | 40 |
| 5 | 50 |
| 6 | 60 |
| 7 | 70 |
| 8 | 80 |
| 9 | 90 |
| 10 | 100 |
| Centimeters (cm) | Millimeters (mm) |
|---|---|
| 12 | 120 |
| 15 | 150 |
| 20 | 200 |
| 25 | 250 |
| 30 | 300 |
| 40 | 400 |
| 50 | 500 |
| 60 | 600 |
| 75 | 750 |
| 100 | 1,000 |
Why drawings use millimeters
Engineering drawings, furniture plans and most product data sheets in metric countries are dimensioned in millimeters only. Using one unit for everything avoids decimal points: a shelf is 800 × 300 × 18 rather than 80 × 30 × 1.8 cm. On many drawings the unit is stated once in the title block, so a figure with no unit is usually millimeters.
Rulers show both units. To read 6.3 cm, count six numbered marks and three short ones, which is 63 mm.
Metric tape measures are often marked in millimeters with a number at every centimeter, so the same tape can be read either way.
A missing unit can mean a tenfold error
If a supplier quotes 25 and you assume centimeters when they meant millimeters, the part will be ten times too large. Confirm the unit whenever a figure is given without one.
Paper and screen sizes in mm
Paper sizes in the ISO 216 series are given in millimeters: A4 is 210 × 297 mm and A5 is 148 × 210 mm. Photo prints and screens are often listed in inches, so the inches to mm converter helps when a size crosses systems. For the reverse of this page, see mm to cm.
Phone cases, screen protectors and camera gear are also listed in millimeters, so a 6.1 in phone screen is about 155 mm across the diagonal. When two sources give the same size in different units, convert both to millimeters before comparing: whole numbers make small gaps, such as 297 versus 300 mm, easy to spot. The prefixes themselves are defined in the BIPM SI Brochure.
Cm to mm questions
How many millimeters is 2.5 cm?
2.5 × 10 = 25 mm.
What is 0.1 cm in mm?
0.1 cm is exactly 1 mm.
How do I read millimeters on a ruler?
The numbered long lines are centimeters. The nine short lines between two numbers are millimeters, so count the centimeters, multiply by 10 and add the short lines.
Is 1 cm exactly 10 mm?
Yes. Both are fixed fractions of the meter (1/100 and 1/1000), so the ratio is exactly 10.
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