A centimeter ruler for your screen
This page opens the online ruler in centimeters. Each numbered mark is 1 cm. Each small mark is 1 mm, and the middle mark between two numbers is 5 mm. The zero mark sits in the top left corner.
To measure, put one end of the object at zero. Then pull a guide out of the ruler and move it to the other end. The tool shows the length, for example "9.21 cm = 92.1 mm = 3 5/8 in". Press Copy to put the text on your clipboard.
For a true size ruler, calibrate first. Press Calibrate at the top right (on a phone, above the ruler), open the Card tab, and match a credit card to the outline. The status then reads "Calibrated".
Centimeters and millimeters
The centimeter is a tenth of a decimeter and a hundredth of a meter. There are 10 millimeters in a centimeter. To turn cm into mm, multiply by 10. To turn mm into cm, divide by 10. So 4.7 cm is 47 mm, and 63 mm is 6.3 cm.
To switch between centimeters and inches, use 2.54: one inch is exactly 2.54 cm. Use the table below for common values. The millimeter ruler shows the same scale with a number every 10 mm, which is easier for small parts.
| Centimeters | Decimal (in) | Nearest 1/16 in |
|---|---|---|
| 1 cm | 0.39 | 3/8 |
| 2 cm | 0.79 | 13/16 |
| 3 cm | 1.18 | 1 3/16 |
| 4 cm | 1.57 | 1 9/16 |
| 5 cm | 1.97 | 1 15/16 |
| 10 cm | 3.94 | 3 15/16 |
| 15 cm | 5.91 | 5 7/8 |
| 20 cm | 7.87 | 7 7/8 |
| 25 cm | 9.84 | 9 13/16 |
| 30 cm | 11.81 | 11 13/16 |
What people measure in centimeters
- Books, notebooks and photo prints.
- Phone cases, cables and small electronics.
- Craft pieces, fabric swatches and patterns.
- Pots, plants and seed spacing, on a flat sheet or card.
- Items to ship, when you want to check a box size before you buy it.
If the object is longer than your screen, measure in parts. Mark the end of the first part with a guide, slide the object, and add the lengths. This adds a small error each time, so use a tape measure for anything long.
Get the most from the cm ruler
- Calibrate with a card, and run the check step with a coin.
- Press F for full screen, so the rulers use the whole display.
- Turn on snap in Options if you want guides to jump to the nearest millimeter.
- Use the arrow keys on a selected guide to move it one millimeter at a time. Hold Shift for steps of 10.
- Need inches too? Switch to "cm + in" to see both scales on one ruler, or open the inch ruler.
The ruler works the same on a phone. Turn the phone sideways for more length. See the phone ruler page for tips.
Reading a centimeter ruler, step by step
Put the object at zero. Find the last numbered mark before the end of the object. That number is the whole centimeters. Then count the small marks after it. Each small mark is one millimeter.
Example: an object ends 3 marks after the 7. That is 7 cm and 3 mm, which you write as 7.3 cm. The tool does this for you when you use a guide, but the same steps work when you read the marks by eye.
Half-centimeter marks help. The middle mark between two numbers is 5 mm, and it is a little longer than the others. If an object ends at that mark, add 0.5 cm to the last whole number.
Common sizes in centimeters
These reference sizes are handy for a quick check. A credit card is 8.56 cm by 5.398 cm. A US quarter is 2.426 cm across. A US dime is 1.791 cm across. A sheet of A4 paper is 21.0 cm by 29.7 cm. A US Letter sheet is 21.59 cm by 27.94 cm. All of these come from standards or the US Mint.
If you have one of these on your desk, lay it on the screen and compare. This is the same idea as the check step, and it takes five seconds.
When centimeters are the right choice
Use centimeters when you want a number that is easy to say and to write. 12.4 cm is quicker to communicate than 124 mm, and it avoids the fractions of an inch. Most countries use metric for clothing, furniture and packaging, so centimeters are the usual unit in shops and online listings.
If a form asks for millimeters, switch to the mm ruler. If a pattern is in inches, switch to the inch ruler. Your calibration stays the same, so you do not need to repeat it.