cm ruler
An actual-size ruler in cm and mm.
This page opens the ruler in centimeters, with every millimeter marked. Like the main online ruler, it calibrates to your screen first, so one centimeter on the scale is one real centimeter on your phone, tablet, laptop or monitor.
Centimeters and millimeters on one scale
Numbered lines are whole centimeters. Halfway between them, a medium line marks 5 mm, and each short line is 1 mm. You read both units at once: an object ending three short lines past the 4 is 4.3 cm, which is also 43 mm. That makes the metric scale the easiest to read precisely, and the best choice for small parts like screws, beads, buttons, SIM cards and jewelry.
A 30 cm ruler on a big screen
A standard school ruler is 30 cm long. Most laptops from 14 inches up and every desktop monitor are at least that wide, so press F for fullscreen and the whole length fits. On a phone, landscape gives you roughly 11 to 14 cm. For anything longer, drag a guide line out of the ruler to mark the end of the screen, slide the object along and add the readings.
Check the calibration with a card
Every bank, debit and ID card is 85.60 mm long and 53.98 mm tall, and the ruler shows a blue mark at both lengths. Hold a card against the ruler with its corner at 0. If its edge lands on the mark, every millimeter on screen is correct. If not, open Calibrate and use the Credit Card method, typically accurate to about 1 mm over 10 cm.
Need inches?
Switch to the inch ruler, or press 2 on this page. The chart below converts common lengths from centimeters to millimeters and inches.
Centimeters to millimeters and inches
| Centimeters | Millimeters | Inches |
|---|---|---|
| 1 cm | 10 mm | 0.394 in |
| 2 cm | 20 mm | 0.787 in |
| 3 cm | 30 mm | 1.181 in |
| 4 cm | 40 mm | 1.575 in |
| 5 cm | 50 mm | 1.969 in |
| 6 cm | 60 mm | 2.362 in |
| 7 cm | 70 mm | 2.756 in |
| 8 cm | 80 mm | 3.150 in |
| 9 cm | 90 mm | 3.543 in |
| 10 cm | 100 mm | 3.937 in |
| 12 cm | 120 mm | 4.724 in |
| 15 cm | 150 mm | 5.906 in |
| 20 cm | 200 mm | 7.874 in |
| 25 cm | 250 mm | 9.843 in |
| 30 cm | 300 mm | 11.811 in |
FAQ
Questions, answered.
Is this cm ruler actual size?
Yes, once it’s calibrated to your screen. Auto-Detect recognizes most phones, tablets and Macs; on other screens, enter your screen diagonal or match a credit card. Check it by holding a card at 0: its long side should end at 8.56 cm, on the blue card mark.
How do I read millimeters on a ruler?
Each numbered line is a centimeter and each short line between them is a millimeter. The medium line halfway between two numbers is 5 mm. Read the last numbered centimeter, then count the short lines past it: 7 cm and 3 lines is 7.3 cm, or 73 mm.
How many millimeters are in a centimeter?
Exactly 10. A centimeter is one hundredth of a meter and a millimeter is one thousandth, so 1 cm = 10 mm and 30 cm = 300 mm.
Can I get a 30 cm ruler on my screen?
Most laptops from about 14 inches up, and every desktop monitor, are wider than 30 cm. Press F for fullscreen to use the whole width. On a phone, turn it to landscape for 11 to 14 cm of ruler, and measure longer items in sections with a guide line.
How do I measure in mm on my phone?
Open this page on your phone and check the status shows your model, or calibrate with a credit card. Lay the object on the screen at 0 and read the far end. Screen calibration is typically within 1 mm over 10 cm, which is fine for screws, beads and jewelry.
What everyday objects can I check centimeters with?
A bank card is 85.60 × 53.98 mm, a 1 euro coin is 23.25 mm across, a 2 euro coin is 25.75 mm and an A4 sheet is 210 × 297 mm. Lay one on the ruler to confirm the scale.
Measure something now.
Free, private and accurate to the millimeter.