Online ruler, actual size
A ruler you can hold things up to. It measures in centimeters, millimeters, inches or pixels, sizes itself to your screen, and becomes exact once you've lined it up with a bank card.
How to measure something
- Choose a unit at the top: cm, mm, in or px.
- Hold the object flat against the screen with one end on marker A.
- Drag marker B to the other end. The distance appears in the box beside the ruler.
Drag the ruler's body to move it. Drag the round handle past its end to turn it or make it longer; the 0° and 90° buttons lay it flat or stand it upright. On a phone, standing it upright gives you the long side of the screen to measure with.
Getting the size right
A web page isn't told how big your screen's pixels really are, so a ruler drawn on it can easily be the wrong size. This one does its best to work it out for you:
- iPhones, iPads and Mac laptops are recognized, and the ruler is exact straight away.
- Android phones get a close estimate from the screen width, usually within a few percent.
- Other screens start from the 96-pixels-per-inch standard, which can be 10% or more off.
For an exact scale, calibrate it once with a card. Bank cards, ID cards and most membership cards are all the same size, 85.60 × 53.98 mm. Tap Calibrate, hold the card against the screen, drag the outline until it lines up, and save. Keep your browser zoom at 100% while you do it. There's more about this in why on-screen rulers are often the wrong size.
Reading the scale
Centimeters and millimeters
Each numbered line is a centimeter, and each small space between them is a millimeter. The slightly longer line in the middle marks 5 mm. Four numbered lines and three small ones is 4.3 cm, which is the same as 43 mm.
Inches
Each numbered line is an inch. This ruler divides every inch into eighths: the longest line in between is ½ inch, the next longest ¼, and the shortest ⅛. Something that ends three short lines past the 2 is 2⅜ inches long. The box beside the ruler shows inches as a decimal, 2.38 in. For more detail and a few common mistakes, see how to read a ruler.
Unit converter
Type a length to see it in millimeters, centimeters, meters, inches, feet and yards, or tap "Use the ruler's reading" to bring in whatever the ruler is measuring.
10 cm=3.93701 in≈ 3 15/16 in
The same length in every unit
Common conversions
An inch is exactly 2.54 cm. To turn inches into centimeters, multiply by 2.54; to go back, divide by 2.54. Millimeters to centimeters is just a divide by ten.
| Inches | Centimeters | Millimeters |
|---|---|---|
| 1 in | 2.54 cm | 25.4 mm |
| 2 in | 5.08 cm | 50.8 mm |
| 3 in | 7.62 cm | 76.2 mm |
| 4 in | 10.16 cm | 101.6 mm |
| 5 in | 12.7 cm | 127 mm |
| 6 in | 15.24 cm | 152.4 mm |
| 8 in | 20.32 cm | 203.2 mm |
| 10 in | 25.4 cm | 254 mm |
| 12 in | 30.48 cm | 304.8 mm |
What it's good for
Anything small enough to hold against the screen: a screw, a bead, a coin, a ring, a photo print, the width of a phone case. In px mode it measures things on screen, which is useful for web and design work. There's also a protractor for angles, a caliper for small gaps, a tape that adds up a path one point at a time, a ring sizer, and a ruler you can print.
It isn't the right tool for anything longer than your screen, or for measurements where a millimeter matters for safety, such as medical, engineering or building work. A screen isn't a calibrated instrument.
Questions
- Is this ruler accurate?
- Once you have calibrated it with a bank card, readings are usually within about a millimeter over 10 cm. Before that it depends on your device: iPhones, iPads and Mac laptops are recognized and sized exactly, Android phones are a close estimate, and other screens can be 10% or more off.
- How do I calibrate it?
- Tap Calibrate at the top right of the ruler, hold any bank, ID or membership card against the screen, and drag the outline until its edge lines up with the card. Save it, and the ruler remembers the setting on this device.
- Does it work on a phone?
- Yes. Everything can be dragged with a finger. A phone screen is narrow, so for longer things turn the ruler to 90° and use the long side of the screen.
- Why does the ruler look the wrong size?
- Usually because the screen has not been calibrated, the browser is zoomed in or out, or the window was moved to a different monitor. Set the zoom back to 100% and calibrate again.
- How long is a 12-inch ruler in centimeters?
- 30.48 cm, or 304.8 mm. One inch is exactly 2.54 cm.
- Can I measure something longer than my screen?
- Not in one go. Measure it in sections, or use the Tape tool, which adds up the lengths between the points you tap. For anything long, a real tape measure is quicker.
- What is the px setting for?
- It measures in screen pixels instead of physical units, which is handy for checking the size of things on a web page or in a design. It is not affected by calibration.
- Is it free, and does it save anything about me?
- It is free and needs no account. Your calibration, unit and theme are saved only in your own browser so they are there next time.