Online Ruler: Measure Actual Size in cm, mm and Inches
Free on-screen ruler that shows real size once calibrated with a bank card or your device model.
What an online ruler does
An online ruler is a ruler drawn on your screen. You hold a small object against the glass, line up one end with the 0 mark and read the number where the other end lands.
A web page can only draw in pixels. To turn pixels into centimeters or inches it needs to know how many pixels fit in one inch of your screen, the PPI. Screens differ a lot, so a ruler that only guesses can be off by several millimeters. This ruler starts with an estimate and lets you set the real value once, by matching a bank card or picking your device. After that it shows true size on that screen.
How to use the online ruler
1
Calibrate once
Press Calibrate and match the outline to a bank card, or pick your phone or laptop model. It is saved for next time.
2
Line up the object
Hold the object flat on the screen with one end exactly on the 0 mark of the ruler edge.
3
Read the length
Read the mark at the other end. Tap the ruler edge twice to drop two guides and get the exact distance.
The ruler starts on the bottom edge on desktop and on the left and bottom edges on phones. Turn on the top, right, bottom or left edge with the edge buttons, or press Vertical to make the ruler area tall for long objects. The fullscreen button gives you the longest possible ruler. Read the full guide to using an online ruler.
How to calibrate the ruler
Calibration tells the ruler how big one inch is on your screen. Do it once per screen. You have four methods:
- Card (most accurate). Any bank card, driving licence or loyalty card is 85.60 x 53.98 mm. Hold it on the screen and move the slider until the dashed outline matches. Step by step: calibrate a screen ruler with a credit card.
- Device. Search for your phone, tablet, laptop or monitor and select it. Values come from public specs, so they are close but not perfect.
- Diagonal. Type your screen size in inches, for example 14 for a 14 inch laptop. The ruler combines it with your resolution. The screen size calculator shows the same math.
- Auto. If you skip calibration, the ruler estimates PPI from your screen. Good enough for a rough check only.
Before you start, set browser zoom to 100% (Ctrl+0, or Cmd+0 on a Mac). Calibrate again if you change zoom, display scaling or monitor.
Units and conversions
One inch is exactly 2.54 centimeters. The inch scale is marked in sixteenths, the metric scale in millimeters.
| Length | cm | mm | inches |
|---|---|---|---|
| 1 millimeter | 0.1 | 1 | 0.0394 |
| 1 centimeter | 1 | 10 | 0.3937 |
| 1 inch | 2.54 | 25.4 | 1 |
| 1 foot | 30.48 | 304.8 | 12 |
Need inches only or centimeters only? Use the inch ruler or the cm ruler, which open in that unit.
Everyday objects to check your calibration
Hold one of these against the screen and check that the ruler reads close to the size listed.
| Object | Metric | Inches |
|---|---|---|
| Bank card | 85.60 x 53.98 mm | 3.370 x 2.125 |
| US dollar bill | 156 x 66.3 mm | 6.14 x 2.61 |
| AA battery | 50.5 x 14.5 mm | 1.99 x 0.57 |
| US quarter, diameter | 24.26 mm | 0.955 |
| 1 euro coin, diameter | 23.25 mm | 0.915 |
| A4 paper (reference) | 210 x 297 mm | 8.27 x 11.69 |
Why a screen ruler can be off
- Screen density. A phone packs far more pixels into an inch than a desktop monitor.
- System scaling. Windows scaling at 125% or 150%, and Mac display scaling, change the size of a CSS pixel.
- Browser zoom. Zooming in or out resizes everything, including the ruler.
- External monitors. A browser often cannot know the physical size of a second display.
Matching a real object once removes these guesses. For a longer object than your screen, use the tips in how to measure without a ruler.
Frequently asked questions
Is this online ruler actual size?
Yes, once you calibrate it. Before that it uses an estimate from your screen. The status line under the ruler says Estimated or Calibrated, so you always know which one you have.
How do I calibrate the ruler with a bank card?
Press Calibrate, hold any bank card, ID card or loyalty card against the screen and move the slider until the dashed outline matches the card exactly. Keep browser zoom at 100% and press Save.
Why is my ruler too big or too small?
The usual causes are no calibration yet, browser zoom that is not 100%, changed display scaling in your system settings, or a window moved to another monitor. Reset zoom to 100% and calibrate again.
Does the online ruler work on a phone?
Yes. On a phone the ruler fills the screen and shows a long ruler on the left edge plus one on the bottom. Use the menu button to switch edges, show pixels or turn the ruler vertical.
Can I measure in inches, centimeters and millimeters?
Yes. Switch between cm, mm and in with one tap. Inches are marked in sixteenths. On desktop you can also pick px, which counts CSS pixels and needs no calibration.
What does PPI mean?
PPI means pixels per inch. The ruler uses CSS pixels per inch, the pixels your browser draws with, which can differ from the hardware PPI on a spec sheet. Our screen size calculator shows both.
Does the ruler upload anything or need an account?
No. The ruler runs entirely in your browser. Your calibration, unit and edges are saved in local storage on your device. Nothing is sent to a server and there is no sign-up.
How do I reset the calibration?
Open Calibrate and press Reset calibration. The ruler goes back to the automatic estimate.
Is the online ruler free?
Yes. It is free with no sign-up, no app and no limits.