Online Ruler (Actual Size)
A free real-size ruler for your screen: calibrate once with any bank card, then measure in cm, mm or inches.
Estimated size, not calibrated. Hold a bank card to the screen for 10 seconds to make this ruler exact.
Drag A and B, or tap the ruler to move the nearest marker. Arrow keys move a focused marker.
An online ruler is a measuring scale drawn on your screen at real-world size, so you can measure small objects such as screws, rings, buttons or printed photos by holding them against the display. This online ruler shows centimetres, millimetres and inches, and becomes exact once you calibrate it with a standard bank card.
Calibration matters because a browser does not know how big your screen physically is. In CSS, one inch is always 96 pixels, but the physical size of a pixel depends on the device, so a ruler drawn at "96 pixels per inch" is usually the wrong size on a phone or a high-resolution laptop. Last updated: 2026-10-01.
How to Calibrate the Online Ruler
- Set the browser zoom to 100% (Ctrl + 0, or Cmd + 0 on a Mac).
- Tap Calibrate and keep Bank card selected.
- Hold any credit, debit, ID or library card flat on the screen with its corner in the corner of the dashed outline. These cards share the ID-1 format of ISO/IEC 7810, which fixes the size at 85.60 × 53.98 mm.
- Drag the slider, then use − and + for fine steps, until the outline's far edge sits exactly on the card's edge.
- Tap Save calibration. It is stored only in this browser on this device, so the ruler opens at the right size next time.
No card nearby? Choose Screen size and enter the diagonal from your device's spec sheet. The ruler divides the screen's pixel resolution by that diagonal. It is quick, but a card is more accurate on laptops that run a scaled display mode.
Online Ruler vs AR Measure Tape
The two tools solve different problems. An on-screen ruler measures things small enough to rest on the glass. A camera-based AR measure tape estimates distances across a room.
| Online ruler | AR measure tape | |
|---|---|---|
| Best for | Screws, jewellery, cards, prints, small parts | Furniture, walls, doors, room sizes |
| Range | Up to your screen size; scroll for longer | Several metres |
| Precision | Down to 1 mm or 1/16 in once calibrated | Depends on camera tracking and lighting |
| Needs | A card to calibrate once | A phone camera |
Tips for Accurate Screen Measurements
- Keep browser zoom at 100%. Pinch-zooming the page changes the scale until you zoom back out.
- Look straight down at the screen. Viewing at an angle shifts the edge you see against the ticks.
- Rest the object flat on the glass and line one end up with the 0 mark or with marker A.
- Use the markers instead of eyeballing the scale: the readout snaps to 1 mm or 1/16 inch.
- Recalibrate after changing monitors or display scaling. The ruler warns you when the screen it was calibrated on changes.
- On a phone held upright the ruler runs vertically (↕), which fits a longer stretch on screen. Switch to ↔ for a horizontal ruler.
Reading Centimetres, Millimetres and Inches
On the metric scale, the longest ticks mark whole centimetres, the middle ticks mark 5 mm and the shortest ticks mark single millimetres. On the inch scale, the ticks get shorter from whole inches to halves, quarters, eighths and sixteenths. The opposite edge of the ruler always shows the other unit, so you can read both systems at once without converting.
Frequently Asked Questions
Is this online ruler actual size?
It is actual size once you calibrate it. Until then it uses a typical scale for your type of device and is marked as estimated. Calibrating with a bank card takes about 10 seconds and is saved in your browser, so the ruler opens at real size from then on.
How do I measure something with a ruler on my phone?
Calibrate once with a card. On a phone held upright the ruler runs vertically, so more of it fits on screen. Lay the object on the glass with one end at the 0 mark, drag marker B to the other end and read the distance. You can also copy the measurement to paste it elsewhere.
What size is a credit card for calibration?
Credit, debit and most ID cards use the ID-1 format of ISO/IEC 7810: 85.60 mm by 53.98 mm, which is about 3.370 by 2.125 inches. Any card of that size works, including library and loyalty cards.
Why is an online ruler the wrong size on my screen?
Browsers treat one inch as 96 CSS pixels, but the physical size of a pixel depends on your screen and display scaling. That is why uncalibrated rulers are often too small or too large. Calibrating with a card measures your actual screen, and the ruler warns you if the screen or zoom level changes afterwards.
Can I measure in centimetres and inches at the same time?
Yes. The main edge shows the unit you pick, and the opposite edge shows the other unit. The readout below the ruler always gives the distance in centimetres, millimetres and inches, including the nearest sixteenth of an inch.
Do I need a ruler app? Is my data private?
No app is needed. The ruler runs entirely in your browser on Android, iPhone, tablets and computers. Nothing is uploaded: the only thing stored is your calibration, which stays on this device.