An online ruler that measures at actual size
This online ruler draws a centimetre, millimetre or inch scale on your screen at true, physical size, so you can measure small objects — a screw, a coin, a ring, a photo, a SIM card — by holding them against the display. Drag anywhere on the ruler to measure the distance between two points, or switch to the protractor to measure angles.
Screens differ in pixel density, so a web page cannot know how big a centimetre is on yours. A quick calibration fixes that: hold any bank card against the screen and match its outline, or type in your screen size in inches. The calibration is remembered in this browser.
If you need a ruler you can carry, use the printable ruler instead — it prints at exact size on A4 or Letter paper.
How to use the Online Ruler
- Under Calibrate to true size, choose Bank card, hold a card against the screen at the outline’s top-left corner and move the slider (or the − and + buttons) until the outline’s right edge matches the card. Press Use this size.
- No card handy? Choose Screen size and enter the diagonal of your display in inches, with browser zoom at 100%.
- Pick cm / mm or inches. Place the object against the ruler’s left edge (zero) and read the scale, or tap the scale to drop a marker.
- To measure any distance, drag from one point to another in the lower part of the ruler; the length and its angle appear below.
- For angles, choose Protractor and drag the two arms to line up with the object’s sides.
Online Ruler features
True-size calibration
Three ways: a standard ID-1 card (85.60 mm wide), your screen diagonal, or an exact pixels-per-centimetre value for fine-tuning.
Metric and imperial
Millimetre ticks with centimetre numbers, or inch scale divided to 1/16 inch, with conversions shown for every measurement.
Drag to measure
Measures straight lines in any direction, not just horizontally, and gives the angle of the line.
Protractor
A half-circle protractor with inner and outer scales and two draggable arms; it also shows the reflex angle and radians.
Zoom check
If you change the browser zoom after calibrating, the page warns you because the scale would no longer be true.
Private and offline
Calibration is stored only in your browser; nothing is uploaded.
How the calibration works
Browsers measure in CSS pixels, nominally 96 per inch, but real screens range from about 90 to over 160 CSS pixels per inch once scaling is applied. Calibration finds your actual value.
With a bank card, the outline width in pixels is divided by 85.60 mm, the width fixed for ID-1 cards by ISO/IEC 7810 — the same size as credit, debit, driving licence and most ID cards in India, the UK, Europe and the US. With the screen size method, the diagonal in CSS pixels is divided by the diagonal in inches; for example a 1920 × 1080 screen with a 24-inch diagonal has √(1920² + 1080²) ÷ 24 ≈ 91.8 pixels per inch, or 36.1 pixels per centimetre.
| Card or object | Standard size |
|---|---|
| Bank, credit or ID card (ID-1) | 85.60 × 53.98 mm |
| A4 paper | 210 × 297 mm |
| US Letter paper | 8.5 × 11 in (215.9 × 279.4 mm) |
| US quarter coin | 24.26 mm diameter |
| Indian ₹10 coin (2019 series) | 27 mm diameter |
Accuracy tips
- Keep browser zoom at 100% (Ctrl/Cmd + 0) and do not change it after calibrating.
- Look straight at the screen: viewing at an angle (parallax) makes edges appear to line up when they do not.
- The card method is usually the most accurate; check it by measuring the card’s short side, which should read 5.40 cm.
- Phones: the screen is small, so measure short items, and use the printable ruler for longer ones.
- For screen details such as resolution and pixel ratio, open the screen resolution checker.
Online Ruler FAQ
Is this online ruler actual size?
Only after calibration. Before that it uses the 96-pixels-per-inch estimate, which can be noticeably off. Calibrate with a bank card or your screen size for true size.
How can I measure something without a ruler?
Calibrate the online ruler with any bank card, then hold the object against the screen at the zero mark. For longer objects, print the printable ruler.
Why is the ruler wrong after I zoomed?
Zooming changes how many screen pixels each CSS pixel uses. Reset zoom to what it was when you calibrated, or calibrate again.
How accurate is it?
With careful card calibration, typically within about half a millimetre per 10 cm on a laptop or monitor. Phones and tablets work too, over shorter lengths.
Can I measure angles?
Yes. Switch to Protractor and drag the two arms; it shows the angle between them from 0° to 180°.
Does it work on iPhone and Android?
Yes. Calibrate on the phone itself, as every screen has a different pixel density.