Online Ruler in Actual Size (cm, mm, inch)

A free online ruler that shows true, real-world size on any screen. Calibrate once against a bank card (or your screen size) and measure objects in centimetres, millimetres and inches — directly on the screen or on a photo you upload.

How to use the online ruler

1
Calibrate once

Hold a credit/bank card (85.6 mm wide) to the screen and drag the slider until the on-screen card matches. Calibration is remembered for next time.

2
Pick units and orientation

The ruler shows centimetres and millimetres on one edge and inches on the other. Switch to vertical for tall objects.

3
Drag to measure

Press and drag across the ruler to read the distance in mm, cm and inches at once.

4
Or measure on a photo

Upload a photo, draw a line along something of known length, enter its real size, then measure anything else in the image to scale.

Measure on your screen or on a photo, calibrated to real size

Calibrate to your screen

Hold a bank or credit card against the screen and drag the slider until the on-screen card matches its real size. The ruler is then true-to-scale.

Match a real card
⚠ Your browser zoom or display scaling changed — re-calibrate for accurate size.
Measured: 0,0 mm 0,00 cm 0,00 in

Drag across the ruler to measure a distance. Switch to vertical for tall objects, or overlay a reference outline to double-check the scale.

🖼️
Drop a photo here or click to choose
Draw a line along an object of known length, then enter its real size.

Features

True-to-size with one-time calibration
Match an ISO ID-1 bank card (85.6 × 53.98 mm) and the ruler renders at real-world scale; the calibration is saved and shared with the site's other measuring tools.
Centimetres, millimetres and inches together
A metric scale on one edge and an imperial scale (down to 1/16") on the other, so you never convert by hand.
Drag-to-measure with a live read-out
Most screen rulers only show a scale — this one returns the exact distance between two points in mm, cm and inches as you drag.
Measure on a photo
Set a known reference length in an image and measure anything else in it to scale — no separate tool needed.
Zoom-change warning
If your browser zoom or display scaling changes after calibration, the tool warns you instead of silently showing the wrong size.
Horizontal and vertical
Rotate the ruler for tall objects — handy on phones where a horizontal-only ruler runs out of room.

FAQ

How accurate is an on-screen ruler?

After calibrating against a real bank card it is accurate to within about a millimetre, as long as you don't change the browser zoom or your operating-system display scaling. The credit-card method is more reliable than entering the screen size, because it sidesteps zoom and scaling guesswork.

Why do I need to calibrate?

Screens differ in pixel density, and browser zoom and OS scaling change how big a pixel looks. Calibration measures how many pixels equal one millimetre on your specific display so the ruler matches reality.

Does the calibration stick?

Yes — it is saved in your browser and reused next time, and it is shared with the site's other on-screen measuring tools so you only calibrate once per device.

Can I measure something in a photo?

Yes. Switch to "Measure on a photo", draw a line along an object whose real length you know, enter that length, and every other line you draw is scaled from it.

Does it work on a phone?

Yes. It works in any modern mobile browser; use the vertical orientation for tall objects and touch-drag to measure.

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Who is this tool for?
✨ Content Creators 🎓 Students 📚 Educators & Teachers 💻 Developers & IT Pros 🔌 Electronics Enthusiasts 📷 Photographers
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