Free Online Ruler
A calibrated on-screen ruler in cm and inches.
What is the Online Ruler?
You need to measure something small — a screw, a photo, a bead — and there's no ruler in sight, but there's a screen right in front of you. This online ruler puts a scale in centimetres or inches on your display. The catch every honest screen ruler has to deal with is that screens vary in pixel density, so it lets you calibrate once against a bank card to make the scale genuinely accurate.
The calibration is saved on your device, and nothing is uploaded.
Last updated: Aug 27, 2026
What does this tool do?
It shows a ruler across your screen with major and minor tick marks in centimetres or inches. Because a fixed pixel scale would be wrong on most displays, you drag a card outline to match a standard bank card's width — a known 85.6 mm — and the ruler recalibrates to your exact screen. After that, hold small objects against the scale to measure them.
It's a handy stand-in when a tape measure isn't to hand.
Key features
cm or inches
Switch units with one tap.
Card calibration
Match a bank card for real accuracy.
Clear tick marks
Major and minor graduations.
Saved setting
Calibrate once per device.
Private
Runs entirely in your browser.
Example
Common use cases
- Crafters & hobbyists — Measure beads, parts and materials.
- Online shoppers — Check a small item's real size.
- Students — A ruler when the pencil case is empty.
- Sewers — Quick measurements of trims and buttons.
- Collectors — Size coins, stamps and small items.
- Anyone — Measure something small without a ruler.
Benefits
- A ruler on any screen, any time.
- Card calibration makes it truly accurate.
- Your setting is remembered per device.
- Free and private.
Tips
- Calibrate with a real bank card before your first measurement — it's what makes the scale correct.
- Recalibrate if you switch to a different screen; pixel density varies between devices.
- Lay the object flat against the screen and view it straight on to avoid parallax error.
- Use it for small things — for anything bigger than the screen, a tape measure is better.
- Zoom your browser to 100% first, since page zoom changes the on-screen scale.
Common mistakes to avoid
Skipping calibration
Fix: Without matching a card the scale is only a guess — calibrate once for accuracy.
Reusing calibration across screens
Fix: Recalibrate on each device; displays differ in pixel density.
Measuring at an angle
Fix: View the object straight on and flat to the screen to avoid parallax.
Browser not at 100% zoom
Fix: Page zoom rescales the ruler — reset to 100% before measuring.
How it works
- 1
Calibrate with a card
Match the outline to a bank card's width.
- 2
Pick your units
Centimetres or inches.
- 3
Measure
Hold objects against the on-screen scale.
Frequently asked questions
Screens differ in pixel density, so a ruler can't be accurate everywhere by default. Matching the outline to a standard bank card sets the true scale for your display.
Yes — your setting is saved in this browser, so you only calibrate once per device.
After calibrating with a card it's close enough for everyday measuring. For precision or anything large, use a physical ruler or tape measure.
Screens differ in pixel density, so a ruler can't be accurate everywhere by default. Matching the outline to a standard bank card sets the true scale for your display.
Yes — your setting is saved in this browser, so you only calibrate once per device.
After calibrating with a card it's close enough for everyday measuring. For precision or large items, use a physical ruler or tape measure.
Yes — free, no sign-up.
Conclusion
An on-screen ruler in centimetres or inches that becomes genuinely accurate once you calibrate it against a bank card. Keep your browser at 100%, measure small objects flat against the screen, and recalibrate per device — a quick fix when no tape measure is nearby.