Tests & Toys
Online Ruler
A screen ruler calibrated against a bank card — honestly accurate.
✓ Free
✓ No account
◉ Runs on this device
Tests & Toys
Powering up the tool…
TOOL ONLINE-RULER
What this tool does
About the online ruler
The credit-card trick beats DPI guessing; the tool says why.
How to use it
- Hold any standard card to the yellow bar.
- Drag the slider until they match exactly.
- Measure against the cm scale.
- Recalibrate per device — every screen differs.
Useful for
- Measure small objects sans tape measure.
- Check a printed dimension quickly.
- Get true on-screen scale for a template.
Straight answers
Online Ruler FAQ
Why can’t the browser just know my screen’s real size?
Browsers report logical pixels, not physical millimetres — the same 96 “DPI” claim covers wildly different panels. The card trick sidesteps the whole lie: every standard card is 85.6mm by ISO spec, making your wallet a calibration instrument.
How accurate after calibration?
Within ~1mm over the visible span if you matched the card carefully — fine for buttons, screws and stamps. Precision engineering still wants callipers; honest scope, stated.