Screen Calibration

A screen has no fixed physical size, so an online ruler is only as good as its screen calibration. This guide explains the three calibration methods built into the tool and which one fits your device best.

Unit
Edges
Click two points to measure — press Esc or double-click to clear. Click the vertex first, then two points on the rays — press Esc or double-click to clear.
Shape
U Unit · M Measure · A Angle · G Grid · Esc

Method 1: Smart auto-detect

The tool matches your screen resolution, pixel ratio, and user agent against a database of 100+ device profiles and sets the correct PPI instantly. It is the fastest screen calibration option and works out of the box for most laptops and phones — press Detect My Device and you are calibrated in about a second.

Method 2: Screen diagonal

Enter your monitor or laptop diagonal in inches — the number printed on the box or in the spec sheet. From the diagonal and your resolution the tool calculates the exact PPI with the Pythagorean formula. This is the most reliable screen calibration for external monitors and TVs that auto-detect cannot identify.

Method 3: Credit card overlay

Place a real credit card (ISO standard 85.6mm × 54mm) on the screen and drag the slider until the outlined card matches it exactly. Because the card size is a physical constant, this is the most accurate screen calibration on any unknown display — it usually takes about twenty seconds.

Calibration methods compared
MethodBest forTime needed
Smart Auto-DetectKnown laptops and phones~1 second
Screen diagonalExternal monitors and TVs~10 seconds
Credit card overlayAny unknown screen~20 seconds

FAQ

Which screen calibration method is most accurate?
Credit card calibration, because it references a physical constant directly against the glass. Auto-detect is nearly as good on known devices, and diagonal entry is reliable for external monitors.
Do I have to calibrate on every visit?
No. Your screen calibration is stored in browser localStorage and reloaded automatically, so the online ruler is accurate from the first click of your next visit.
What happens if I never calibrate?
The tool falls back to a default PPI estimate, so cm, mm, and inch readings can drift on non-standard displays. Pixel measurement is unaffected, since pixels need no calibration.

Related guides

Open the online ruler and start measuring

Free, no install, calibrate once and measure forever.

Back to the online ruler