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Verify the reference span
Worked exampleA displayed 50 mm span measuring 48 mm is 4% too short
Compare with a trusted physical ruler
Measure from the screen origin to the target. Use a span that fits, not a clipped line. Enter the physical distance you observed, not the screen label.
This does not change calibration or certify the screen’s accuracy.
| Displayed span | Observed distance | Scale difference |
|---|---|---|
| 25 mm reference | 24 mm observed | −4% scale difference |
| 50 mm reference | 50 mm observed | 0% at this checked span |
| 100 mm reference | 103 mm observed | +3% scale difference |
| 50 mm reference | 48 mm observed | −2 mm; recalibrate |
What “actual size” really means
An actual-size ruler needs the correct number of CSS pixels per physical inch for the display and browser configuration being used. A nominal 96 PPI assumption is only a starting estimate. Device detection may find a likely model or screen profile, but it cannot establish the physical size of every external monitor. Verify an automatic result before relying on it.
Run an independent scale check
First calibrate with a known reference, such as a standard bank-card outline. Then choose a span above that fits your screen, and compare the origin-to-target distance with a trusted physical ruler. Enter the physical distance you observed. If a displayed 50 mm span measures 48 mm, the display is 4% short. The check reports the difference; it does not silently relabel the screen as verified or apply an unconfirmed correction.
Recheck after the display changes
Browser zoom, operating-system scaling, moving to another monitor, and device orientation can affect the relationship between screen coordinates and physical size. The tool resets its scale estimate when it detects a changed screen signature, but not every physical configuration change is observable in a browser. Recalibrate and repeat the independent check whenever the setup changes. A successful single-span check is useful evidence, not a certification of every length or viewing angle.
How this ruler works
The scale uses your current calibration in CSS pixels per inch. Metric ticks divide that density by 25.4; inch ticks divide it by 16. Conversions use the exact definition 1 inch = 25.4 mm. The target keeps its entered length rather than stretching to fill the display.
Keep browser zoom at 100%, verify with a known reference, and recheck after changing screens. Inputs stay in your browser; no camera, account, or upload is required. For critical dimensions, use a suitable calibrated physical instrument.
Methodology, calibration help, and measurement support: Accuracy methodology · Calibrate screen · Contact us. Unit definitions: NIST.