Calibrate an online ruler and check its error
A web page cannot automatically know whether its displayed centimetre is a physical centimetre. Apps24 matches an on-screen rectangle to an object known to be 85.60 mm wide. The table verifies implementation calculations, not a physical measurement.
Input and result
240 CSS px / 8.56 cm | 1 cm = 28.04 CSS px | 5 cm = 140.19 CSS px |
342 CSS px / 8.56 cm | 1 cm = 39.95 CSS px | 5 cm = 199.77 CSS px |
342 CSS px / 8.56 cm | 1 in = 101.48 CSS px | 1 mm = 4.00 CSS px |
What the result means
At a reference width of 240 CSS pixels, 1 cm is 240÷8.56, about 28.04 CSS pixels. At 342, 1 cm is about 39.95 and 5 cm about 199.77. This proves the ticks follow the setting, not the screen's physical accuracy. CSS pixels and hardware pixels can differ.
What to do next
1. Prepare a reference known to be 85.60 mm wide, not a similarly sized business card. 2. Match both edges of the screen rectangle and confirm calibration. No card photo or upload is needed. 3. Cross-check the displayed 5 cm with a separate physical ruler. If it measures 5.2 cm, (5.2−5)÷5×100=4% too long: recalibrate. That is a worked example, not our measurement. 4. Recheck after changing screen, zoom or orientation.
Use the toolLimits and test conditions
On 2026-10-07, automated tests checked calculations and tick positions at 240 and 342 CSS pixels. No physical ruler or card was measured; no ± accuracy range is certified. Restored settings must be confirmed on the current display. Use a physical instrument for precision or safety decisions. The reference area scrolls on narrow phones, but do not estimate hidden edges to calibrate.
References
Apps24 checks examples against the implementation and regression tests. Browser experiments state their conditions; they do not certify every device. Report an error with the page URL, input, expected result and browser. Never send passwords, private documents or personal data.