Guides · The reasoning under the instruments
Why screen measurement is hard, and what makes it honest
The rulers on this site print a state and an error band beside every reading, which is unusual enough to need explaining. These guides are that explanation: what a browser actually knows about your display (almost nothing), what a physical object fixes, and how much each object can be trusted. Start with the first if you came here wondering why an on-screen ruler was wrong; start with the second if you came here wanting a number.
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How screens measure
Why an online ruler is wrong: a CSS inch is not an inchCSS fixes an inch at exactly 96 pixels by definition, so a browser draws a ruler without ever measuring the screen. The cost, on seven real displays: from 4.6% too long to 66% too short, and typically a quarter to a third short on a modern machine. Also why no browser API can resolve it, and why a device-size database is the wrong fix.
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Reference objects
Credit card size, and the other objects you can measure withA bank card is 85.60 × 53.98 mm — 3.370 × 2.125 in — by ISO/IEC 7810, and that makes it the most useful measuring reference most people own. The nominal size of sixteen everyday objects in millimetres, centimetres and inches, how far each is permitted to vary, and why only two of them are offered here as references.
What these guides are for
Every instrument on this site is built on two facts that are easy to state and easy to get wrong. The first is that a browser has no idea how large your display is, so an uncalibrated ruler is not a ruler with a small error — it is a ruler whose error is whatever the gap happens to be between your panel and 96 pixels per inch. The second is that the fix is a physical object of known size, which means the quality of a calibration is the quality of the object and of your hand, both of which can be written down as numbers.
So these pages exist to make the instruments checkable rather than to sell them. Where a figure appears, the arithmetic that produced it appears beside it; where an object is refused, the reason recorded against it is printed in full. If you would rather read the method than the reasoning, the calibration page carries the procedure, the ISO clauses and every source behind the numbers.