Real SizerGuides · The reasoning under the instruments
Why screen measurement is hard, and how a reference object fixes it
Start with the first guide if an on-screen ruler showed you the wrong size, and with the second if you want the size of a card or another everyday object. The rulers on this site print a state and an error band beside every reading, and these guides explain both: what a browser knows about your display (almost nothing), what a physical object fixes, and how far each object can be trusted.
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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. In seven calculated examples, a CSS inch runs from 4.6% too long to 66% too short. The guide also covers why no browser API can supply the missing size, and why this site does not use a device-size database.
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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, which makes it the most useful measuring reference most people own. The guide gives the nominal size of sixteen everyday objects in millimeters, centimeters and inches, how far the four with verified limits may vary, and why only two of them are offered here as references.
What these guides are for
The guides explain 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 off by 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, so a calibration is only as good as the object and your hand, and both can be written down as numbers.
They show their working so you can check the instruments yourself: where a figure appears, the arithmetic that produced it appears beside it, and where an object is refused, the reason recorded against it is printed in full. For the method itself, the calibration page carries the procedure, the ISO clauses and every source behind the numbers.