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A screen ruler is only as good as its scale. Here is how this one works it out, how to correct it in a few seconds, and when to reach for paper instead.
No browser can ask a screen how big it physically is. That number is not available to a web page at all, because it can be used to identify people. So every screen ruler works the density out some other way, and every one of them is guessing to some degree.
We recognise your screen from its pixel signature. That is exact on iPhone and iPad, close on Android, and unreliable on laptops and external monitors, where the same resolution can be a 13 inch laptop or a 32 inch display. When we are not sure we say so, rather than showing a confident wrong answer.
In pixels the rule is exact on every device, because a pixel is the one unit a browser does know.
| Device | How the scale is worked out | Typical error |
|---|---|---|
| iPhone, iPad | Pixel signature, near unique per model | 0% |
| Android | 160 dp per inch, by definition of the unit | ~3% |
| Mac at default scaling | Panel matched on mode list and aspect | 0% |
| Windows, external monitor | Not inferable; calibration needed | up to 70% |
Every bank, debit and credit card in the world is made to the same standard, 85.60 by 53.98 millimetres. That makes one the most widely owned accurate reference object there is, and it is why you do not need a ruler to set up a ruler.
Hold a card against the outline and move the slider until they match exactly. The setting is kept in your own browser, so the ruler is right when you come back, and no copy of it is sent anywhere.
A card lying flat is wider than most phones. If the outline runs off the screen, turn it upright rather than shrinking it: an outline squeezed to fit would calibrate the ruler wrong while looking entirely correct.
It does not tell you how long your object is. It has no camera and no sensor, so it cannot see what you put against it. It is a ruler: it shows a true scale and you read it, exactly like the one in a drawer.
Do not rely on it where being wrong carries a cost: medical decisions, engineering tolerances, construction, or anything bought and sold by size. Use a physical ruler, or print one, where a printed millimetre really is a millimetre.
On paper, physical units really are physical, so a printed ruler is exact by construction. No calibration, no device detection, nothing to check except the printer.
Its one enemy is the printer’s own scaling. Most print dialogues default to fit to page, which shrinks the sheet by a few per cent and makes every ruler on it wrong. Set scaling to 100%, then measure the 50 mm check square before you trust the sheet.
A 30 cm ruler does not fit on A4 or US Letter, so it prints as strips that join end to end, numbered continuously rather than starting again at zero.
Every bank card is 85.60 by 53.98 mm. Hold one on the outline and move the slider until they match.
Too wide for this screen. Try the other orientation.