Ruler at actual size

How to use the ruler at real size

1. Take any bank card, ID card, driving licence or loyalty card: they share one size, 85.60 × 53.98 mm (ISO/IEC 7810 ID-1).
2. Lay it flat on the screen with its corner in the outline's corner. Drag the outline, or use the slider and the ±1 px buttons, until the outline's edges just meet the card's edges.
3. Press Save calibration. The page shows a card-sized box: the card should now fit it exactly.
4. Hold the object against the ruler, with one end on 0. Read centimetres on one edge and inches on the other. For something longer, press Next and read on from the new start.
5. To read a distance as a number, drag from one point to the other in the measure area. Press Full screen for the longest ruler your screen can show.

Why a screen ruler has to be calibrated

Browsers treat 96 pixels as one inch, but no rule makes a screen agree. Worked example: a 15.6-inch laptop with 1920 × 1080 pixels has a diagonal of √(1920² + 1080²) = 2,203 pixels, so 141 pixels per inch, 5.56 per millimetre. At 100 % display scaling a "96-pixel inch" on that screen is 17.3 mm long; at 125 % scaling it is 21.6 mm. A ruler that trusts the browser is off by 15 % or more in either direction. Matching a real card measures your screen's own pixels per millimetre instead, and the page stores that one number.

Which method to pick: the card is the most exact because it is the largest common object, 85.60 mm long. A coin works when no card is to hand, but it is about 24 mm across, so the same 1 px slip makes an error about 3.5 times larger. Typing the diagonal needs nothing at all, but it depends on the size the browser reports (see below).

What it cannot do

• It is only as exact as your calibration. On this screen a 1 px mismatch in the card outline makes every length off by about 0.2 %. Matching to the nearest pixel is the limit; the ruler's ticks are drawn to the pixel too.
• Browser zoom changes it. Zooming makes every pixel of the page bigger or smaller, so the page watches the pixel ratio and, if it changes, asks you to reset the zoom (Ctrl+0) or recalibrate. Moving the window to another monitor needs a new calibration, too.
• The screen-size method trusts what the browser reports. On a Mac set to a "scaled" resolution, and on many phones, the reported pixel count is not the panel's real one, so that method can be several percent off. A quoted diagonal is rounded as well: 0.05 in on 15.6 in is already 0.3 %. Use a card when it matters.
• It is not for anything that must be certified or legally measured: it is not a calibrated instrument, and glass thickness and viewing angle (parallax) add their own error when you hold an object against a screen.
• A phone or laptop screen is short. Longer lengths need the Next button and careful marking, and each step adds error.

Privacy

Nothing leaves your device. The only thing saved is the calibration, as one number of pixels per millimetre with the pixel ratio and method, in this browser's local storage under polyatic-ruler; Clear saved calibration deletes it. The page sends an anonymous page-view count and one of two fixed words, ruler-calibrate and ruler-measure, never a measurement.

Questions

Is an online ruler actual size?

Only after calibration. A web page cannot read how many pixels your screen packs into a millimetre, and the browser's own idea (96 pixels to the inch) is wrong on almost every screen. Match a bank card to the outline once and the ruler is real size to within about the width of one pixel on the card, typically 0.2 % to 0.3 %.

Can I use a ruler on my phone?

Yes. On a phone held upright the ruler runs down the long side of the screen. Calibrate with a card: the outline stands upright so a card fits, and you slide until its edges meet. Then lay the object along the screen. Tap Full screen for the longest ruler the phone allows; on iPhone the page fills Safari's window instead, because iPhone Safari has no full-screen mode for web pages.

How do I measure in mm on screen?

The top edge (the left edge on a phone) is in centimetres with a tick for every millimetre and a longer one at each half centimetre. For a figure, use the measure area: drag from one point to the other and it shows the distance in millimetres to one decimal, in centimetres and in inches.

How do I see inches at actual size?

Calibrate first, then read the bottom edge (the right edge on a phone): whole inches are numbered, with ticks at each 1/2, 1/4, 1/8 and 1/16 inch. The measure tool also gives inches to two decimals and to the nearest sixteenth, for example 3.94 in, about 3 15/16 in.

How do I measure something longer than my screen?

Press Next. The ruler moves on by a whole number of centimetres and shows its new start, for example From 20.0 cm. Mark the object where the last visible tick was, slide that mark back to the start of the ruler and read on. Each move adds a little error, so for long lengths a tape measure is better.