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Le conversioni in pixel usano il PPI attuale del tuo righello (96).
Last updated: October 13, 2026 · By the TrueRuler team
An online ruler turns your screen into a real measuring tool. TrueRuler draws centimetre, millimetre and inch scales at true physical size by detecting your device's pixel density — or by calibrating once with your screen size or a standard bank card. It is free, needs no installation, and works on any phone, tablet, laptop or monitor.
An online ruler is a web-based measuring tool that displays a ruler on your screen at real physical size. It calculates your display's pixels-per-inch (PPI) and draws every centimetre, millimetre and inch to match a physical ruler held against the screen. After a one-time calibration, you can measure small objects directly on your display — no app to install.
Your screen does not know what an inch is — it only knows pixels. The bridge between the two is pixel density, measured in PPI (pixels per inch). If your screen shows 141 pixels per inch, the ruler draws one inch as exactly 141 pixels, one centimetre as 55.6 pixels, and one millimetre as 5.56 pixels.
There is a catch: two monitors can share the same 1920×1080 resolution yet be 22 or 32 inches wide — the same pixels, very different real sizes. That is why an online ruler must learn your screen's true PPI. TrueRuler does this in two ways: it recognises common phones, tablets and laptops from a device database and applies their factory PPI, or it computes PPI from the screen's diagonal size.
One more detail: browsers work in CSS pixels, which can differ from physical pixels on high-density ("Retina") displays. TrueRuler reads your device's pixel ratio and converts automatically, and it warns you if browser zoom is not at 100%, because zooming changes the size of a CSS pixel.
When you enter your screen's diagonal size, the ruler computes PPI with the Pythagorean theorem:
PPI = √(width² + height²) ÷ diagonal inches
Worked example. A 15.6-inch laptop with a 1920×1080 screen: √(1920² + 1080²) = √4,852,800 ≈ 2202.9 pixels diagonally. Divide by 15.6 inches → 141.2 PPI. One inch is therefore drawn as 141.2 pixels, one centimetre as 55.6 pixels (141.2 ÷ 2.54), and one millimetre as 5.56 pixels.
The ruler shows metric and imperial scales side by side. Use millimetres for precision work, centimetres for everyday measuring, and inches for imperial measurements. The built-in converter above also handles metres, feet, yards and points.
| Unit | In centimetres | In millimetres | In inches |
|---|---|---|---|
| 1 inch | 2.54 cm | 25.4 mm | 1 in |
| 1 centimetre | 1 cm | 10 mm | 0.3937 in |
| 1 millimetre | 0.1 cm | 1 mm | 0.03937 in |
| 12 inches | 30.48 cm | 304.8 mm | 12 in |
Press Rotate to swap the horizontal and vertical rulers — handy for measuring height or when your screen is narrow. The vertical ruler works exactly like the horizontal one, in centimetres, millimetres and inches.
Press Measure and drag a box over anything visible on screen — a photo, a diagram, a product image. The box shows live width × height in cm, mm and inches. Drag the handles to resize, drag the centre to move it, and use your arrow keys for precise nudging. Copy puts the reading on your clipboard.
To find your ring size, wrap a thin strip of paper around your finger, mark where it overlaps, then lay the strip flat against the on-screen ruler and read the length in millimetres — most jewellers size rings from the finger's circumference in mm. For a ring you already own, open the Measure box and drag it across the ring's inner diameter. Finger size shifts slightly during the day, so measure in the evening for the most reliable result.
Press Protractor to overlay a full 180° protractor. Drag either arm to the angle you want to measure; the readout shows the degrees live. Drag the centre to move the whole protractor. It uses the same calibration as the ruler, so angles stay true on any screen.
Press Print for a paper ruler at exact true size: 18 cm on one edge and 7 inches on the other. Print at 100% scale (never "fit to page"), cut along the edge, and check it against a known object. A printed ruler is handy when you need to measure something away from the screen.
| Method | Best for | Watch out for |
|---|---|---|
| Physical ruler | Everyday accuracy you can trust | Not always at hand |
| Tape measure | Large objects and rooms | Poor precision below a millimetre |
| Phone AR measuring apps | 3D spaces and furniture | Camera distortion; needs installing |
| Printable ruler | True size on paper | Needs a printer and the 100% scale setting |
| TrueRuler (online ruler) | Quick measuring on any screen | Needs one-time calibration; keep zoom at 100% |
Yes — once calibrated, it is accurate to about a millimetre on most screens. The green status dot means your device was recognised and the factory pixel density is applied. If your device is not recognised, calibrate with the bank-card method, which is the most precise option.
Pick any of three methods: choose your device from the list, enter your screen's diagonal size, or hold a standard bank card (85.60 mm wide) against the blue guide and drag the slider until they match. Your calibration is saved automatically.
Almost always because two monitors share a resolution but not a physical size — a 1920×1080 panel can be 22 or 32 inches. The ruler cannot guess, so calibrate that monitor once with the bank-card method and it will stay correct.
As long as your screen — a 12-inch ruler fits on most laptops, while phones show less. For a fixed-length reference, print the paper ruler: exactly 18 cm on one edge and 7 inches on the other at 100% scale.
Yes. Switch the toolbar to mm and the rulers redraw with millimetre ticks and numbering — ideal for precise small measurements.
Yes. The layout adapts to small screens, and most popular phones and tablets are in the device database for automatic calibration. On touch screens, drag the measure box with your finger.
No. Unlike AR measuring apps, TrueRuler never uses your camera — it measures from your screen's pixel density, so there is nothing to point and no permission to grant.
Yes, completely free with no account and no installation. All processing happens in your browser; nothing you measure is uploaded anywhere.
Yes. Wrap a paper strip around your finger, mark the overlap, lay it flat against the on-screen ruler and read the millimetres — jewellers size rings from finger circumference. Or drag the Measure box across a ring's inner diameter.
Press Print, then print at 100% scale — never "fit to page", which would shrink it. Cut along the edge and verify against a bank card (85.60 mm) before trusting it.
Your calibration, unit choice and theme are saved in your browser's local storage on your own device. Clearing site data resets them; nothing is sent to any server.
PPI means pixels per inch — your screen's pixel density. A ruler must know it to convert pixels into real centimetres and inches. That single number is the whole difference between a decorative ruler image and a true-size measuring tool.
Stampa questa pagina in scala 100% (mai «adatta alla pagina»). Ritaglia lungo il bordo e avrai un vero righello da 18 cm / 7 inch.
Scegli un metodo. La calibrazione con carta è la più precisa.
Inserisci la diagonale dalle specifiche del tuo dispositivo (confezione o impostazioni → info).
Appoggia una qualsiasi carta di pagamento sul riquadro blu e trascina il cursore finché il riquadro non ha esattamente le dimensioni della carta. Tutte le carte misurano 85.60 × 53.98 mm.