A free, true-to-life ruler right in your browser. Calibrate it to your screen once with any bank card, then measure real objects in centimeters, millimeters and inches — or switch to the pixel ruler for design work. No app, no sign-up. 📏
Actual size • cm / mm / inch • pixel mode • calibrates to your screen
Drag the ruler to lift it out of the box and place it anywhere on your screen — lay it over a photo, a window or any object and measure it. Drag the two orange handles to measure a span; press Dock to return it. Lay a real object against the ruler from 0 to read its size.
Every monitor and phone has a different pixel density, so a ruler is only truly accurate after a one-time calibration. Hold a standard bank, credit or ID card (or a driver's licence) flat against your screen and drag the slider until the on-screen card is exactly the same width as the real one. We save it on this device only.
We pre-fill your physical resolution automatically. Just add the diagonal size and press Apply.
Calibrated to your exact screen, so a centimetre on the ruler is a real centimetre — not an approximation.
Metric on top, imperial on the bottom, with inch fractions down to 1/16 — no switching back and forth.
Flip to pixel mode to measure on-screen elements in px for web and graphic design — handy for developers.
Runs entirely in your browser. Your calibration is stored only on your device — nothing is uploaded.
This free online ruler turns any screen into a real, life-size measuring tool, so you can measure a small object in seconds without digging out a tape measure. Use it as an inch ruler, a cm ruler, or a precise mm ruler — and because it draws to actual size, what you see on screen is the real length. Most on-screen rulers are inaccurate because every display has a different pixel density, so a ruler that looks correct on one laptop can be 20% off on a phone or a 4K monitor. Our real-size ruler solves that with a quick, one-time calibration, giving you a trustworthy virtual ruler in both imperial and metric units.
The tool shows a full metric scale along the top — numbered centimeters with fine millimeter ticks — and an imperial scale along the bottom with inches divided down to sixteenths. You never have to switch units or do the math in your head: read the same edge as a ruler in inches and a ruler in cm at the same time. Need a quick 12 inch ruler, a 6 inch ruler or a 30 cm ruler? It's right here, on demand, with no printing required. Drag the two markers to measure a span and the live readout shows it three ways at once — for example 12.4 cm · 124 mm · 4 7/8 in — which is far faster than reading a paper ruler.
The web has no way of knowing how physically big your screen is. Browsers assume a notional 96 pixels per inch, but real monitors range from about 80 to over 300 pixels per inch, and phones pack in even more — which is why an uncalibrated "actual size ruler" is really just a guess. A standard bank, credit or ID card is a fixed 85.6 mm wide everywhere in the world (the ISO/IEC 7810 ID-1 standard), so it makes a perfect reference. Hold your card to the screen, drag the on-screen card until it matches, and the tool calculates exactly how many pixels equal one inch on your display, then redraws every millimeter and inch tick to scale. You only calibrate once per device — the setting is saved in your browser for next time.
A screen ruler is handy for all the moments you need to measure something small but the real ruler or online tape measure isn't nearby. People use it to size a photo or print, a screw or bolt, a SIM card, a ring or other jewellery, a phone case, washi tape and fabric, beads, stamps and coins, a watch band, a wound or rash, or a pet's paw before ordering supplies. Crafters, sewers, students, online shoppers comparing product dimensions, and hobbyists all reach for a digital ruler like this because it's always in your pocket. Lay the object flat against the glass, line one edge up with the 0 mark, and read off the size in whichever unit you prefer.
Switch to pixel mode and the same tool becomes a precise pixel ruler measured in px, with major marks every 100 pixels. It's ideal for designers and developers checking spacing, icon sizes, banner dimensions and layout gaps when building or reviewing a web page, a slide or a graphic. The readout reports the exact pixel distance between the markers, and the tool shows your device pixel ratio so you can see how CSS pixels map to the physical pixels on your display — a small detail that matters on Retina and high-DPI screens.
The online ruler is fully responsive and touch-friendly, so it works on iPhone, iPad, Android, Windows and Mac in Chrome, Safari, Edge and Firefox. On a phone or tablet you calibrate the same way with a card and then measure objects directly on the glass with your finger. The fullscreen button stretches the ruler across the whole display when you want the longest possible measuring length, and it works even on iPhone, where Safari handles fullscreen differently. Everything runs locally in your browser — there's no app to download, no sign-up, and no data ever leaves your device, so it's completely free and private.
For the best results, calibrate at the browser zoom you normally use and keep it at 100%, since zooming changes the on-screen scale. Lay flat objects directly against the screen and align one edge with the 0 mark. For thicker objects, look straight down to avoid parallax error. If you move from your laptop to your phone, just recalibrate on the new screen — each device keeps its own saved calibration, so your real size ruler stays accurate everywhere you use it.