A real-size ruler on your screen in centimetres, millimetres, inches and pixels: drag to measure, type a length to see it at actual size, calibrate with a bank card for exact results.
Horizontal or vertical, phone and desktop - nothing to install, nothing uploaded
The online ruler turns your screen into a measuring stick. Open it and a ruler runs the full width of the window - centimetres and millimetres along the top edge, inches along the bottom, like a ruler on a desk. On a phone held upright it runs down the side of the screen, the longest ruler the phone has room for. Drag along it and the line follows your finger or mouse: hold a cable, a screw, a ring or a phone case against the screen, line one end up with 0 and read the other off. The big number at the top is the length; the line under it gives the same length in the other units, so 5 cm also reads 50 mm, 1.97 in and 1 15/16″.
Type into the big number to see a length at real size - 1 cm, 10 cm, 1.5 or 2.5 inches, 300 px. A centimeter or an inch drawn to scale, a visual measurement without a tape measure: the band between the two blue lines is exactly that long and runs across the whole screen, so an object lying beside the ruler lines up too. Write it as 10cm, 10,5 or 3 1/2 if you like; the box takes the number. The arrows switch between a horizontal and a vertical ruler, and fullscreen gives the whole width of the screen.
A browser draws 96 pixels to a CSS inch, and on most screens that is not a real inch: on an iPhone it is about 0.63 inches, on a MacBook about 0.75, so a ruler drawn from that number alone comes out a third too small on a phone. This one checks which screen it is on first. iPhones, iPads and MacBooks are recognised from their screen size and pixel ratio and drawn at their real pixel density - an iPhone at 153 or 163 pixels per inch, a MacBook Pro at 127. Android phones are estimated from the width of the screen, usually within a few percent. The Calibrate button at the bottom says which screen the ruler is set for.
Desktop monitors and Windows laptops cannot be recognised this way, so there the ruler starts at the browser's 96 pixels per inch and the Calibrate button asks for a card. Bank card draws a blue box the size of a standard card (85.6 x 54 mm - any bank, ID or transit card): lay your card on it and move the slider until the box matches its width. On a phone the box stands upright, since a card is wider than the screen. Screen size takes the diagonal in inches - 15.6 for a laptop, 27 for a monitor - and works out the density from the resolution. A calibration is remembered for the screen it was made on, so a laptop and the monitor beside it each keep their own, and it survives zooming the page; Forget clears everything kept on this device.
The unit you pick goes on the top edge and into the big number; the other edge shows inches, or centimetres when you pick inches. Centimetres label every 10 mm with millimetre ticks between; the millimetre scale labels the same ticks 10, 20, 30; inches carry 1/16 ticks with the fraction next to the decimal, so 3.94 in also reads 3 15/16. Pixels measure the screen itself in CSS pixels, the unit web pages are laid out in - useful for checking a layout, a banner or an ad slot without a screenshot. The calibration panel lists the pixels per inch and per centimetre in use, the screen size and the pixel ratio.
Nothing leaves the browser because nothing is sent anywhere: the ruler is drawn on a canvas from your screen's pixel density, worked out from the screen size and pixel ratio your browser already reports. The calibration and the last length stay in the browser's local storage on this device. No account, no download, no extension - the page works offline once it has loaded.
As accurate as the pixel density it is drawn with. On an iPhone, iPad or MacBook this ruler knows the density of the panel and matches a physical ruler to within about a millimetre over 10 cm. On an Android phone it is usually within a few percent, and on a desktop monitor it can be off by 10 to 20% until it is calibrated. Lay a bank card on the blue box once and it is exact on any screen.
Yes. Type the number into the big length box at the top and the ruler marks that span at real size, with the same length in the other units underneath. Any length can be visualized on-screen this way - 10mm, 18cm, 20cm or 30cm, centimeters or inches. If the length is longer than the ruler, the page says so - open fullscreen for the full width of the screen (a 30cm ruler needs it on most laptops), or turn a phone sideways.
Yes, and a phone is where a screen ruler most needs to know the device: a CSS inch on a phone is only about 0.6 real inches, so a ruler that trusts the browser draws a "5 cm" line that measures 3 cm. This one recognises iPhones exactly, estimates Android phones closely and runs down the long side of the screen.
No. There is no image and no file here at all; the ruler is drawn in your browser from the screen's pixel density, and your calibration is stored on your own device.