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
Drag along the ruler to measure - or type a length in the panel
Remembered on this device ·
The online ruler turns your screen into a measuring stick. Open it and a ruler in centimetres and millimetres runs across the page at real size, with inches and pixels one tap away in the Unit control. Drag along the ruler and the readout shows the length in all four units at once - hold a cable, a screw, a ring or a phone case against the screen and read it off. Type a number into Show a length to see exactly how long 1 cm, 10 cm, 1.5 or 2.5 inches or 300 px is on this screen - a centimeter or an inch drawn to scale, a visual measurement without a tape measure. Write it as 10cm or 1cm if you like; the box takes the number. The ruler runs the full width of the page, or the full width of the screen in fullscreen; switch to Vertical for a ruler down the side.
Every screen has its own pixel density, and a browser assumes 96 pixels per inch unless told otherwise. That is exact on most phones and Macs and up to about 10% off on many monitors, so the ruler opens with that browser default and offers two ways to correct it. Bank card draws a dashed box the size of a standard card (85.6 mm wide - any bank, ID or transit card): hold your card against the screen and move the slider until the box matches its width, and the ruler follows. Screen size takes the diagonal of your screen in inches - 6.1 for a phone, 15.6 for a laptop, 27 for a monitor - and works out the density from the resolution. Both settings, and the last length you measured, are remembered on the device, so the ruler is right the next time you open it; a forget link under the calibration clears them if you would rather nothing is kept.
The unit changes the scale and the readout together. 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 shown 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 details card on the ruler lists the calibration in use, pixels per inch and per centimetre, the length of the ruler as shown, and the screen size and pixel ratio the browser reports.
Nothing leaves the browser because nothing is sent anywhere: the ruler is drawn on a canvas from one number, your screen's pixel density, and the calibration you set is kept in the browser's local storage on this device only. No account, no download, no extension - the page works offline once it has loaded.
As accurate as the pixel density it is drawn with. With the browser default of 96 pixels per inch it is exact on most phones and Apple displays and within a few percent on most laptops; a large desktop monitor can be off by up to 10%. Calibrate once with a bank card and the ruler matches a physical ruler to within a millimetre.
Yes. Type the number into Show a length and the ruler highlights 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, whichever you think in. 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 switch to the vertical ruler on a phone.
Yes. The ruler fits the screen width, the vertical ruler runs most of the screen height, and dragging with a finger measures the same way a mouse does. Phone screens report their pixel density accurately, so the browser default is usually already correct - hold a card against the screen to check.
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.