Online Ruler
A ruler in centimetres and inches. Calibrate it once for your screen and the markings become real.
How to use
Switch units with the buttons at the top. To make the ruler show true size, press Calibrate, hold any bank card against the screen and drag the slider until the card matches the outline exactly. Bank cards are the same size everywhere by standard — 85.6 mm — which makes one a reliable reference.
The calibration value is stored in your browser, so you only do this once per device. Without calibration the ruler still works, but only approximately: the browser cannot know your screen's physical size and assumes an inch is 96 dots, which is true for very few monitors.
FAQ
Why is it inaccurate without calibration?
The browser has no access to your monitor's physical dimensions. It works with a nominal inch of 96 CSS pixels, while real pixel density ranges from roughly 90 to 220 per inch. Every on-screen ruler is approximate until it is calibrated.
Why a bank card?
Its size is standardised worldwide at 85.6 by 53.98 mm. Any bank, transit or loyalty card in that format works — they are all identical.
Do I have to calibrate every time?
No, the value is saved in your browser. You will need to redo it if you clear site data, switch browser or device, or connect a monitor with different pixel density.
Can I measure something bigger than the screen?
The ruler scrolls sideways, but its length is limited by your window. For larger objects, measure in sections or reach for a tape measure.