Measure a small object in a pinch
When there is no ruler to hand, calibrate with a bank card and hold the object against the scale.
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Guide
The screen ruler converts DPI into pixels per millimetre and draws millimetre and centimetre marks along the top and inch with 1/16 inch marks along the bottom, within the page width available. It defaults to 96 DPI; calibration mode uses the 85.60 × 53.98 mm width of a standard ID-1 bank card or ID card as a physical reference.
Updated 2026-09-271 min read
pxPerMm = dpi / 25.4; the calibration value is saved in the current browser and has to be redone after changing device or changing the system scaling.
When there is no ruler to hand, calibrate with a bank card and hold the object against the scale.
See roughly how many millimetres a button in a design file occupies on a real screen.
Move the DPI slider and watch the centimetres for the same pixel length change.
Every screen has a different real pixel density, so it should be recalibrated on the new device.
Calibration and drawing happen entirely in the browser; no screen or camera data is read and nothing is uploaded.
Updated 2026-09-27
Centimeter and inch ruler on screen, calibrated with a bank card
The browser cannot know the physical size of the screen, so the ruler is only accurate after calibrating with a bank card
Currently using the default 96 DPI estimate, which can be off by a lot on some screens — turning on calibration mode is recommended.
Note: this tool is a screen-estimated ruler,not a certified measuring instrument. Results depend on the screen, the system scaling and the calibration accuracy; for everyday reference only, and not for metrology, trade or other formal use. Screen info: resolution - · device pixel ratio -