Online ruler
Not calibrated yet, so the scale is only approximate. Calibrate once and it is remembered on this device.
Runs in your browser · nothing is uploaded
A ruler on your screen, in centimetres or inches. It needs calibrating once against an object of known size — every bank card is exactly 85.6 mm wide, which makes a reliable reference.
How to use the online ruler
A browser genuinely cannot know how big your screen is. A CSS inch is defined as exactly 96 CSS pixels regardless of the display, so on a phone at 460 pixels per inch a "1 inch" line is about a fifth of a real inch, and on a large 1080p television it is nearly double. There is no API that reports physical size, because manufacturers do not reliably report it. Any on-screen ruler that claims accuracy without asking you to calibrate is guessing.
The calibration reference used here is the ID-1 card format from ISO/IEC 7810: 85.60 by 53.98 mm. Every bank card, credit card and most national ID cards are made to it, which makes one the most reliably-sized object in almost anyone’s pocket. Line the long edge up against the scale and adjust until it reads 8.56 cm, and the whole ruler is then correct on that screen.
The calibration is stored in your browser and applies to this device only — which is right, since a different screen needs a different number. Clearing site data or using a private window loses it, and the ruler falls back to the uncalibrated CSS default and says so rather than quietly being wrong.
One limitation no calibration fixes: browser zoom. Zooming changes the CSS pixel size, so a ruler calibrated at 100% is wrong at 110%. If the numbers stop matching, check the zoom level first.
Questions
A CSS inch is fixed at 96 pixels regardless of the screen, and no API reports physical size. Without calibration any on-screen ruler is guessing.