What it is
Life Size Ruler is a free online ruler. It runs in your browser, has no sign up and does not install anything. It is an independent project, not connected with any card issuer, mint or phone maker named on this site.
How it works
The ruler is drawn on a canvas at a scale of pixels per millimetre. That scale starts from the browser's 96 pixels per inch and is replaced by your own calibration: the width of the card outline you matched, divided by 85.6 mm (or the coin diameter you chose).