Short, verifiable explanations behind the tools: how calibration works, how to read a ruler, and where errors come from. Each guide links to the tool it explains.
ISO/IEC 7810 ID-1 credit card dimensions, rounded inch conversions, and how to use a card to check any ruler
A4 is 210 × 297 mm; here is its size in pixels at every common DPI, the formula behind it, A3 and A5 values, and a US Letter comparison
Understand the three layers behind every on-screen measurement, why 96 CSS pixels per inch is only a convention, and how calibration closes the gap
Learn the PPI formula, work through monitor, laptop, and phone examples, and see why a browser needs CSS pixels per inch rather than PPI alone
Calibrate a browser screen ruler with a bank card or your display diagonal, verify the result, and learn what makes the calibration go stale
Set a scale with a reference object, keep everything in one plane, shoot square to the subject, and know when a photo measurement should not be trusted
Learn the tick hierarchy on an inch ruler, count sixteenths, reduce fractions, and convert any mark to decimal inches and millimeters
See how page zoom and pinch zoom rescale CSS pixels, how to detect the change through devicePixelRatio, and a checklist to restore an actual-size ruler