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.
A standard credit card is 85.60 × 53.98 mm (3.370 × 2.125 in) under ISO/IEC 7810 ID-1. Also: thickness, corner radius, and how to check a ruler with it.
A4 is 210 × 297 mm: 2480 × 3508 px at 300 DPI, 794 × 1123 at 96 DPI and 595 × 842 at 72 DPI. Plus the formula, A3/A5 sizes and US Letter.
Why 96 CSS pixels rarely equal one real inch on your screen, how device pixels and devicePixelRatio fit in, and how calibration closes the gap.
PPI = diagonal pixels ÷ diagonal inches. Worked examples (a 24-inch 1080p monitor is 91.8 PPI) and why a browser ruler still needs calibration.
Make an on-screen ruler match real size: line it up with a bank card or enter your screen diagonal, check the result, and know when to recalibrate.
Measure anything in a photo with a known reference like a bank card: set the scale, avoid perspective errors, and know when the result can't be trusted.
What every line on an inch ruler means: count sixteenths, simplify fractions like 6/16 = 3/8, and convert any mark to decimal inches and millimeters.
Page zoom and pinch zoom quietly resize an on-screen ruler. Spot the change with devicePixelRatio and use a quick checklist to get true size back.