Measurement Guides

Longer explainers behind the tools on this site: what a measurement actually means, why screens disagree about it, and how to get it right without special equipment.

Every tool on Online Ruler Tools is built around one problem: a screen does not know its own size, and most on-screen rulers guess rather than ask. These guides are the reasoning behind that, written to stand on their own rather than as instructions for a calculator. Read one end to end, or jump to the topic you came for.

All eight

• Pixel Density (PPI) Explained: Why Screens Draw Sizes Differently

  What PPI actually measures, why a laptop and a phone disagree about an inch, and how to work out your own screen's density.

• Why Two On-Screen Rulers Never Agree

  The CSS pixel is defined as 1/96 of an inch. Almost no screen matches that. Here is what that means and the one fix that actually works.

• How to Measure Something Without a Ruler

  Cards, coins, banknotes and paper are all built to fixed, published sizes. Used right, they measure almost anything to within a millimetre.

• How to Find Out Someone's Ring Size Without Asking

  For a proposal or a surprise gift. Borrowing a ring is the reliable method; here is how to do it, and what to avoid.

• How to Read a Protractor

  Vertex, baseline, inner scale, outer scale: the four things that trip people up, explained with the reasoning, not just the steps.

• Screen Resolution Explained: 1080p, 1440p, 4K and What They Actually Mean

  Resolution, pixel density and physical size are three different things. Knowing which one you are asking about answers most screen questions.

• The ISO 216 Paper Standard: Why A4 Is the Shape It Is

  One ratio, one starting size, and every A, B and C paper size in the world falls out of it. The history and the maths behind A4.

• A4 vs Letter: Why Two Standards Exist and How to Not Get Caught Out

  A4 and US Letter are close enough to cause real printing problems and different enough to matter. Here is what changes between them.