Measuring guides

Notes from a woodworker who keeps needing a ruler when the workshop is somewhere else. Each guide covers one job you can do with a screen in your hand, and is honest about where a screen stops being the right tool.

• Everyday objects with known sizes

  Bank cards, paper and coins whose dimensions are fixed by a standard or a mint, and which of them make a trustworthy calibration reference.

• Identifying screws, bolts and drill bits

  Read a metric thread size, count the pitch, convert a wood-screw gauge and match a drill bit, all with the calipers on your screen.

• Inch fractions to millimetres

  A conversion table in 1/32" steps, how to read a fractional rule, and when to round to the nearest 1/16".

• Why a browser can't tell how big your screen is

  Pixels, pixel density, CSS pixels and device pixel ratio, and why every on-screen ruler needs calibrating.

Using the ruler itself

• How to use Screen Ruler

  Set up your screen, then use the calipers, protractor and bubble level, with troubleshooting.

• Accuracy and calibration

  Calibrate against a known length, check both axes and understand read precision versus physical accuracy.

Changes to the site and these guides are listed on What's new. If a guide is wrong or missing something, tell me.