millimeter

The millimetre owns the drawing office. Metric engineering and construction drawings are dimensioned in millimetres with the unit symbol left off entirely, a convention the ISO drafting standards make explicit, so a bare "1200" on a plan is 1.2 m and the letters "mm" appear nowhere on the sheet. Trade stock follows the same habit: plasterboard at 12.5 mm, structural plywood at 18 mm, an A4 sheet at 210 by 297 mm, and camera lenses named by their focal length in millimetres.

The identity worth carrying in your head is rainfall: 1 mm of rain is exactly 1 litre per square metre, because a litre is a cubic decimetre and the arithmetic collapses to nothing. A 10 mm storm on a 100 m² roof is 1,000 litres, a tonne of water, computed without a calculator, and that link between depth, area and volume is what the site's hydrology and irrigation solvers run on. The millimetre also marks the floor of unaided measurement: it is about the finest division a person can read reliably by eye, which is why rules and tape measures stop subdividing there and anything finer moves to callipers and micrometers.