The map of measurement
Every number arrives
wearing a unit.
And the unit carries half the meaning — "0.03" says nothing; "0.03 °C" is a margin of error. 32 units people actually meet, each explained, each cited, each convertible with the factor shown.
Grouped by the thing being measured — one color per quantity.
Time how long something takes
Length how far or how big
Mass how much stuff
Temperature how hot or cold
Amount of substance how many particles
Electric current how much charge flows
Luminous intensity how bright, to human eyes
Volume how much space
Force how hard a push
Pressure how concentrated a push
Energy how much work can be done
Power how fast energy flows
Frequency how often per second
Sound level how loud, on a power ruler
The foundation
7
SI base units — since 2019, every one pinned to a constant of nature instead of an object in a vault
Exact, not approximate
30
of the convertible units here have factors exact by definition — an inch is 25.4 mm, not "about." Only 1 (the atomic mass unit) is measured.
Quantities
14
kinds of "how much" on this map, from time to sound — each with its own color
How units work
Seven base units, constants of nature, and why the kilogram lived in a vault until 2019 →
Where the definitions come from