The five families of units this site converts between: the SI system, the Imperial system, the US customary system, astronomical units, and the units that belong to no system at all.
What the International System of Units is, its seven base units, the 2019 redefinition that tied them to physical constants, and how its prefixes work.
What the Imperial system is, where it came from, how its units are defined today, and how it differs from US customary measure.
What US customary measure is, why it split from British Imperial units in 1824, how its gallon and pint differ, and where it is defined in metric terms today.
The astronomical unit, the light year and the parsec: how each is defined, which distances they suit, and how astronomers measure them.
The calorie, the kilowatt-hour and the electronvolt belong to no measurement system. What each one means, why it survives, and how it relates to the joule.