Guide
The systems, not just the units.
Most converters hide the fact that “imperial” is not one thing. Fathom keeps the families labelled so a UK pint never pretends to be a US pint.
SI and everyday metric
The International System of Units is coherent: derived units are products of base units with no extra factors. The metre, kilogram, second, ampere, kelvin, mole, and candela sit at the root. Everyday metric (litres, hectares, tonnes, Celsius) is SI plus a few named multiples that people actually say out loud.
US customary and UK imperial
Length and mass have been aligned since 1959: the international yard is 0.9144 m and the pound is 0.45359237 kg, on both sides of the Atlantic. Volume is where the split remains. A US liquid gallon is 231 cubic inches (3.785 L). An imperial gallon is 4.54609 L. Fluid ounces, pints, and quarts follow their gallon, so a US pint is 16 US fl oz and an imperial pint is 20 imperial fl oz.
Tons split too: a US short ton is 2,000 lb, a UK long ton is 2,240 lb, a tonne is 1,000 kg. Stone (14 lb) is still a UK body-weight unit and rare in the US.
CGS
Centimetre-gram-second units — dynes, ergs, gauss, poise, gal — still appear in older papers and some lab instruments. They are metric, but they are not SI. Electromagnetic CGS even splits into electrostatic and electromagnetic flavours, which is why statvolts and abamperes look unhinged next to SI.
IEC binary prefixes
A kilobyte is 1,000 bytes. A kibibyte is 1,024 bytes. Drive manufacturers use the first; memory and many operating systems historically used the second while printing “KB” anyway. Fathom labels kB/MB/GB as SI and KiB/MiB/GiB as IEC so the 7.3% gap at the gigabyte is visible instead of surprising.
Fuel, food, and type
Some families invert. Fuel economy in mpg is distance per volume; L/100 km is volume per distance. Cooking cups are national. Typography points were metal, then DTP (1/72 in), then CSS pixels (1/96 in). If more than one system exists, Fathom includes it.