Comment by Balgair

A good metric I once heard was:

Fahrenheit is what humans feel

Celsius is what water feels

Kelvin is what atoms feel

Also, for converting between km and miles, using the Fibonacci sequence is a 'good enough' estimator.

5 miles is ~8 km, 21 miles is ~34 km. 13 km is ~8 miles, 55 km is ~34 miles.

Drop or add decimals to get to numbers not in the sequence. If you want to know what 6 km, use 5.5km and you see that its a bit over 3.4 miles.