What is a customary unit of measerment? Give an example?

This depends entirely on the calculations on hand.

Normally you're going to be using Metres when doing mathermatics-based calculations for units of measurement.

1km = 1000 metres; 1 metre = 100 cm = 1,000 mm

Alternatively, you could be using Miles (especially if you're in America or England)

1 Mile = 1.2km