metric
It is customary.
A "gran" is a German measurement, it is neither customary or metric. A "gram" is a metric unit.
metric and customary
They are both systems of measurement except that the metric system is simpler and used by the majority of the world.
No a quart is a customary measurement for the capacity of liquid as where a Kilometer is a metric measurement for length
using classical rather than metric units of measurement
You use conversion factors.
There are thousands of measurement units in both systems and it is not possible to list them all.
It is a customary unit.
It's neither people use centuries and decades every where
There are different ways of converting weight, length, volume etc.
1 foot = 304.8 millimeters