metric
It is customary.
To convert a metric measurement to US customary measurement, you need to use specific conversion factors for each unit. For example, to convert meters to feet, multiply the number of meters by approximately 3.281. For converting liters to gallons, multiply by about 0.264. It's helpful to have a conversion chart or calculator handy for accurate conversions.
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.