Metric.
Pro-tip:
If you can add mili- centi- deci- to the front...
its metric.
++++
Not sure that's much of a tip. The prefixes are simply divisions, but it is true that they are used normally only for metric units. However, the litre is indeed part of the metric system: it is the basic unit of fluid volume.
Fahrenheit is a customary unit
Use whatever is common in your country. In the long term, however, the entire world should switch to metric, for reasons of standardization, so if you happen to live in a country that uses non-metric units, you might try to push for metrification.
What is the metric unit of a orange? What is the metric unit of a pencil? What iw the customary unit amount of water a bathtub will hold?
126.8 cups 1 liter = 4.22 cups 1 cup = 0.23 liter
There are 1000 milliliters in 1 liter. One milliliter is 0.001 liter.
Metric.
liter(metric) gallon(customary)
It is a customary unit.
The opposing systems are imperial and metric, not customary. But, meters are metric.
The pound is NOT a metric unit.
It is customary.
It can be more or less. An once (customary) is bigger than a gram (metric) but a pound (customary) is smaller than a kilogram (metric).
Metric.
Metric
customary
It is a customary unit.
A "gran" is a German measurement, it is neither customary or metric. A "gram" is a metric unit.