If you mean capacity as in volume: the basic unit is cubic meters, since lengths are measured in meters. Of course, cubic centimeters, cubic millimeters, or cubic kilometers can be used as well, depending on the application.
If you mean capacitance: the modern version of the metric system, the SI, the basic unit is the Farad. Since in practice this is a very large unit, usually smaller multiples are used, such as microfarad, nanofarad, or picofarad.
Answer
There are just seven base units in the SI system. 'Capacity', in the sense of volume, is measured in cubic metres which is a derived unit, not a base unit.
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The standard liquid for measuring volume in the metric system is water.
The standard unit in the metric system for measuring mass is the kilogram (kg).
The standard unit for measuring length and distance is the meter (m) in the metric system.
The metric system is not the standard measuring system in America primarily due to historical reasons and inertia. The US has not fully adopted the metric system because of the cost and effort involved in converting all practices, standards, and infrastructure. Additionally, there may be resistance to change from some sectors of society.
You would use meters for measuring length in the metric system, which is the standard unit for length. Inches are used in the imperial system for measuring length.