liters or milliliters or cups.
Volume of cup ca be measured in milliliter. It can also be measured in decimal liter.
gallons
Kilometers are a measure of distance. Even if you substituted kilograms, there would be a fair bit of math involved, as gallon is a unit of volume (as you indicated), while kilogram is a unit of mass. Is this what you were intending to ask?
You would use Volume.
millimetres (centimetres could also be used)
millilitres
milliliter
The millimeter is a unit of length. The liter is a unit of volume. The two units of measure can not be equated.
You would typically use liters or gallons to measure the volume of a hot tub.
there are many unit which measure volume .SI units that measure volume is cubic meter .
The unit of measurement for milk is litre (L) in the SI.
You would use liters (L) to measure the volume of a soda container in the metric system.