Volume is basically the three dimensional space that something fills. So you can place the liquid in a container with standardized markings.
Volume can change based on temperature and pressure though.
The SI unit of volume is the cubic meter. Other units used can be cubic centimeters, liters, etc. Often we choose the units based on the situation. To use units in an equation though, it’s often better to start with SI units.
You can convert between units of volume using ratios. Another simple way to convert units is just to use Google,
You can input something like this:
“Two liters is how many gallons”
And Google will tell you.
You can also calculate the volume if you know the density and mass. (Again, temperature and pressure are also factors).
Density = mass/volume
Volume = mass/density