Cubic Millimeters
A cubic millimeter is the volume of a cube 1 millimeter on each side, so 1,000 of them make a cubic centimeter. It is the same as a microliter, and a drop of water is roughly 50 cubic millimeters.
Barrels
It is important to note that there are different types of barrels used for different purposes:
The US liquid barrel is defined as 31.5 US liquid gallons, which is equivalent to about 119 liters: US liquid barrel
One US dry barrel is equivalent to 115.628 liters: US dry barrel
One oil barrel equaling approximately 42 US gallons or 159 liters: US Oil barrel
A Federal US Barrel typically contains 31 gallons, which is equivalent to approximately 117.35 liters: US federal barrel
One UK barrel is equivalent to around 164 liters: UK barrel