The capacity of a container is another word for the volume of material it can hold. It is usually measured in liters or gallons. This is not the same as the volume that would displace the container if you submerged it in water. The difference between these two quantities is the thickness of the walls of the container.
This difference is negligible if the container is made of a thin material, but in the case of wooden or concrete containers with walls that can be several centimeters thick, it is not. When measuring capacity, it's always best to measure interior dimensions. If you don't have access to the inside, you need to know the thickness of the container walls to get an accurate result. If you have access to the inside of the container, you can measure the interior dimensions and calculate the capacity directly, using the volume formula.
Calculate the capacity of a container by measuring its dimensions and using the volume formula appropriate to the shape of the container.