Measure the dimensions of the tank. If the tank is a cuboid shape, measure the height, the width and the length. If the tank is a cylindrical shape, measure the diameter of the circular face and the depth. If the tank is an irregular shape, divide it up into regular segments, which you can then add together.
Multiply the dimensions together. For a cuboid, multiply the height by the width by the length. This will give you the volume of the total tank in cubic units, depending on which units you have used. For a cylinder, halve the diameter to find the radius. Multiply this by 3.14 and then by the depth of the tank. This will be the total volume.
Measure the height that the water comes up the tank. If your tank is four feet high, and the water extends two feet up the side, to work out the total volume of water, you divide two by four and multiply this by the total volume of the tank. In this case, the volume of water in the tank will be half the total volume of the tank.