Measure the surface area of the lake. Rough approximations can be made by using aerial maps or by measuring the distance around the lake if it is small enough to walk around. Many approximations are available online or in atlases.
Measure the depth of the lake. Because the depth is rarely constant, calculating an average depth helps approximate the volume. To calculate average depth, take depth readings from various parts of the lake and average them. Average depths can also be found by referring to lake depth charts available online.
Multiply the depth, or average depth, by the surface area. The result is the cubic volume of the lake because depth multiplied by surface area equals cubic volume.