This calculator will do the math for you if you feed it your measurements of the length, width and height of your forms and click on Compute. It will give an answer in cubic yards or feet; meters or liters or number of 80, 60, 55 or 40 pound bags of pre-mixed concrete with the Metric equivalent.
We have kept the results simple because concrete is not that exact to work with. (Anything smaller than a tenth of a yard might be cute for math professors, but not useful for construction.)