Pi * D^2 / 4 * H * 62.4
Where D is tank diameter in ft H is tank height in ft and 62.4 is the unit weight of water in lbs per cubic foot. Divide by 144 of D is in inches and 12 if H is in inches.
Or if you want to be real lazy you can see Pi/4*62.4 is ~49 and round to 50.