I am trying to calculate the volume in gallons a cylindrical tank can hold with the dimensions 5' diameter and 10' length. V = D x D x 0.7854 x L works and yields 1,468 Gallons US either by measuring the tank in inches and dividing by 231 or measuring the tank in feet and multiplying by 7.48 but when using the formula V = pir^2l it doesn't work out.