Total volume of a cylinder shaped tank is the area, A, of the circular end times the height, h. A = πR2 where R is the radius which is equal to D/2. Therefore:
V(tank) = πR2 L

The filled volume of a vertical cylinder tank is just a shorter cylinder with the same radius, R, and diameter, D, but height is now the fill height or H.  Therefore:
V(fill) = πR2 H