volume = length x width x heigth x 0.52
Source
Arch Ital Urol Androl. 2005 Mar;77(1):60-2.
Measurements of urinary bladder volume: comparison of five ultrasound calculation methods in volunteers.
Dicuio M, Pomara G, Menchini Fabris F, Ales V, Dahlstrand C, Morelli G.