where
t = minimum required pipe wall thickness (nominal wall thickness less manufacturing tolerance) ; mm (in.)
t0 = minimum required pipe wall thickness due to internal pressure only; mm (in.)
P = design pressure; bar (kgf/cm2, psi)
D = outside diameter of pipe; mm (in.)
K = 20 (200, 2) for SI (MKS, US) units of measure, respectively
S = permissible stress; N/mm2 (kgf/mm2, psi); to be determined by a) or b) below