t = Pd / 2SE
Where:
d = Inside diameter in inches;
E = 1.0 welded joint efficiency; except for heads with seams = 0.9;
P = Minimum required bursting pressure in p.s.i.;
S = Minimum tensile strength of plate material in p.s.i., as prescribed in § 179.100-7 ;
t = Minimum thickness of plate in inches after forming.
49 C.F.R. §§179.100-6