P = K[RHO]gL
Where-
P = fuel pressure at each point within the tank
[RHO] = typical fuel density
g = acceleration due to gravity
L = a reference distance between the point of pressure and the tank farthest boundary in the direction of loading
K = 4.5 for the forward loading condition for those parts of fuel tanks outside the fuselage pressure boundary
K = 9 for the forward loading condition for those parts of fuel tanks within the fuselage pressure boundary, or that form part of the fuselage pressure boundary
K = 1.5 for the aft loading condition
K = 3.0 for the inboard and outboard loading conditions for those parts of fuel tanks within the fuselage pressure boundary, or that form part of the fuselage pressure boundary
K = 1.5 for the inboard and outboard loading conditions for those parts of fuel tanks outside the fuselage pressure boundary
K = 6 for the downward loading condition
K = 3 for the upward loading condition
14 C.F.R. §25.963