a 6 inch pipe full of water extending vertically for 100' what is the pressure at the bottom
P = Po + (rho)*g*d
The water density, rho, has to be in mass units (slugs/volume), not pounds/volume
rho = 62.4 lb/ft^3/g = 1.938 slug/ft^3
d = 100 ft is the water column height
Po = 14.7 lb/in^2 = 2117 lb/ft^2 is the atmospheric pressure above.
P = 2117 + (1.938)*(32.2)(100)
= 8357 lb/ft^2
= 58.0 lb/in^2
Note that the diameter of the pipe doesn't matter.