The diameter of a pipe is its width across an open end of the pipe. The inside diameter is the width measuring on the inside of the pipe; the outside diameter is the width measuring on the outside.
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The inside diameter (ID) is 8 inches. The nominal pipe size (NPS) and outside diameter (OD) depend on the thickness of the walls of the pipe.
Three-quarter inch diameter pipe refers to the outside diameter. Different pipe types of the same outside diameter usually have different inside diameters. ASTM SCH 40 pipe of 3/4 inch diameter has an inside diameter of 0.824 inches. ASTM CL 200 pipe of 3/4 inch diameter has an inside diameter of 0.930 inches because of the thinner wall.
That would depend on the wall thickness of the pipe. 54 inches is the outside diameter so subtract the pipe wall thickness (x2) adn you have the inside diameter.
diameter = 2 * radius
When dealing with pipe the volume is more important, therefore, I would use the inside diameter.