Internal diameter is calculated using nominal outside diameter and wall thickness as per ANSI B36.10: 1970 and BS 1600: Part 2: 1970.

Nominal pipe outside diameter. Possible sizes: 1/8",1/4", 3/8", 1/2", 3/4", 1", 1 1/4", 1 1/2", 2", 3", 4", 5", 6", 8", 10", 12", 14", 16", 18", 20", 22", 24", 26", 28", 30", 32", 34", 36", 38", 40", 42" and 44".

Possible schedule numbers: 5S, 10S, 10, 20, 30, STD/40S, 40, 60, XS/80S, 80, 100, 120, 140, 160 and XXS