Pipe Thread Sizing

How to Determine Pipe Thread Size

Pipe thread sizes do not refer to any physical dimensions. The outside diameter of each pipe fitting must be measured and compared to the table for size identification. For example, a 1" NPT pipe thread has an outside diameter of 1.315".

NPT size
thread diameter
(O.D. of MPT or I.D. of FPT)
1/8" NPT
0.405"
1/4" NPT
0.540"
3/8" NPT
0.675"
1/2" NPT
0.840"
3/4" NPT
1.050"
1" NPT
1.315"
1-1/4" NPT
1.660"
1-1/2" NPT
1.900"
2" NPT
2.375"
2-1/2" NPT
2.875"
3" NPT
3.5"
4" NPT
4.5"