NPT - American Standard Pipe Thread/Taper Guide:


NPT is not a measured size, but rather an industry call out size.
The NPT call out designation is not the true thread diameter inside or out.

PROBE THREAD SIZE

ACTUAL OUTSIDE DIAMETER

1/8" NPT 0.405" (Approx. 13/32")
1/4" NPT 0.540" (Approx. 1/2")
3/8" NPT 0.675" (Approx. 5/8")
1/2" NPT 0.840" (Approx. 7/8")