The default American tapered pipe thread. Male and female threads are both cut on a 1:16 taper, so they wedge together and seal on the thread flanks themselves rather than on a face or gasket.
Standard
ANSI/ASME B1.20.1
Flank angle
60° included
Taper
1 : 16 on diameter
Sealing
Interference between mating flanks as the taper draws up, usually with thread sealant or PTFE tape.
Thread profile
A 60° included flank angle with a flat-truncated crest and root -- the single fastest way to tell a Unified/American pipe thread from the other family by eye.
1 : 16 taper on diameter (0.75″ per foot of length)Angle exaggerated for visibility — the real taper is far shallower.
Size chart
Basic dimensions, NPT · tap drill at 75% thread
Nominal
TPI
Pitch
Major Ø
Pitch Ø
Minor Ø
Tap drill
1/16
27
0.0370"
0.3125"
0.2884"
0.2644"
J (0.2764")
1/8
27
0.0370"
0.4050"
0.3809"
0.3569"
U (0.3689")
1/4
18
0.0556"
0.5400"
0.5039"
0.4678"
31/64" (0.4859")
3/8
18
0.0556"
0.6750"
0.6389"
0.6028"
5/8" (0.6209")
1/2
14
0.0714"
0.8400"
0.7936"
0.7472"
49/64" (0.7704")
3/4
14
0.0714"
1.0500"
1.0036"
0.9572"
63/64" (0.9804")
1
11.50
0.0870"
1.3150"
1.2585"
1.2020"
1-15/64" (1.2303")
1-1/4
11.50
0.0870"
1.6600"
1.6035"
1.5470"
1-37/64" (1.5753")
1-1/2
11.50
0.0870"
1.9000"
1.8435"
1.7870"
1-13/16" (1.8153")
2
11.50
0.0870"
2.3750"
2.3185"
2.2620"
2" (2.2903")
2-1/2
8
0.1250"
2.8750"
2.7938"
2.7126"
2" (2.7532")
3
8
0.1250"
3.5000"
3.4188"
3.3376"
2" (3.3782")
4
8
0.1250"
4.5000"
4.4188"
4.3376"
2" (4.3782")
Tap drill calculator
NPT is tapered: engagement here depends on how far the joint is driven onto the taper, not diameter alone. Treat this as a straight-thread approximation and check ANSI/ASME B1.20.1 or your tap manufacturer’s chart before cutting production parts.
Calculated live
Tap drillJ0.2764" calculated · J (0.2764") nearest standard
MAN4MACHINE