Reference chart

NPT Pipe Thread Dimensions Chart

American National Standard taper pipe threads (NPT, and NPTF, which shares the same basic dimensions) from 1/16 to 4 NPS: threads per inch, pipe outside diameter, hand-tight and effective thread lengths, pitch diameters at both gauge planes and the tap drill. Inches on top, millimeters underneath.

Need ported manifolds, fittings or threaded pipe parts? .

NPT taper pipe thread dimensions, 1/16 to 4 NPS, in inches and millimeters
NPS	TPI	Pipe OD (D)	Hand-tight L1	Effective thread L2	Pitch dia at pipe end E0	Pitch dia at L1 E1	Tap drill (no reamer)
1/16	27	0.31257.94 mm	0.16004.06 mm4.32 turns	0.26116.63 mm	0.271186.89 mm	0.281187.14 mm	D (0.2460)6.25 mm
1/8	27	0.405010.29 mm	0.16154.10 mm4.36 turns	0.26396.70 mm	0.363519.23 mm	0.373609.49 mm	R (0.3390)8.61 mm
1/4	18	0.540013.72 mm	0.22785.79 mm4.10 turns	0.401810.21 mm	0.4773912.13 mm	0.4916312.49 mm	7/16" (0.4375)11.11 mm
3/8	18	0.675017.15 mm	0.24006.10 mm4.32 turns	0.407810.36 mm	0.6120115.55 mm	0.6270115.93 mm	37/64" (0.5781)14.68 mm
1/2	14	0.840021.34 mm	0.32008.13 mm4.48 turns	0.533713.56 mm	0.7584319.26 mm	0.7784319.77 mm	23/32" (0.7188)18.26 mm
3/4	14	1.050026.67 mm	0.33908.61 mm4.75 turns	0.545713.86 mm	0.9676824.58 mm	0.9888725.12 mm	59/64" (0.9219)23.42 mm
1	11.5	1.315033.40 mm	0.400010.16 mm4.60 turns	0.682817.34 mm	1.2136330.83 mm	1.2386331.46 mm	1-5/32" (1.1563)29.37 mm
1-1/4	11.5	1.660042.16 mm	0.420010.67 mm4.83 turns	0.706817.95 mm	1.5571339.55 mm	1.5833840.22 mm	1-1/2" (1.5000)38.10 mm
1-1/2	11.5	1.900048.26 mm	0.420010.67 mm4.83 turns	0.723518.38 mm	1.7960945.62 mm	1.8223446.29 mm	1-47/64" (1.7344)44.05 mm
2	11.5	2.375060.33 mm	0.436011.07 mm5.01 turns	0.756519.22 mm	2.2690257.63 mm	2.2962758.33 mm	2-7/32" (2.2188)56.36 mm
2-1/2	8	2.875073.03 mm	0.682017.32 mm5.46 turns	1.137528.89 mm	2.7195369.08 mm	2.7621670.16 mm	2-5/8" (2.6250)66.68 mm
3	8	3.500088.90 mm	0.766019.46 mm6.13 turns	1.200030.48 mm	3.3406384.85 mm	3.3885086.07 mm	3-1/4" (3.2500)82.55 mm
3-1/2	8	4.0000101.60 mm	0.821020.85 mm6.57 turns	1.250031.75 mm	3.8375097.47 mm	3.8888198.78 mm	3-3/4" (3.7500)95.25 mm
4	8	4.5000114.30 mm	0.844021.44 mm6.75 turns	1.300033.02 mm	4.33438110.09 mm	4.38713111.43 mm	4-1/4" (4.2500)107.95 mm

Per ASME B1.20.1. D and L1 are the standard's tabulated values; E0 = D - (0.050D + 1.1)P, E1 = E0 + L1/16 and L2 = (0.80D + 6.8)P are computed from its formulas. Taper is 1 in 16 on the diameter (0.0625 in/in, 3/4 in per foot), a half angle of 1 degree 47 minutes. Thread height is 0.8P. Tap drills are for tapping without a taper reamer; common alternates are 1/4 in for 1/16 NPT and 11/32 in for 1/8 NPT. Confirm with your tap maker, and gauge the tapped hole.

Reading the columns

L1, hand-tight engagement is how far an external thread screws into a mating internal thread by hand. The pitch diameter at that point is E1, the gauge plane that L1 plug and ring gauges check. L2, effective thread length is the length of usable thread on the pipe end, measured from the end: the complete threads plus the ones with full roots and slightly flattened crests. Past L2 the thread vanishes. E0 is the pitch diameter right at the pipe end.

Tapered threads are checked by position, not by go and no-go size: a product thread passes when the gauge notch lands within one turn either side of the part face. That is why a tapped port is taken to depth with a plug gauge in hand rather than by counting turns of the tap.

NPT vs NPTF vs BSPT

NPT (ASME B1.20.1) seals on the taper: the flanks wedge together as the joint is tightened, but the crests and roots are not controlled to touch, which leaves a spiral leak path. Tape or pipe dope fills it. It is the general-purpose US pipe thread for water, air, gas and hydraulics.

NPTF (Dryseal, ASME B1.20.3) has the same taper, pitch and basic sizes, but the crest and root truncation is held so the crests crush into the roots and seal metal to metal. It is used on fuel and high-pressure hydraulic fittings. NPTF and NPT will thread together, but only an NPTF pair seals dry.

BSPT (ISO 7-1, formerly BS 21) is the British and international taper pipe thread. The taper is the same 1 in 16, but the thread angle is 55 degrees with rounded crests and roots, and the pitch differs at most sizes. It is called out as R (external taper), Rc (internal taper) or Rp (internal parallel). BSPP (G threads, ISO 228-1) is the parallel version and seals on a washer or O-ring instead of the thread.

Threads per inch, NPT compared with BSPT, by nominal size
Size	NPT TPI	BSPT TPI
1/16	27	28
1/8	27	28
1/4	18	19
3/8	18	19
1/2	14	14
3/4	14	14
1	11.5	11
1-1/4	11.5	11
1-1/2	11.5	11
2	11.5	11
2-1/2	8	11
3	8	11
4	8	11

Frequently asked questions

What is the actual size of a 1/2 NPT thread?: About 0.840 in (21.3 mm) across the threads at the pipe end, with 14 threads per inch. The nominal pipe size names the pipe, not the thread diameter: 1/2 NPS pipe has a 0.840 in outside diameter, and the tapered thread is cut on that.
What drill size do I use for a 1/4 NPT tap?: A 7/16 in drill when you tap straight from the drilled hole. If you run a taper pipe reamer first, start a little smaller so the reamer has stock to cut. Tap to depth with a plug gauge rather than by counting turns.
How many turns is hand-tight on an NPT fitting?: About 4 to 5 turns for 1/16 through 2 NPS, rising to about 7 turns at 4 NPS; the table lists L1 in inches and turns. A wrench then adds a few more turns to wedge the flanks, and sealant fills the spiral gap at the crests and roots.
Can I thread an NPT fitting into a BSPT port?: Not reliably. BSPT uses a 55 degree thread angle and different pitches at most sizes (28 TPI at 1/8 versus 27 for NPT). At 1/2 and 3/4 the pitches match at 14 TPI, so the parts can start, but the flank angles do not match and the joint is prone to leaking.
What is the difference between NPT and NPTF?: Same taper, pitch and basic dimensions, but NPTF (Dryseal, ASME B1.20.3) controls the crest and root truncation so the crests crush into the roots and seal metal to metal without sealant. NPT relies on tape or paste. An NPT part will thread into an NPTF part, but only NPTF to NPTF is a dry seal.

Related charts and guides

Pipe schedule chart Unified thread dimensions Tap and drill size chart CNC turning Screw machining Welding C360 brass 316 stainless steel CNC machining shops

Straight threads for bolts and screws are on the Unified thread dimensions chart.

NPT Pipe Thread Dimensions Chart

Need ported manifolds, fittings or threaded pipe parts? .

NPT taper pipe thread dimensions, 1/16 to 4 NPS, in inches and millimeters
NPS	TPI	Pipe OD (D)	Hand-tight L1	Effective thread L2	Pitch dia at pipe end E0	Pitch dia at L1 E1	Tap drill (no reamer)
1/16	27	0.31257.94 mm	0.16004.06 mm4.32 turns	0.26116.63 mm	0.271186.89 mm	0.281187.14 mm	D (0.2460)6.25 mm
1/8	27	0.405010.29 mm	0.16154.10 mm4.36 turns	0.26396.70 mm	0.363519.23 mm	0.373609.49 mm	R (0.3390)8.61 mm
1/4	18	0.540013.72 mm	0.22785.79 mm4.10 turns	0.401810.21 mm	0.4773912.13 mm	0.4916312.49 mm	7/16" (0.4375)11.11 mm
3/8	18	0.675017.15 mm	0.24006.10 mm4.32 turns	0.407810.36 mm	0.6120115.55 mm	0.6270115.93 mm	37/64" (0.5781)14.68 mm
1/2	14	0.840021.34 mm	0.32008.13 mm4.48 turns	0.533713.56 mm	0.7584319.26 mm	0.7784319.77 mm	23/32" (0.7188)18.26 mm
3/4	14	1.050026.67 mm	0.33908.61 mm4.75 turns	0.545713.86 mm	0.9676824.58 mm	0.9888725.12 mm	59/64" (0.9219)23.42 mm
1	11.5	1.315033.40 mm	0.400010.16 mm4.60 turns	0.682817.34 mm	1.2136330.83 mm	1.2386331.46 mm	1-5/32" (1.1563)29.37 mm
1-1/4	11.5	1.660042.16 mm	0.420010.67 mm4.83 turns	0.706817.95 mm	1.5571339.55 mm	1.5833840.22 mm	1-1/2" (1.5000)38.10 mm
1-1/2	11.5	1.900048.26 mm	0.420010.67 mm4.83 turns	0.723518.38 mm	1.7960945.62 mm	1.8223446.29 mm	1-47/64" (1.7344)44.05 mm
2	11.5	2.375060.33 mm	0.436011.07 mm5.01 turns	0.756519.22 mm	2.2690257.63 mm	2.2962758.33 mm	2-7/32" (2.2188)56.36 mm
2-1/2	8	2.875073.03 mm	0.682017.32 mm5.46 turns	1.137528.89 mm	2.7195369.08 mm	2.7621670.16 mm	2-5/8" (2.6250)66.68 mm
3	8	3.500088.90 mm	0.766019.46 mm6.13 turns	1.200030.48 mm	3.3406384.85 mm	3.3885086.07 mm	3-1/4" (3.2500)82.55 mm
3-1/2	8	4.0000101.60 mm	0.821020.85 mm6.57 turns	1.250031.75 mm	3.8375097.47 mm	3.8888198.78 mm	3-3/4" (3.7500)95.25 mm
4	8	4.5000114.30 mm	0.844021.44 mm6.75 turns	1.300033.02 mm	4.33438110.09 mm	4.38713111.43 mm	4-1/4" (4.2500)107.95 mm

Reading the columns

NPT vs NPTF vs BSPT

Threads per inch, NPT compared with BSPT, by nominal size
Size	NPT TPI	BSPT TPI
1/16	27	28
1/8	27	28
1/4	18	19
3/8	18	19
1/2	14	14
3/4	14	14
1	11.5	11
1-1/4	11.5	11
1-1/2	11.5	11
2	11.5	11
2-1/2	8	11
3	8	11
4	8	11

Frequently asked questions

What is the actual size of a 1/2 NPT thread?

About 0.840 in (21.3 mm) across the threads at the pipe end, with 14 threads per inch. The nominal pipe size names the pipe, not the thread diameter: 1/2 NPS pipe has a 0.840 in outside diameter, and the tapered thread is cut on that.

What drill size do I use for a 1/4 NPT tap?

A 7/16 in drill when you tap straight from the drilled hole. If you run a taper pipe reamer first, start a little smaller so the reamer has stock to cut. Tap to depth with a plug gauge rather than by counting turns.

How many turns is hand-tight on an NPT fitting?

About 4 to 5 turns for 1/16 through 2 NPS, rising to about 7 turns at 4 NPS; the table lists L1 in inches and turns. A wrench then adds a few more turns to wedge the flanks, and sealant fills the spiral gap at the crests and roots.

Can I thread an NPT fitting into a BSPT port?

Not reliably. BSPT uses a 55 degree thread angle and different pitches at most sizes (28 TPI at 1/8 versus 27 for NPT). At 1/2 and 3/4 the pitches match at 14 TPI, so the parts can start, but the flank angles do not match and the joint is prone to leaking.

What is the difference between NPT and NPTF?

Same taper, pitch and basic dimensions, but NPTF (Dryseal, ASME B1.20.3) controls the crest and root truncation so the crests crush into the roots and seal metal to metal without sealant. NPT relies on tape or paste. An NPT part will thread into an NPTF part, but only NPTF to NPTF is a dry seal.