American National Pipe – NPT/NPS

American National pipe (NPT, NPS), Like British Standard Pipe (BSP), is designated by trade size, rather than actual diameter, as shown in the table below.

NPT Thread – American Standard Taper Pipe Thread (ANSI/ASME B1.20.1), Taper 1:16 Thread Form Data – Basic Dimensions of American National Standard Taper Threads, NPT (National Pipe Tapered)

There are two basic types of National pipe threads:

• NPT: National Pipe Taper
• NPS: National Pipe Straight

NPT threads are also sometimes referred to as

• MIP (Male Iron Pipe)
• FIP (Female Iron Pipe)
• IPT (Iron Pipe Thread)
• FPT (Female Pipe Thread)
• MPT (Male Pipe Thread)

Note that these references are somewhat casual, and might possibly be used in reference to NPS instead of NPT.

Both NPT and NPS have the same thread angle, shape, and pitch (threads per inch). However, NPT threads are tapered and NPS threads are straight (parallel). Both threads have a 60° included angle and have flat peaks and valleys (this is a Sellers thread form).

If you’ve worked with pipe much at all, you’ve probably noticed that the size of the pipe isn’t really what size the pipe is. Unlike tubing, which is generally specified by its OD, or hose, which is generally specified by its ID, pipe is specified by something else… its Trade Size. So when you say “3/4 pipe,” you’re actually saying “pipe whose OD is a little more than an inch, and whose ID is about 53/64.” -that is, if you are talking about schedule 40 pipe, which is generally what is used for most plumbing applications.

Pipe dimensions are specified by trade size and schedule, according to the following table. Note that while British Standard Pipe dimensions are similar, they are not equivalent to the American Standard Pipe Sizes. See NPT vs. BSP Pipe for thread comparisons.

Nominal Pipe Size (inch)	Number of threads per inch	Outside Diameter (inch)	Outside Diameter (mm)	Pitch of thread		Depth of thread		Truncation, max.”**		Pitch diameter at plane of hand-tight engagement		Length from end of pipe to plane of hand-tight engagement		Length of useful thread		Length of washout thread		Pitch Diameter at Beginning of External Thread Start (inch)	Effective Thread, External Diameter (inch)
				P		h		l		E		L1		L2
				in	mm	in	mm	in	mm	in	mm	in	Threads	in	Threads	in	Threads
1⁄16	27	0.3125	7.938	0.03704	0.941	0.02963	0.753			0.28118		0.16		0.2611		0.1285	3.47	0.27118	0.28750
1/8″	27	0.405	10.287	0.03704	0.941	0.02963	0.753	0.00360	0.091	0.37360	9.489	0.162	4.36	0.2639	7.12	0.1285	3.47	0.36351	0.38000
1/4″	18	0.540	13.716	0.05556	1.411	0.04444	1.129	0.00490	0.124	0.49163	12.487	0.228	4.10	0.4018	7.23	0.1928	3.47	0.47739	0.50250
3/8″	18	0.675	17.145	0.05556	1.411	0.04444	1.129	0.00490	0.124	0.62701	15.926	0.240	4.32	0.0478	7.34	0.1928	3.47	0.61201	0.63750
1/2″	14	0.840	21.336	0.07143	1.814	0.05714	1.451	0.00560	0.142	0.77843	19.772	0.320	4.48	0.5337	7.47	0.2478	3.47	0.75843	0.79179
3/4″	14	1.050	26.670	0.07143	1.814	0.05714	1.451	0.00560	0.142	0.98887	25.117	0.339	4.75	0.5457	7.64	0.2478	3.47	0.96768	1.00179
1″	11.5	1.315	33.401	0.08696	2.209	0.06957	1.767	0.00630	0.160	1.23863	31.461	0.400	4.60	0.6828	7.85	0.3017	3.47	1.21363	1.25630
1-1/4″	11.5	1.660	42.164	0.08696	2.209	0.06957	1.767	0.00630	0.160	1.58338	40.218	0.420	4.83	0.7068	8.13	0.3017	3.47	1.55713	1.60130
1-1/2″	11.5	1.900	48.260	0.08696	2.209	0.06957	1.767	0.00630	0.160	1.82234	46.287	0.402	4.83	0.7235	8.32	0.3017	3.47	1.79609	1.84130
2″	11.5	2.375	60.325	0.08696	2.209	0.06957	1.767	0.00630	0.160	2.29627	58.325	0.436	5.01	0.7565	8.70	0.3017	3.47	2.26902	2.31630
2-1/2″	8	2.875	73.025	0.12500	3.175	0.10000	2.540	0.00780	0.198	2.76215	70.159	0.682	5.46	1.1375	9.10	0.4337	3.47	2.71953	2.79062
3″	8	3.500	88.900	0.12500	3.175	0.10000	2.540	0.00780	0.198	3.38850	86.068	0.766	6.13	1.2000	9.60	0.4337	3.47	3.34062	3.41562
3-1/2″	8	4	101.600	0.12500	3.175	0.10000	2.540	0.00780	0.198	3.88881	98.776	0.821	6.57	1.2500	10.00	0.4337	3.47	3.83750	3.91562
4″	8	4.5	114.300	0.12500	3.175	0.10000	2.540	0.00780	0.198	4.38712	111.433	0.844	6.75	1.3000	10.40	0.4337	3.47	4.33438	4.41562
5″	8	5.563	141.300	0.12500	3.175	0.10000	2.540	0.00780	0.198	5.44929	138.412	0.937	7.50	1.4063	11.25	0.4337	3.47	5.39073	5.47862
6″	8	6.625	168.275	0.12500	3.175	0.10000	2.540	0.00780	0.198	6.50597	165.252	0.958	7.66	1.5125	12.10	0.4337	3.47	6.44609	6.54062
8″	8	8.625	219.075	0.12500	3.175	0.10000	2.540	0.00780	0.198	8.50003	215.901	1.630	8.50	1.7125	13.70	0.4337	3.47	8.43359	8.54062
10″	8	10.750	273.050	0.12500	3.175	0.10000	2.540	0.00780	0.198	10.62094	296.772	1.210	9.58	1.9250	15.40	0.4337	3.47	10.54531	10.66562
12″	8	12.750	323.850	0.12500	3.175	0.10000	2.540	0.00780	0.198	12.61781	320.493	1.360	10.88	2.1250	17.00	0.4337	3.47	12.53281	12.66562
14″	8	14	355.600	0.12500	3.175	0.10000	2.540	0.00780	0.198	13.87262	352.365	1.562	12.50	2.2500	18.00	0.4337	3.47	13.77500	13.91562
16″	8	16	406.400	0.12500	3.175	0.10000	2.540	0.00780	0.198	15.87575	403.244	1.812	14.50	2.4500	19.60	0.4337	3.47	15.76250	15.91562
18″	8	18	457.200	0.12500	3.175	0.10000	2.540	0.00780	0.198	17.87500	454.025	2.000	16.00	2.6500	21.20	0.4337	3.47	17.75000	17.91562
20″	8	20	508.000	0.12500	3.175	0.10000	2.540	0.00780	0.198	19.87031	504.706	2.125	17.00	2.8500	22.80	0.4337	3.47	19.73750	19.91562
24″	8	24	609.600	0.12500	3.175	0.10000	2.540	0.00780	0.198	23.68094	606.066	2.375	19.00	3.2500	26.00	0.4337	3.47	23.71250	23.91562

Nominal Pipe Size (inch)	Number of threads per inch	Tapping Drills: Taper NPT with Reamer *			Tapping Drills: Taper NPT without Reamer			Tapping Drills: Straight NPS
		Drill Size	Equivalents (in)	Equivalents (mm)	Drill Size	Equivalents (in)	Equivalents (mm)	Drill Size	Equivalents (in)	Equivalents (mm)
1⁄16	27	15/64″	0.2344	5.9531	C	0.242	6.1468	1/4″	0.25	6.35
1/8″	27	21/64″	0.3281	8.3344	Q	0.332	8.4328	11/32″	0.3438	8.7313
1/4″	18	27/64″	0.4219	10.7156	7/16″	0.4375	11.1125	7/16″	0.4375	11.1125
3/8″	18	9/16″	0.5625	14.2875	37/64″	0.5781	14.6844	19/32″	0.5938	15.0813
1/2″	14	11/16″	0.6875	17.4625	45/64″	0.7031	17.8594	23/32″	0.7188	18.2563
3/4″	14	57/64″	0.8906	22.6219	29/32″	0.9063	23.0188	15/16″	0.9375	23.8125
1″	11.5	1-1/8″	1.125	28.575	1-9/32″	1.2813	32.5438	1-5/32″	1.1563	29.3688
1-1/4″	11.5	1-15/32″	1.4688	37.3063	1-31/64″	1.4844	37.7031	1-1/2″	1.5	38.1
1-1/2″	11.5	1-23/32″	1.7188	43.6563	1-23/32″	1.7188	43.6563	1-3/4″	1.75	44.45
2″	11.5	2-5/32″	2.1563	54.7688	2-3/16″	2.1875	55.5625	2-3/16″	2.1875	55.5625
2-1/2″	8	2-25/32″	2.7813	70.6438	2-5/8″	2.625	66.675	2-5/8″	2.625	66.675
3″	8	3-9/32″	3.2813	83.3438	3-1/4″	3.25	82.55	3-1/4″	3.25	82.55
3-1/2″	8	3-3/4″	3.75	95.25	3-3/4″	3.75	95.25	3-3/4″	3.75	95.25
4″	8	4-1/4″	4.25	107.95	4-1/4″	4.25	107.95	4-1/4″	4.25	107.95
5″	8	5-1/4″	5.25	133.35	5-9/32″	5.2813	134.1438	5-5/16″	5.3125	134.9375
6″	8	6-1/4″	6.25	158.75	6-11/32″	6.3438	161.1313	6-3/8″	6.375	161.925

Notes: 

* Ream the hole before tapping with a reamer having a taper of 3/4 inch per foot. Pipe Size is the accepted industry designation and does not refer to either the inside or the outside diameter (ID or OD) of a pipe or a fitting. Dimensions in Inches except where statedE= Pitch diameter at hand-tight plane. This is also the pitch diameter at the gauge plane.

**’= Truncation from point of thread triangle to flat (not shown in diagram). Minimum = 0.033P for all pitches. See table for maximum.
Tolerances:

When using L1 gauges to check threads, the thread is within permissible tolerance if the ring gauge face, or plug gauge notch, is +/- 1 turn from being flush with the end of the thread.Example designation:

• 3/8 – 18 NPT where
• 3/8 = nominal pipe size
• 18 = number of threads per inch

NPT = symbol for the thread series and form (i.e., National (American)Standard Pipe, Taper)

Note- Basic dimensions are given to four or five decimal places to eliminate errors when calculating gauge dimensions, they do not imply a greater degree of precision than is normally obtainable.

L1= Length of normal hand-tight engagement. This is also the L1 gauge length.

(Longer thread engagement may be used in special applications, such as flanges for high pressure use. In such cases the pitch diameter, E, remains as specified
and the diameter at the end of the pipe is proportionally smaller.)

L2= Effective Length of thread