NPT Thread Chart
NPT thread dimensions from 1/16" to 4": threads per inch, outside diameter, tap drill, hand-tight engagement and effective thread length, in inches and mm, per ASME B1.20.1.
NPT thread chart per ASME B1.20.1
Showing 14 of 14 sizes
| Size | Threads per inch | Outside diameter | Tap drill (no reamer) | Hand-tight L1 | Effective length L2 | Pitch diameter E0 |
|---|---|---|---|---|---|---|
| 1/16" NPT | 27 TPI | 7.94 mm0.3125″ | D6.25 mm0.246″ | 4.06 mm0.16″ | 6.63 mm0.2611″ | 6.89 mm0.27118″ |
| 1/8" NPT | 27 TPI | 10.29 mm0.405″ | R8.61 mm0.339″ | 4.10 mm0.1615″ | 6.70 mm0.2639″ | 9.23 mm0.36351″ |
| 1/4" NPT | 18 TPI | 13.72 mm0.54″ | 7/1611.11 mm0.4375″ | 5.79 mm0.2278″ | 10.21 mm0.4018″ | 12.13 mm0.47739″ |
| 3/8" NPT | 18 TPI | 17.14 mm0.675″ | 37/6414.68 mm0.5781″ | 6.10 mm0.24″ | 10.36 mm0.4078″ | 15.55 mm0.61201″ |
| 1/2" NPT | 14 TPI | 21.34 mm0.84″ | 23/3218.26 mm0.7188″ | 8.13 mm0.32″ | 13.56 mm0.5337″ | 19.26 mm0.75843″ |
| 3/4" NPT | 14 TPI | 26.67 mm1.05″ | 59/6423.42 mm0.9219″ | 8.61 mm0.339″ | 13.86 mm0.5457″ | 24.58 mm0.96768″ |
| 1" NPT | 11.5 TPI | 33.40 mm1.315″ | 1-5/3229.37 mm1.1563″ | 10.16 mm0.4″ | 17.34 mm0.6828″ | 30.83 mm1.21363″ |
| 1-1/4" NPT | 11.5 TPI | 42.16 mm1.66″ | 1-1/238.10 mm1.5″ | 10.67 mm0.42″ | 17.95 mm0.7068″ | 39.55 mm1.55713″ |
| 1-1/2" NPT | 11.5 TPI | 48.26 mm1.9″ | 1-47/6444.05 mm1.7344″ | 10.67 mm0.42″ | 18.38 mm0.7235″ | 45.62 mm1.79609″ |
| 2" NPT | 11.5 TPI | 60.32 mm2.375″ | 2-7/3256.36 mm2.2188″ | 11.07 mm0.436″ | 19.22 mm0.7565″ | 57.63 mm2.26902″ |
| 2-1/2" NPT | 8 TPI | 73.02 mm2.875″ | 2-5/866.67 mm2.625″ | 17.32 mm0.682″ | 28.89 mm1.1375″ | 69.08 mm2.71953″ |
| 3" NPT | 8 TPI | 88.90 mm3.5″ | 3-1/482.55 mm3.25″ | 19.46 mm0.766″ | 30.48 mm1.2″ | 84.85 mm3.34062″ |
| 3-1/2" NPT | 8 TPI | 101.60 mm4″ | 3-3/495.25 mm3.75″ | 20.85 mm0.821″ | 31.75 mm1.25″ | 97.47 mm3.8375″ |
| 4" NPT | 8 TPI | 114.30 mm4.5″ | 4-1/4107.95 mm4.25″ | 21.44 mm0.844″ | 33.02 mm1.3″ | 110.09 mm4.33438″ |
Dimensions per ASME B1.20.1. Tap drill is for direct tapping without reaming. With a taper pipe reamer a smaller drill is used: for example 27/64" for 1/4" NPT, 11/16" for 1/2" and 57/64" for 3/4". Tap maker charts vary slightly; the right size is the one that keeps the L1 gauge within ±1 turn.
What Is an NPT Thread
NPT (National Pipe Taper) is the US tapered pipe thread defined in ASME B1.20.1. It is the most common thread on hydraulic, pneumatic, gas and water fittings in North America.
It has a 1:16 taper: the diameter drops 1/16" per inch of length (3/4" per foot, about 1°47' per side). The thread angle is 60° and it seals as the flanks tighten, usually with PTFE tape or pipe dope.
The nominal size is not the thread diameter: it comes from the pipe. A 1/2" NPT thread is 0.840" (21.34 mm) across, and a 1" NPT is 1.315" (33.40 mm).
How to Read the Chart
Threads per inch: how many threads fit in one inch. "1/2-14 NPT" is the 1/2" thread with 14 threads per inch.
Tap drill: the hole drilled before the tapered tap, without reaming. If the hole is reamed with a taper pipe reamer first, a smaller drill is used and the tap works less, which helps in stainless steel.
Hand-tight engagement L1: how far the male thread goes into the female by hand. Wrench tightening adds about 2 to 3 more turns. Effective length L2: the length of full thread the male part needs.
Pitch diameter E0: the pitch diameter at the small end of the male thread. It is the reference used to make the gauges.
NPT, NPTF, NPS and BSPT Are Not the Same
NPTF (Dryseal, ASME B1.20.3) has the same taper and threads per inch as NPT but controls crest and root to seal metal to metal without sealant. It is used in fuel and hydraulic lines.
NPS is straight pipe thread, not tapered; it does not seal on its own and is used with a gasket or seal.
BSPT (ISO 7-1, marked R or Rc) is the British pipe thread: 55° angle and different pitches (1/8" BSPT has 28 threads and 1/4" has 19). It looks like NPT but they are not interchangeable: a mixed joint leaks.
How to Call Out an NPT Thread on Your Part
On the drawing give the size and pitch, for example 1/2-14 NPT, and whether it is male or female. If depth matters, state it or call for L1 gauge verification.
To quote, upload your STEP and the PDF drawing; the thread goes on the drawing. If you need NPTF instead of NPT, say so explicitly: one does not replace the other.
About NPT Threads
- What does 1/2-14 NPT mean?
- A 1/2" NPT tapered pipe thread with 14 threads per inch. Its outside diameter is 0.840" (21.34 mm), not half an inch: the nominal size comes from the pipe.
- What drill size for a 1/4 NPT tap?
- For direct tapping without reaming, a 7/16" (11.11 mm) drill. If you ream the hole with a taper pipe reamer first, use 27/64" (10.72 mm).
- What drill size for a 1/2 NPT tap?
- Without reaming, 23/32" (18.26 mm). With a taper pipe reamer, 11/16" (17.46 mm).
- How many threads per inch does NPT have?
- 27 on 1/16" and 1/8"; 18 on 1/4" and 3/8"; 14 on 1/2" and 3/4"; 11.5 from 1" to 2"; and 8 from 2-1/2" up.
- Are NPT and BSP threads the same?
- No. NPT has a 60° thread angle and BSPT 55°, and several sizes have different threads per inch (1/8" NPT has 27, 1/8" BSPT has 28). They look alike, but a mixed joint will not seal.
- What is the difference between NPT and NPTF?
- Both are tapered with the same threads per inch. NPTF (Dryseal) controls crest and root to seal metal to metal without sealant; NPT needs PTFE tape or pipe dope.
- How far should an NPT thread engage?
- By hand it goes in the L1 engagement from the chart (for example 0.320" on 1/2" NPT), and a wrench adds about 2 to 3 more turns. The thread is checked with an L1 gauge, with a ±1 turn tolerance.
A 1/2" NPT tapered pipe thread with 14 threads per inch. Its outside diameter is 0.840" (21.34 mm), not half an inch: the nominal size comes from the pipe.
For direct tapping without reaming, a 7/16" (11.11 mm) drill. If you ream the hole with a taper pipe reamer first, use 27/64" (10.72 mm).
Without reaming, 23/32" (18.26 mm). With a taper pipe reamer, 11/16" (17.46 mm).
27 on 1/16" and 1/8"; 18 on 1/4" and 3/8"; 14 on 1/2" and 3/4"; 11.5 from 1" to 2"; and 8 from 2-1/2" up.
No. NPT has a 60° thread angle and BSPT 55°, and several sizes have different threads per inch (1/8" NPT has 27, 1/8" BSPT has 28). They look alike, but a mixed joint will not seal.
Both are tapered with the same threads per inch. NPTF (Dryseal) controls crest and root to seal metal to metal without sealant; NPT needs PTFE tape or pipe dope.
By hand it goes in the L1 engagement from the chart (for example 0.320" on 1/2" NPT), and a wrench adds about 2 to 3 more turns. The thread is checked with an L1 gauge, with a ±1 turn tolerance.
Got a part in mind?
Upload it and we’ll make it real. One part or a thousand.