BSP, NPT, UNF/JIC and Metric Threads: How to Identify and Match Them
BSP threads have a 55° profile, while NPT, UNF and metric threads have a 60° profile. BSPT and NPT tapered threads seal on the thread itself; on…
ASME B1.20.1 · ASME B1.20.3
NPT Thread Size Chart
Threads per inch, pitch, pipe OD, DN equivalent and approximate tap drill sizes for ASME B1.20.1 NPT and ASME B1.20.3 NPTF tapered pipe threads from 1/8" to 2".
Quick answer
NPT is the American tapered pipe thread with a 60° profile and a 1:16 taper (ASME B1.20.1). 1/2" NPT has 14 threads per inch (pitch 1.814 mm) and a pipe OD of 21.336 mm (0.840 in); because the thread is tapered, a caliper typically reads 21.14 mm at the 4th–5th thread. 1/8" NPT is 27 TPI, 1/4" and 3/8" are 18 TPI, and 1" to 2" are 11.5 TPI (2-1/2" and above are 8 TPI).
ASME B1.20.1 · ASME B1.20.3
Size chart
NPT and NPTF tapered pipe thread dimensions (ASME B1.20.1 / B1.20.3)
| Nominal size (NPS)in | Designation | Threads/in (TPI) | Pitchmm | Tube / pipe ODin | Tube / pipe ODmm | Pipe DN equivalent | Male thread, typical reading (4th–5th thread)mm | Female thread, typical reading (4th–5th thread from the mouth)mm | Tap drillin | Tap drillmm |
|---|---|---|---|---|---|---|---|---|---|---|
| 1/8 | 1/8-27 NPT | 27 | 0.941 | 0.405 | 10.287 | DN6 | 10.18 | 8.54 | 0.339 | 8.6 |
| 1/4 | 1/4-18 NPT | 18 | 1.411 | 0.54 | 13.716 | DN8 | 13.54 | 11.07 | 7/16 | 11.1 |
| 3/8 | 3/8-18 NPT | 18 | 1.411 | 0.675 | 17.145 | DN10 | 16.98 | 14.49 | 37/64 | 14.7 |
| 1/2 | 1/2-14 NPT | 14 | 1.814 | 0.84 | 21.336 | DN15 | 21.14 | 17.9 | 23/32 | 18.3 |
| 3/4 | 3/4-14 NPT | 14 | 1.814 | 1.05 | 26.67 | DN20 | 26.49 | 23.21 | 59/64 | 23.4 |
| 1 | 1-11.5 NPT | 11.5 | 2.209 | 1.315 | 33.401 | DN25 | 33.14 | 29.15 | 1-5/32 | 29.4 |
| 1-1/4 | 1-1/4-11.5 NPT | 11.5 | 2.209 | 1.66 | 42.164 | DN32 | 41.9 | 37.87 | 1-1/2 | 38.1 |
| 1-1/2 | 1-1/2-11.5 NPT | 11.5 | 2.209 | 1.9 | 48.26 | DN40 | 47.97 | 43.94 | 1-47/64 | 44.1 |
| 2 | 2-11.5 NPT | 11.5 | 2.209 | 2.375 | 60.325 | DN50 | 60 | 55.95 | 2-7/32 | 56.4 |
Chart notes
- Thread angle 60°, flat crests and roots. Taper 1:16 (half angle about 1°47').
- Pipe OD is not the reading at the start of the thread; because the thread is tapered, the caliper reading is smaller than the OD (e.g. 1/2" NPT pipe OD 21.336 mm, typical male reading 21.14 mm).
- NPT seals on the thread and needs a sealant. NPTF (Dryseal, ASME B1.20.3) has the same TPI and diameters; its crests and roots are made with interference for a dry seal. NPT and NPTF can be assembled together, but a mixed pair should not be expected to seal dry.
- 1/2" and 3/4" NPT and BSPT threads are very similar (both 14 TPI). The differences are the thread angle (60° / 55°) and crest shape (flat / rounded); a definitive check may require an optical comparator.
- The NPS → DN mapping is for pipe (1/2" pipe = DN15). Hydraulic hose DN is defined differently; see the hose size chart.
- Do not confuse with NPSM (straight mechanical pipe thread, 60°): it has the same TPI but is parallel and is used on swivel nuts with a 30° cone seat.
- Tap drill sizes are approximate and apply to unreamed holes.
Reference standards
- ASME B1.20.1
- General purpose tapered pipe threads (NPT)
- ASME B1.20.3
- Dryseal tapered pipe threads (NPTF)
Values are nominal; for critical applications, verify against the current standard and the manufacturer's catalog.
Description
Description
NPT (National Pipe Taper) is the American tapered pipe thread defined in ASME B1.20.1: the profile angle is 60° and the taper is 1:16, with flat crests and roots. Its Dryseal version, NPTF, is defined in ASME B1.20.3 and uses the same TPI and diameters. The chart gives the threads per inch, pitch, pipe OD, DN equivalent and approximate tap drill for sizes from 1/8" to 2".
How to read the chart
- Nominal size (NPS) is the pipe size. The designation is written as "1/2-14 NPT": 1/2" nominal size, 14 threads per inch.
- Pipe OD is not the value you read with a caliper. Because the tapered thread narrows towards the end, the measured diameter is smaller: for 1/2" NPT the pipe OD is 21.336 mm, while the typical reading at the 4th–5th thread is 21.14 mm.
- DN equivalent is for pipe (1/2" = DN 15). Hydraulic hose DN is defined differently; a 1/2" hose is written as DN12.
- Tap drill is an approximate value for an unreamed hole; if a taper reamer is used, a smaller drill is chosen.
NPT vs NPTF
NPT leaves a spiral leak path between the thread flanks; PTFE tape or liquid thread sealant is required to seal it. In NPTF, the crest and root heights are designed to interfere during tightening, so that this path is closed by metal contact. The two threads screw together mechanically, but with a mixed pair the dry-seal assurance of NPTF no longer applies.
Don't confuse it with NPSM
NPSM has the same TPI but is a parallel mechanical pipe thread. The thread does not seal; the seal is made on a 30° cone seat, and it is found on swivel nut ends. A diameter that stays the same along the thread indicates NPSM; one that gets smaller towards the end indicates NPT.
Measuring tips
Confirm the taper by measuring the diameter at the first, fourth and last full thread; a 1:16 taper means a diameter change of 62.5 mm per meter. Measure the diameter at the 4th–5th full thread and compare it with the typical reading in the chart. Find the pitch with a pitch gauge: 1/8" is 27 TPI; 1/4" and 3/8" are 18 TPI; 1/2" and 3/4" are 14 TPI; 1" to 2" are 11.5 TPI.
Commonly confused threads
- 1/2" and 3/4" NPT vs BSPT: The same 14 TPI; told apart by the 60° / 55° profile and the crest shape.
- 1/4-18 NPT vs 9/16-18 UNF: The same 18 TPI; diameters of 13.716 and 14.288 mm. NPT is tapered, UNF is parallel.
- 1/4-18 NPT vs M14×1.5, and 1/8-27 NPT vs M10×1: The diameter difference is under 0.3 mm; pitch and taper tell them apart.
- 1-11.5 NPT vs 1-5/16-12 UN: Diameters of 33.401 and 33.338 mm; a pitch difference of 4.3%. Use a pitch gauge and a taper check together.
On stainless steel NPT connections
Tapered threads seat a little deeper with each tightening and partly deform as they are tightened; at points that are taken apart often, a parallel-threaded connection with an O-ring is more suitable. To counter the risk of galling on AISI 316/316L threads, use a sealant compatible with stainless steel and do not overtighten the thread.
FAQ
Frequently asked questions
If your question isn't answered here, message us on WhatsApp with dimensions or a photo, and our technical team will answer directly.
How many mm is 1/2 NPT?
1/2" NPT has a pipe OD of 21.336 mm (0.840 in), 14 threads per inch and a pitch of 1.814 mm. Because the thread is tapered, a caliper typically reads 21.14 mm at the 4th–5th full thread. On a female thread the minor diameter typically reads 17.90 mm at the 4th–5th thread from the opening and about 18.3 mm at the first full thread at the opening.
What is the difference between NPT and NPTF?
Both are 1:16 tapered threads with a 60° profile and the same TPI. In NPTF (ASME B1.20.3), the crest and root heights are designed to interfere on tightening, closing the spiral leak path with metal contact. In NPT this path stays open and a sealant is required. With a mixed pair, the dry-seal assurance of NPTF does not apply.
Can NPT and BSP be connected?
No. NPT has a 60° profile and BSP a 55° profile; at 1/8", 1/4", 3/8" and at 1" and above, the threads per inch also differ. Because both are 14 TPI at 1/2" and 3/4", the part screws in a few turns and the wrong match is noticed late. If a transition is needed, use an adapter with NPT at one end and BSP at the other.
Is sealing tape needed on NPT threads?
On NPT, yes: PTFE tape or liquid thread sealant closes the spiral gap left between the thread flanks. Wrap the tape in the direction of the thread, leaving the first thread clear, and keep residue out of the line. NPTF is designed for a dry seal, but this assurance is lost when it is paired with NPT.
What drill size is needed for an NPT tap?
Approximate values for an unreamed hole: 7/16 in (≈11.1 mm) for 1/4" NPT, 37/64 in (≈14.7 mm) for 3/8", 23/32 in (≈18.3 mm) for 1/2" and 59/64 in (≈23.4 mm) for 3/4". If a taper reamer is used, a smaller drill is chosen; check the value against your workshop standard.
From chart to quote
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