NPT is the American tapered pipe thread with a 60° thread angle; BSP is the British pipe thread with a 55° angle, made either parallel (BSPP, "G") or tapered (BSPT, "R" and "Rc"). The two look alike, and in 1/2 and 3/4 in sizes they even share 14 threads per inch, but they do not interchange.
This chart gives, for each size from 1/16 to 4 in, the threads per inch, the key diameters, the tap drill and the thread engagement. The NPT table is in inches with mm below; the BSP table is in mm with inches below. Further down you will find a side-by-side comparison for telling the two apart and a guide to which threads seal on the thread and which need a washer or O-ring. Use the search box to filter every table, for example 1/2.
NPT Pipe Thread Chart (ASME B1.20.1)
| Size | TPI | Pipe OD | Hand-tight L1 | Effective thread L2 | Tap drill (no reamer) | Tap drill (with reamer) |
|---|---|---|---|---|---|---|
| 1/16 NPT | 270.0370 in pitch | 0.3125 in7.94 mm | 0.1600 in4.06 mm, 4.3 turns | 0.2611 in6.63 mm | D 0.2460 in6.25 mm | 15/64 0.2344 in5.95 mm |
| 1/8 NPT | 270.0370 in pitch | 0.4050 in10.29 mm | 0.1615 in4.10 mm, 4.4 turns | 0.2639 in6.70 mm | R 0.3390 in8.61 mm | 21/64 0.3281 in8.33 mm |
| 1/4 NPT | 180.0556 in pitch | 0.5400 in13.72 mm | 0.2278 in5.79 mm, 4.1 turns | 0.4018 in10.21 mm | 7/16 0.4375 in11.11 mm | 27/64 0.4219 in10.72 mm |
| 3/8 NPT | 180.0556 in pitch | 0.6750 in17.14 mm | 0.2400 in6.10 mm, 4.3 turns | 0.4078 in10.36 mm | 37/64 0.5781 in14.68 mm | 9/16 0.5625 in14.29 mm |
| 1/2 NPT | 140.0714 in pitch | 0.8400 in21.34 mm | 0.3200 in8.13 mm, 4.5 turns | 0.5337 in13.56 mm | 23/32 0.7188 in18.26 mm | 11/16 0.6875 in17.46 mm |
| 3/4 NPT | 140.0714 in pitch | 1.0500 in26.67 mm | 0.3390 in8.61 mm, 4.7 turns | 0.5457 in13.86 mm | 59/64 0.9219 in23.42 mm | 57/64 0.8906 in22.62 mm |
| 1 NPT | 11.50.0870 in pitch | 1.3150 in33.40 mm | 0.4000 in10.16 mm, 4.6 turns | 0.6828 in17.34 mm | 1-5/32 1.1562 in29.37 mm | 1-1/8 1.1250 in28.57 mm |
| 1-1/4 NPT | 11.50.0870 in pitch | 1.6600 in42.16 mm | 0.4200 in10.67 mm, 4.8 turns | 0.7068 in17.95 mm | 1-1/2 1.5000 in38.10 mm | 1-15/32 1.4688 in37.31 mm |
| 1-1/2 NPT | 11.50.0870 in pitch | 1.9000 in48.26 mm | 0.4200 in10.67 mm, 4.8 turns | 0.7235 in18.38 mm | 1-47/64 1.7344 in44.05 mm | 1-23/32 1.7188 in43.66 mm |
| 2 NPT | 11.50.0870 in pitch | 2.3750 in60.32 mm | 0.4360 in11.07 mm, 5.0 turns | 0.7565 in19.22 mm | 2-7/32 2.2188 in56.36 mm | 2-3/16 2.1875 in55.56 mm |
| 2-1/2 NPT | 80.1250 in pitch | 2.8750 in73.02 mm | 0.6820 in17.32 mm, 5.5 turns | 1.1375 in28.89 mm | 2-5/8 2.6250 in66.67 mm | Bore or thread-mill; check fitting maker |
| 3 NPT | 80.1250 in pitch | 3.5000 in88.90 mm | 0.7660 in19.46 mm, 6.1 turns | 1.2000 in30.48 mm | 3-1/4 3.2500 in82.55 mm | Bore or thread-mill; check fitting maker |
| 3-1/2 NPT | 80.1250 in pitch | 4.0000 in101.60 mm | 0.8210 in20.85 mm, 6.6 turns | 1.2500 in31.75 mm | 3-3/4 3.7500 in95.25 mm | Bore or thread-mill; check fitting maker |
| 4 NPT | 80.1250 in pitch | 4.5000 in114.30 mm | 0.8440 in21.44 mm, 6.8 turns | 1.3000 in33.02 mm | 4-1/4 4.2500 in107.95 mm | Bore or thread-mill; check fitting maker |
Pipe OD is the nominal outside diameter of the pipe (ASME B1.20.1 “D”). L1 is the hand-tight engagement; L2 is the effective thread length, L2 = (0.80 D + 6.8) × pitch. Wrench make-up beyond hand-tight is about 3 more turns for sizes up to 2 in. Tap drills are typical shop values; for 2-1/2 in and above most shops bore and taper-ream or thread-mill.
BSP Pipe Thread Chart (ISO 228-1 G and ISO 7-1 R/Rc)
| Size | TPI | Major dia | Pitch dia | Minor dia | Gauge length (ISO 7-1) | Tap drill G (parallel) | Tap drill Rc (taper) |
|---|---|---|---|---|---|---|---|
| 1/16 BSP | 280.907 mm pitch | 7.723 mm0.3041 in | 7.142 mm0.2812 in | 6.561 mm0.2583 in | 4.0 mm4.4 turns | 6.80 mm0.2677 in | See tap maker |
| 1/8 BSP | 280.907 mm pitch | 9.728 mm0.3830 in | 9.147 mm0.3601 in | 8.566 mm0.3373 in | 4.0 mm4.4 turns | 8.80 mm0.3465 in | 8.40 mm0.3307 in |
| 1/4 BSP | 191.337 mm pitch | 13.157 mm0.5180 in | 12.301 mm0.4843 in | 11.445 mm0.4506 in | 6.0 mm4.5 turns | 11.80 mm0.4646 in | 11.20 mm0.4409 in |
| 3/8 BSP | 191.337 mm pitch | 16.662 mm0.6560 in | 15.806 mm0.6223 in | 14.950 mm0.5886 in | 6.4 mm4.8 turns | 15.25 mm0.6004 in | 14.75 mm0.5807 in |
| 1/2 BSP | 141.814 mm pitch | 20.955 mm0.8250 in | 19.793 mm0.7793 in | 18.632 mm0.7335 in | 8.2 mm4.5 turns | 19.00 mm0.7480 in | 18.25 mm0.7185 in |
| 3/4 BSP | 141.814 mm pitch | 26.441 mm1.0410 in | 25.279 mm0.9952 in | 24.118 mm0.9495 in | 9.5 mm5.2 turns | 24.50 mm0.9646 in | 23.75 mm0.9350 in |
| 1 BSP | 112.309 mm pitch | 33.249 mm1.3090 in | 31.770 mm1.2508 in | 30.292 mm1.1926 in | 10.4 mm4.5 turns | 30.75 mm1.2106 in | 30.00 mm1.1811 in |
| 1-1/4 BSP | 112.309 mm pitch | 41.910 mm1.6500 in | 40.431 mm1.5918 in | 38.953 mm1.5336 in | 12.7 mm5.5 turns | 39.50 mm1.5551 in | 38.50 mm1.5157 in |
| 1-1/2 BSP | 112.309 mm pitch | 47.803 mm1.8820 in | 46.324 mm1.8238 in | 44.846 mm1.7656 in | 12.7 mm5.5 turns | 45.25 mm1.7815 in | 44.50 mm1.7520 in |
| 2 BSP | 112.309 mm pitch | 59.614 mm2.3470 in | 58.135 mm2.2888 in | 56.657 mm2.2306 in | 15.9 mm6.9 turns | 57.00 mm2.2441 in | 56.00 mm2.2047 in |
| 2-1/2 BSP | 112.309 mm pitch | 75.184 mm2.9600 in | 73.705 mm2.9018 in | 72.227 mm2.8436 in | 17.5 mm7.6 turns | Bore or thread-mill; check fitting maker | Bore or thread-mill; check fitting maker |
| 3 BSP | 112.309 mm pitch | 87.884 mm3.4600 in | 86.405 mm3.4018 in | 84.927 mm3.3436 in | 20.6 mm8.9 turns | Bore or thread-mill; check fitting maker | Bore or thread-mill; check fitting maker |
| 4 BSP | 112.309 mm pitch | 113.030 mm4.4500 in | 111.551 mm4.3918 in | 110.073 mm4.3336 in | 25.4 mm11.0 turns | Bore or thread-mill; check fitting maker | Bore or thread-mill; check fitting maker |
Major, pitch and minor diameters are the ISO 228-1 / ISO 7-1 basic sizes at the gauge plane (Whitworth form, h = 0.640327 P). Gauge length is the ISO 7-1 basic distance from the end of the male thread to the gauge plane, roughly the hand-tight engagement of R into Rc. Tap drills are typical values for G (parallel) and Rc (taper, without reaming) taps.
How to Tell NPT from BSP
- Count the threads per inch with a pitch gauge or over a known length. Most sizes differ: 27 vs 28, 18 vs 19, 11.5 vs 11, 8 vs 11.
- For 1/2 and 3/4 in, check the angle. Both are 14 TPI. A 60° centre gauge or screw-pitch gauge sits flat in an NPT thread and rocks in a 55° BSP thread. BSP crests and roots are rounded; NPT crests and roots are flat (truncated).
- Measure the OD. NPT male threads are slightly larger: 1/2 NPT is about 21.3 mm at the large end, 1/2 BSP is 20.96 mm.
- Look for taper. A thread that gets visibly smaller towards the end is NPT or BSPT (both 1:16 on diameter). A constant diameter is BSPP (G). Many BSPP fittings also have a sealing face, O-ring groove or bonded washer.
- Check markings and origin. “NPT”, “NPTF” or no marking on US equipment usually means NPT; “G”, “R”, “Rc”, “Rp” or “BSP” on European, UK, Asian or Australian equipment means BSP.
NPT vs BSP side by side
| Size | NPT TPI | BSP TPI | NPT male OD | BSP major dia | Tell-apart |
|---|---|---|---|---|---|
| 1/16 | 27 | 28 | 7.94 mm | 7.72 mm | Very close: count carefully |
| 1/8 | 27 | 28 | 10.29 mm | 9.73 mm | Very close: count carefully |
| 1/4 | 18 | 19 | 13.72 mm | 13.16 mm | Very close: count carefully |
| 3/8 | 18 | 19 | 17.14 mm | 16.66 mm | Very close: count carefully |
| 1/2 | 14 | 14 | 21.34 mm | 20.95 mm | Same TPI: check angle |
| 3/4 | 14 | 14 | 26.67 mm | 26.44 mm | Same TPI: check angle |
| 1 | 11.5 | 11 | 33.40 mm | 33.25 mm | Very close: count carefully |
| 1-1/4 | 11.5 | 11 | 42.16 mm | 41.91 mm | Very close: count carefully |
| 1-1/2 | 11.5 | 11 | 48.26 mm | 47.80 mm | Very close: count carefully |
| 2 | 11.5 | 11 | 60.32 mm | 59.61 mm | Very close: count carefully |
| 2-1/2 | 8 | 11 | 73.02 mm | 75.18 mm | Clearly different |
| 3 | 8 | 11 | 88.90 mm | 87.88 mm | Clearly different |
| 4 | 8 | 11 | 114.30 mm | 113.03 mm | Clearly different |
Which Pipe Threads Seal?
| Thread | Form | Seals on | What to use |
|---|---|---|---|
| NPT | Tapered 1:16, 60° | The threads (flank wedge) | PTFE tape or pipe-thread sealant on the male thread; leave the first thread bare. |
| NPTF | Tapered 1:16, 60° (Dryseal) | The threads (crest and root crush) | Designed to seal without sealant; sealant is still often used for reliability. |
| BSPT R / Rc | Tapered 1:16, 55° | The threads | PTFE tape or thread sealant. R (male) seals into Rc (taper female) or Rp (parallel female). |
| BSPP G | Parallel, 55° | A washer, O-ring or face, not the threads | Bonded (Dowty) washer, O-ring, gasket or cutting face (e.g. ISO 1179 ports). Tape on the threads does not make it seal. |
- Do not over-tighten tapered threads. Hand-tight plus about 2 to 3 turns with a wrench is typical. Extra torque splits cast fittings and aluminium ports.
- Do not mix tape and O-rings. On BSPP the tape can stop the face or washer from seating.
- Use a taper reamer before tapping NPT in tough materials; it reduces tap torque and gives a cleaner taper.
How to Use This Chart
Worked example: tapping a 1/2 NPT port in aluminium. Drill 23/32 in (0.7188 in, 18.26 mm), or 11/16 in if you will taper-ream first. Tap until a fitting goes in hand-tight about 4.5 turns, matching L1 = 0.320 in at 14 TPI. Full effective thread length is 0.534 in, so the port needs at least that much tapped depth plus room for the tap's chamfer.
- Pipe size is not thread size. A 1/2 in pipe thread is about 0.84 in (21 mm) across; the name comes from the old nominal bore of the pipe.
- Gauge, do not measure, a tapered thread. Use an L1 plug or ring gauge, or count hand-tight turns with a known-good fitting.
- The Taper Calculator handles the 1:16 (3° 34′ included) taper if you single-point pipe threads on a lathe.
Frequently Asked Questions
Use a 7/16 in drill (0.4375 in, 11.11 mm) when tapping without a taper reamer. If you taper-ream the hole first, drill 27/64 in (0.4219 in).
Not reliably. NPT has a 60 degree thread angle and BSP has 55 degrees, and most sizes have a different pitch. In 1/2 and 3/4 in both have 14 TPI, so they will start and may turn a few times, but the flanks do not match and the joint is likely to leak. Use the correct adapter.
Count the threads per inch and measure the outside diameter. 1/8 in is 27 TPI for NPT but 28 TPI for BSP, 1/4 and 3/8 in are 18 vs 19 TPI, and 1 to 2 in are 11.5 vs 11 TPI. For 1/2 and 3/4 in, where both are 14 TPI, check the thread angle with a 55 or 60 degree gauge, or measure: a 1/2 NPT male is about 21.3 mm OD at the large end, a 1/2 BSP male is about 20.9 mm.
No. BSPP (G) parallel threads do not seal on the threads. They seal with a bonded washer, an O-ring, a gasket or a sealing face on the fitting. Tapered threads (NPT and BSPT R/Rc) are the ones that seal in the threads, with PTFE tape or a thread sealant.
G is a BSP parallel thread to ISO 228-1, used where the seal is made by a washer or O-ring. R is a BSP tapered male thread to ISO 7-1, Rc is a tapered female thread and Rp is a parallel female thread made to mate with R for a sealing joint.
Related Calculators and Charts
NPT: ASME B1.20.1 (pipe ODs, hand-tight engagement L1, effective thread length L2 from the standard's formula). BSP: ISO 228-1 (parallel, G) and ISO 7-1 (taper R/Rc/Rp) basic sizes and gauge lengths. Tap drills are typical shop values from common tap-drill charts, not part of the standards; check critical values against the standard, the tap maker or the fitting manufacturer.