Npt Pipe Chart
A “1/2-inch” pipe thread measures about 0.84 inches across — which is why NPT sizes confuse almost everyone. This chart lists every common NPT size with its real outside diameter, threads per inch and tap drill, and the identifier below matches a thread you’ve measured to NPT or BSP.
Pipe thread identifier
Pipe Thread Reference Pack
Printable NPT thread chart, NPT vs BSP comparison chart, tap drill card and a thread identification worksheet.
- NPT chart (PDF/XLSX)
- NPT vs BSP (PDF)
- Tap drill card (PDF)
- Thread ID worksheet (PDF/DOCX)
Formats: PDF, XLSX, DOCX. Instant download after payment (link valid 72 hours, up to 5 downloads). AI-assisted: the templates were drafted with AI help and reviewed and laid out by Kedop.
$3.00 USD, one-time
Secure card checkout by Stripe. Full refund within 7 days — see the refund policy and license.
NPT thread chart
| Nominal size | Threads per inch | Outside diameter (in) | OD (mm) | Tap drill | Metric DN |
|---|---|---|---|---|---|
| 1/16 in | 27 | 0.313 | 7.9 | 1/4 | — |
| 1/8 in | 27 | 0.405 | 10.3 | R (0.339) | DN 6 |
| 1/4 in | 18 | 0.540 | 13.7 | 7/16 | DN 8 |
| 3/8 in | 18 | 0.675 | 17.1 | 37/64 | DN 10 |
| 1/2 in | 14 | 0.840 | 21.3 | 23/32 | DN 15 |
| 3/4 in | 14 | 1.050 | 26.7 | 59/64 | DN 20 |
| 1 in | 11.5 | 1.315 | 33.4 | 1-5/32 | DN 25 |
| 1-1/4 in | 11.5 | 1.660 | 42.2 | 1-1/2 | DN 32 |
| 1-1/2 in | 11.5 | 1.900 | 48.3 | 1-47/64 | DN 40 |
| 2 in | 11.5 | 2.375 | 60.3 | 2-7/32 | DN 50 |
| 2-1/2 in | 8 | 2.875 | 73.0 | 2-5/8 | DN 65 |
| 3 in | 8 | 3.500 | 88.9 | 3-1/4 | DN 80 |
| 4 in | 8 | 4.500 | 114.3 | 4-1/4 | DN 100 |
The outside diameter is the pipe’s actual OD, which is larger than the nominal size for small pipes — a legacy of old pipe wall thicknesses. Tap drill sizes are the common recommendations for tapping without a taper reamer; check your tap maker’s chart.
The taper
NPT threads taper at 1 in 16 on the diameter — the diameter changes by 1/16 inch for every inch of length, or 3/4 inch per foot, a half-angle of about 1° 47′. As a male and female thread tighten, the flanks wedge together; with a sealant such as PTFE tape or pipe dope, that wedge forms the seal. Thread dimensions and gauging for NPT are defined in ASME B1.20.1.
NPT vs BSP
| Size | NPT TPI | BSP TPI | NPT OD (in) | BSP OD (in) |
|---|---|---|---|---|
| 1/8 in | 27 | 28 | 0.405 | 0.383 |
| 1/4 in | 18 | 19 | 0.540 | 0.518 |
| 3/8 in | 18 | 19 | 0.675 | 0.656 |
| 1/2 in | 14 | 14 | 0.840 | 0.825 |
| 3/4 in | 14 | 14 | 1.050 | 1.041 |
| 1 in | 11.5 | 11 | 1.315 | 1.309 |
| 1-1/4 in | 11.5 | 11 | 1.660 | 1.650 |
| 1-1/2 in | 11.5 | 11 | 1.900 | 1.882 |
| 2 in | 11.5 | 11 | 2.375 | 2.347 |
British Standard Pipe threads (BSPT tapered, BSPP parallel) use a 55° angle and rounded crests; NPT uses 60° with flat crests. At 1/2 and 3/4 inch both have 14 TPI and nearly the same diameter, so they can screw together a few turns — but they won’t seal reliably. Use an adapter.
Worked example
A plumber has an unmarked male fitting. Across the thread crests it measures 0.84 in (21.3 mm), and a thread gauge shows 14 TPI. The identifier matches 1/2 in NPT (OD 0.840 in, 14 TPI, DN 15), with 1/2 in BSP (0.825 in, 14 TPI) as the close alternative. A 60° thread gauge fits the profile snugly, confirming NPT. For a female thread in a new part, the chart gives a 23/32 in tap drill.
How to measure a pipe thread
- Clean the thread.
- For a male thread, measure the outside diameter across the crests with calipers, a few threads from the end.
- For a female thread, measure the inside diameter at the opening and compare with the tap drill sizes.
- Count threads per inch with a thread pitch gauge, or count crests along one inch.
- Check the thread angle with a 60° or 55° gauge if unsure.
- Remember the taper: diameters change along the thread, so measure near the end.
NPT, NPTF, NPS and BSPP
| Type | Taper | Seals by |
|---|---|---|
| NPT | Tapered 1:16 | Thread wedging plus sealant |
| NPTF (dryseal) | Tapered 1:16, controlled crests and roots | Thread interference, often without sealant |
| NPS / NPSM | Straight | A gasket or O-ring, not the threads |
| BSPT (R / Rc) | Tapered 1:16 | Threads plus sealant (55° angle) |
| BSPP (G) | Straight | Bonded washer or O-ring (55° angle) |
Straight threads and tapered threads of the same size can sometimes be screwed together, but they won’t seal unless the joint is designed for it.
Sealing NPT threads
- Use PTFE tape or thread sealant rated for the fluid and pressure (gas, water, oil).
- Wrap tape clockwise when looking at the end of the male thread, so tightening doesn’t unwind it.
- Keep sealant off the first thread to avoid contaminating the system.
- Hand-tighten, then tighten with a wrench as the fitting maker recommends — over-tightening can crack female fittings.
- NPTF (dryseal) threads are designed to seal without sealant in some applications.
Privacy
All calculations run in your browser.
Frequently asked questions
What does NPT stand for?
National Pipe Taper — the US standard tapered pipe thread.
What is the OD of 1/2 inch NPT?
0.840 inches (21.3 mm).
How many threads per inch is 3/4 NPT?
14 threads per inch.
What size drill for 1/4 NPT?
7/16 inch is the common recommendation.
Are NPT and BSP compatible?
No — they differ in angle and, at most sizes, pitch. Use an adapter.
What is the NPT taper?
1 in 16 on the diameter (3/4 inch per foot).
Why is 1/2 inch NPT not 1/2 inch across?
Nominal sizes come from historic pipe bore sizes; the thread is cut on the pipe’s outside diameter of 0.84 inch.
Can I use PTFE tape on gas lines?
Use tape or sealant specifically rated for gas and follow local codes.