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NPT size lookup
Pick a nominal size to get its outside diameter, threads per inch, pitch, tap drill, and thread length. Nominal size is not a physical dimension — a 1/2″ pipe never measures 1/2″ across. Measure the OD and match it to the chart.
REFERENCE CHART · CONSTRUCTION · PLUMBING
NPT Pipe Thread Chart: OD, TPI & Tap Drill Sizes
Every NPT size from 1/16″ to 4″ nominal — outside diameter, threads per inch, pitch, tap drill, and thread length — cross-checked against ASME B1.20.1 consensus sources. The lookup defaults to 1/2″ NPT: OD 0.840″, 14 TPI, tap drill 23/32″. Print-friendly: use your browser’s print to get the chart on one page.
The full NPT chart (ASME B1.20.1)
All dimensions in inches. Tap drill is the hole to drill before tapping the female (internal) thread. Thread length and makeup are approximate — the taper means engagement varies with how hard the joint is wrenched.
| Nominal size | OD (in) | TPI | Pitch (in) | Tap drill | Thread length (approx) | Threads to cut | Makeup, hand + wrench (in) |
|---|---|---|---|---|---|---|---|
| 1/16″ | 0.313 | 27 | 0.0370 | C″ (0.242″) | — | — | — |
| 1/8″ | 0.405 | 27 | 0.0370 | Q″ (0.332″) | 3/8″ | 10 | 1/4″ |
| 1/4″ | 0.540 | 18 | 0.0556 | 7/16″ (0.438″) | 5/8″ | 11 | 3/8″ |
| 3/8″ | 0.675 | 18 | 0.0556 | 37/64″ (0.578″) | 5/8″ | 11 | 3/8″ |
| 1/2″ | 0.840 | 14 | 0.0714 | 23/32″ (0.719″) | 3/4″ | 10 | 7/16″ |
| 3/4″ | 1.050 | 14 | 0.0714 | 59/64″ (0.922″) | 3/4″ | 10 | 1/2″ |
| 1″ | 1.315 | 11-1/2 | 0.0870 | 1-5/32″ (1.156″) | 7/8″ | 10 | 9/16″ |
| 1-1/4″ | 1.660 | 11-1/2 | 0.0870 | 1-1/2″ (1.500″) | 1″ | 11 | 9/16″ |
| 1-1/2″ | 1.900 | 11-1/2 | 0.0870 | 1-47/64″ (1.734″) | 1″ | 11 | 9/16″ |
| 2″ | 2.375 | 11-1/2 | 0.0870 | 2-7/32″ (2.219″) | 1″ | 11 | 5/8″ |
| 2-1/2″ | 2.875 | 8 | 0.1250 | 2-5/8″ (2.625″) | 1-1/2″ | 12 | 7/8″ |
| 3″ | 3.500 | 8 | 0.1250 | 3-1/4″ (3.250″) | 1-1/2″ | 12 | 1″ |
| 3-1/2″ | 4.000 | 8 | 0.1250 | 3-3/4″ (3.750″) | 1-5/8″ | 13 | 1-1/16″ |
| 4″ | 4.500 | 8 | 0.1250 | 4-1/4″ (4.250″) | 1-5/8″ | 13 | 1-1/16″ |
On the 2″ tap drill: one popular source prints 2-7/64″ — it is the outlier. The ASME B1.20.1 consensus (and every other major reference) is 2-7/32″ (2.219″). The 1/16″ tap drill C (0.242″) is the industry-common value; makers vary slightly on the smallest size. On the 1/8″ tap drill: most modern references print Q (0.332″), while Engineering ToolBox lists R — Q is the tighter consensus.
How to identify a fitting: measure the OD
Pipe sizes do not refer to any physical dimension. A 3/4″ NPT fitting measures 1.050″ across the threads — not 3/4″. To identify an unknown fitting: 1. measure the outside diameter of the male threads with calipers; 2. count threads per inch (a thread gauge, or count crests across one inch); 3. match both numbers to the chart — more than one size shares a TPI, so OD alone isn’t enough. A 1/2″ NPT male thread reads about 0.840″ with 14 TPI.
The taper: 1:16, and why NPT seals
NPT threads taper at 1 in 16 on diameter (3/4″ per foot) — the male and female threads wedge together as they’re wrenched, which is what makes the joint pressure-tight. The thread form is a 60° V with flat crests and roots. The taper alone doesn’t guarantee a seal: NPT needs PTFE tape or pipe thread sealant ("dope") to fill the spiral leak path. (NPTF/Dryseal threads are the interference-fit variant that seals without dope.)
Output · formula · meaning
| Output | Formula | What it means |
|---|---|---|
| Outside diameter | ASME B1.20.1 table value | 0.840″ for 1/2″ — the measurement you match with calipers. |
| Threads per inch | ASME B1.20.1 table value | 14 TPI for 1/2″ — needed alongside OD because TPI repeats across sizes. |
| Pitch | 1 ÷ TPI | 0.0714″ for 14 TPI — the distance from one crest to the next. |
| Tap drill | ASME consensus drill size | 23/32″ (0.719″) for 1/2″ — drill this hole before tapping the female thread. |
| Thread length / makeup | Approximate, from the standard | ~3/4″ of thread, ~7/16″ makeup hand + wrench for 1/2″ — approximate because the taper varies engagement. |
Who uses this
- The DIY plumber: measures a mystery fitting at 1.050″ OD, counts 14 TPI, and confirms it’s 3/4″ NPT before the hardware-store run.
- The machinist tapping a hole: looks up the tap drill — 23/32″ (0.719″) for 1/2″ NPT — and drills before running the taper tap.
- The homeowner swapping a water-heater fitting: learns the old 3/4″ fitting needs PTFE tape or dope, and that a BSPT fitting from an import kit won’t mate with it.
- The gas fitter: double-checks the 2″ tap drill is 2-7/32″, not the outlier 2-7/64″ that one popular chart prints.
Accuracy & limitations
Dimensions follow the ASME B1.20.1 consensus as published by independent references, cross-checked 2026-10-24 — OD and TPI agree across sources. Thread lengths and makeup figures are approximate: the 1:16 taper means actual engagement depends on wrenching, and tolerances vary by manufacturer. The 1/16″ size’s tap drill is the industry-common C (0.242″); small-size practice varies. This chart is a reference for identification and shop use, not an engineering drawing — verify critical dimensions against the standard itself.
Cite this calculatorBright Side Kit. “NPT Pipe Thread Chart: OD, TPI, Tap Drill Sizes (2026).” brightoolbox.com, 2026.
Bright Side Kit. "NPT Pipe Thread Chart: OD, TPI, Tap Drill Sizes (2026)." https://brightoolbox.com/npt-pipe-thread-chart/ Accessed October 24, 2026.Data sources- Engineering ToolBox, “ANSI B1.20.1 - NPT - American National Standard Taper Pipe Threads” — OD, TPI, thread lengths, tap drills, taper and thread-angle characteristics.
- Newman Tools Inc., “Pipe Taps: Inch Sizes” — nominal sizes, TPI, and outside diameters for NPT and NPS taps.
Frequently asked questions
NPT vs NPS: what's the difference?
NPT is tapered; NPS is straight. Both share the 60° thread form and the same TPI for a given size, but NPT wedges tighter as it’s wrenched (1:16 taper) while NPS runs parallel. They are not interchangeable for pressure-tight joints — an NPS male in an NPT female won’t seal properly.
Can I mix NPT with BSPT (British) threads?
No. BSPT uses a 55° thread angle (vs NPT’s 60°) with different crest/root shapes and usually metric-pitch spacing. They may start threading by hand and feel close — then leak, gall, or crack under pressure. If a fitting came from an import kit, check the standard before forcing it.
What does the 1:16 taper actually mean?
The diameter shrinks 1 inch for every 16 inches of length (3/4″ per foot), or about 1°47′ from the center axis. That wedge is the sealing mechanism: as you wrench the joint, the flanks crush together and the spiral leak path closes — with tape or dope filling what’s left.
Why doesn't a 1/2" pipe measure 1/2" anywhere?
Nominal size is a trade name, not a dimension — it traces back to old iron-pipe inside diameters. A 1/2″ NPT thread actually measures 0.840″ OD. Always identify fittings by measured OD + TPI against the chart, never by what the size "sounds like."
Do I need PTFE tape, pipe dope, or both?
Use one of them on every NPT joint — tape (PTFE/Teflon tape) or thread sealant compound ("dope"). The taper wedges the threads, but a spiral leak path remains; sealant fills it. Wrap tape clockwise (viewed from the fitting’s end) so tightening doesn’t unwrap it. Don’t use both tape and dope on the same joint unless a manufacturer says to.
How do I measure threads per inch?
Use a thread pitch gauge, or lay a ruler across the threads and count crests in one inch — 14 crests = 14 TPI. On small fittings, count crests across 1/2″ and double it. Combine the TPI with a caliper OD reading to pin the size from the chart.
Which tap drill is right for 2" NPT — I've seen two values?
2-7/32″ (2.219″) is the ASME B1.20.1 consensus. One popular online source prints 2-7/64″ — a 1/32″ difference that makes the tap cut oversize threads. When references disagree on pipe threads, trust the consensus of multiple sources over any single chart.
1/8" NPT tap drill: Q or R? I've seen both.
Q (0.332″) is the tighter consensus across modern references — but Engineering ToolBox’s chart lists R, and both will tap a working 1/8″-27 thread. The two drills differ by only 0.007″, well within what a taper tap tolerates. When charts disagree by a hair like this, pick the size the majority of sources print and test-fit the joint.