Reference & Charts
NPT Pipe Thread Chart
The NPT (National Pipe Taper) reference: threads per inch, actual pipe outside diameter, and the typical tap drill for each nominal size from 1/8" to 4". Note that NPT sizes are named by nominal bore, not the measured thread diameter.
9 entries · Last verified: 2026-07-10
NPT (National Pipe Taper) threads seal on the thread flanks: the 1°47' taper (3/4" per foot) wedges the threads together as they tighten, usually with a sealant or PTFE tape. Sizes are named by nominal bore, so the measured thread diameter is always larger than the name — 1/2" NPT actually measures about 0.840" across. Threads per inch drop as size grows (27 at 1/8", down to 11.5 at 1" and above).
| NPT Size | TPI | Pipe OD(in) | Tap drill |
|---|---|---|---|
| 1/8" NPT | 27 | 0.405 in | R (0.339") |
| 1/4" NPT | 18 | 0.54 in | 7/16" (0.4375") |
| 3/8" NPT | 18 | 0.675 in | 37/64" (0.578") |
| 1/2" NPT | 14 | 0.84 in | 23/32" (0.719") |
| 3/4" NPT | 14 | 1.05 in | 59/64" (0.922") |
| 1" NPT | 11.5 | 1.315 in | 1-5/32" (1.156") |
| 1-1/4" NPT | 11.5 | 1.66 in | 1-1/2" (1.500") |
| 1-1/2" NPT | 11.5 | 1.9 in | 1-47/64" (1.734") |
| 2" NPT | 11.5 | 2.375 in | 2-7/32" (2.219") |
Select a npt size for its full spec sheet.
Frequently asked
- Why is a 1/2" NPT fitting bigger than 1/2 inch?
- NPT sizes are nominal — named after the approximate bore of the pipe, not the thread's measured diameter. A 1/2" NPT thread measures about 0.840" on the outside. Always go by the size stamped on the fitting rather than measuring, or measure the OD and match it to the chart.
- Do NPT threads need tape or sealant?
- Yes — NPT seals on the thread flanks, and PTFE tape or a pipe-thread sealant fills the small spiral clearance to make it leak-tight. NPTF (Dryseal) is a variant designed to seal without sealant, but general NPT should be sealed.
- Are these tap drill sizes exact?
- They're the typical published tap drills for cutting NPT threads. The exact drill depends on the specific tap and how much thread engagement you want before the taper bottoms out — follow the tap manufacturer's recommendation for critical work.
Sources
- ANSI/ASME B1.20.1 — pipe threads, general purpose (NPT): TPI and pipe OD
- Typical NPT tap-drill charts (machining practice)