NPT pipe thread chart
National Pipe Taper threads from 1/16 to 2 inch: threads per inch, the actual pipe outside diameter, and the tap drill to use without reaming. NPT tapers 1 in 16 on diameter (3/4 in per foot).
| Nominal size | TPI | Pipe OD (in) | Tap drill | Decimal (in) |
|---|---|---|---|---|
| 1/16-27 NPT | 27 | 0.313 | D | 0.2460 |
| 1/8-27 NPT | 27 | 0.405 | Q | 0.3320 |
| 1/4-18 NPT | 18 | 0.540 | 7/16 | 0.4375 |
| 3/8-18 NPT | 18 | 0.675 | 9/16 | 0.5625 |
| 1/2-14 NPT | 14 | 0.840 | 45/64 | 0.7031 |
| 3/4-14 NPT | 14 | 1.050 | 29/32 | 0.9062 |
| 1-11.5 NPT | 11.5 | 1.315 | 1-9/64 | 1.1406 |
| 1-1/4-11.5 NPT | 11.5 | 1.660 | 1-31/64 | 1.4844 |
| 1-1/2-11.5 NPT | 11.5 | 1.900 | 1-47/64 | 1.7344 |
| 2-11.5 NPT | 11.5 | 2.375 | 2-13/64 | 2.2031 |
| 2-1/2-8 NPT | 8 | 2.875 | 2-5/8 | 2.6250 |
| 3-8 NPT | 8 | 3.500 | 3-1/4 | 3.2500 |
| 3-1/2-8 NPT | 8 | 4.000 | 3-3/4 | 3.7500 |
| 4-8 NPT | 8 | 4.500 | 4-1/4 | 4.2500 |
Common questions
- What tap drill for 1/4 NPT?
- 7/16 in (0.4375 in) without reaming.
- Why is a 1/2 NPT fitting not 1/2 inch across?
- Pipe sizes are nominal. A 1/2 NPT pipe is 0.840 in in outside diameter.
- What is the NPT taper?
- 1 in 16 on diameter, which is 3/4 in per foot, or 1°47′ per side.
Calculators that use this data
Tap Drill & Thread % CalculatorPercent of thread for any drill (cut and form taps), 75 / 65 / 50% tap drills, UNC / UNF / metric chart with thread limits, every thread to 4 in and M64, pipe taps and STI / Helicoil.Drilling CalculationsDrilling RPM, feed per revolution, point allowance and cutting time.
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