NPT stands for National Pipe Taper. NPT fittings are used in many different applications, such as:
- Fuel Systems
- Pumps
- Regulators
- Filters
- Aftermarket Brake Calipers
- Oil & Coolant Passages in
- Intake Manifolds
- Cylinder Heads
- Engine Blocks
- Gauges
- Etc.
The threads on an NPT fitting are tapered. As the fitting is tightened, the threads compress. These "smashed" threads seal the connection. Because of this, the use of Thread Sealant is highly recommended.
How are they measured?
NPT fittings are listed by the "standard" inner diameter (I.D.) of the fitting. (We put "standard" in quotes because the I.D. of the fitting will vary, depending on the manufacturer.)
The outer diameter (O.D.) of the threads is usually about 1/4 (0.250) in. larger than the stated I.D. size. This measurement will also vary, depending on where you take the measurement.
Despite the variation, you can still measure an NPT fitting. For the male end, measure across the bottom O.D. of the threads. For the female end, measure across the top I.D. of the threads.
Your measurement may be different than what is listed in the table. However, it should be close.
| NPT Size | Male O.D. | Female I.D. | Threads per Inch | ||
| Fraction (in.) | Decimal (in.) | Fraction (in.) | Decimal (in.) | ||
| 1/8 in. | 13/32 | 0.410 | 3/8 | 0.380 | 27 |
| 1/4 in. | 17/32 | 0.540 | 1/2 | 0.490 | 18 |
| 3/8 in. | 11/16 | 0.680 | 5/8 | 0.630 | 18 |
| 1/2 in. | 27/32 | 0.840 | 25/32 | 0.770 | 14 |
| 3/4 in. | 1 1/16 | 1.050 | 1 | 0.980 | 14 |
| 1 in. | 1 5/16 | 1.320 | 1 1/4 | 1.240 | 11.5 |