Pipe Thread Size Chart (NPT)
National Pipe Thread (NPT) size chart showing nominal pipe size, outside diameter, threads per inch, and tap drill size for common pipe sizes.
NPT vs BSP
NPT (National Pipe Thread) is the North American standard with a 60° thread angle and 1:16 taper ratio. BSP (British Standard Pipe) is used internationally with a 55° thread angle. The two standards are not interchangeable.
| NPT Size | Outside Diameter (in) | Threads per Inch | Tap Drill Size |
|---|---|---|---|
| 1/16" | 0.3125 | 27 | C (0.242) |
| 1/8" | 0.4050 | 27 | R (0.339) |
| 1/4" | 0.5400 | 18 | 7/16" (0.4375) |
| 3/8" | 0.6750 | 18 | 37/64" (0.5781) |
| 1/2" | 0.8400 | 14 | 23/32" (0.7188) |
| 3/4" | 1.0500 | 14 | 59/64" (0.9219) |
| 1" | 1.3150 | 11.5 | 1-5/32" (1.1563) |
| 1-1/4" | 1.6600 | 11.5 | 1-1/2" (1.5000) |
| 1-1/2" | 1.9000 | 11.5 | 1-23/32" (1.7188) |
| 2" | 2.3750 | 11.5 | 2-3/16" (2.1875) |
| 2-1/2" | 2.8750 | 8 | 2-5/8" (2.6250) |
| 3" | 3.5000 | 8 | 3-1/4" (3.2500) |
| 4" | 4.5000 | 8 | 4-1/4" (4.2500) |
How to Read This Chart
Find the NPT pipe size in the left column. The outside diameter (OD) is the measurement across the outside of the pipe. Threads per inch (TPI) indicates thread pitch. The tap drill size is the hole diameter needed before cutting internal NPT threads.
Frequently Asked Questions
What does NPT stand for?
NPT stands for National Pipe Thread (also called National Pipe Taper). It is the U.S. standard for tapered pipe threads used in plumbing and industrial piping. The taper is 1/16 inch per inch (3/4 inch per foot), which creates a seal as the threads are tightened.
What is the difference between NPT and BSP threads?
NPT (National Pipe Thread) is the North American standard with a 60-degree thread angle and 1:16 taper. BSP (British Standard Pipe) is the international standard with a 55-degree thread angle. NPT and BSP threads are not compatible and will cross-thread if forced together.
Why is a 1/2" NPT pipe not actually 1/2 inch?
NPT sizes are based on the nominal inside diameter (bore) of the pipe, not the outside diameter. A 1/2" NPT pipe has an outside diameter of 0.840 inches. The naming convention dates back to when pipe walls were thicker.
Do I need thread sealant on NPT fittings?
Yes. NPT threads are designed to be used with thread sealant (Teflon tape or pipe dope) to create a leak-proof seal. Apply 3-5 wraps of Teflon tape clockwise on the male threads before assembly.