NPT Thread Size Calculator

NPT (National Pipe Taper) is the standard tapered pipe thread used in the United States for joining pipes and fittings. The "taper" means the thread diameter decreases along the length of the thread, creating a wedging action that forms a pressure-tight seal when properly assembled.

Defined by ASME B1.20.1 standard, NPT threads are used extensively in plumbing, hydraulics, pneumatics, and industrial piping systems. The tapered design allows the threads to seal by interference fit, though thread sealant (pipe dope or Teflon tape) is typically applied to ensure a leak-tight connection.

The defining characteristic of NPT threads is the 1:16 taper rate. This means:

• The diameter changes by 1 inch for every 16 inches of thread length
• Equivalent to 0.0625 inches per inch, or 3/4 inch per foot
• Thread angle is 60 degrees (measured on the thread flanks)
• Creates increasing interference as the fitting is tightened

This taper design creates a metal-to-metal seal when hand-tightened 4 turns, followed by 2 additional wrench-tightened turns. The tapered interface distributes sealing pressure across multiple threads rather than relying on a single sealing surface.

Identifying NPT thread size requires measuring both the outer diameter and counting threads per inch:

Important: The nominal size (e.g., "1/2 NPT") does NOT match the major diameter. For example, 1/2" NPT has a major diameter of 0.840", which matches 1/2" Schedule 40 pipe OD. Always use the major diameter and TPI to positively identify thread size.

While NPT threads are tapered, NPS (National Pipe Straight) threads are parallel. Understanding the difference is critical for proper fitting selection:

Critical Warning: Never mate NPT (tapered) with NPS (straight) threads. This combination will leak and can damage threads. Always verify both the male and female connections are the same thread type before assembly.