NPT Thread Sizes: Pipe Dimensions, TPI & Identification Chart
NPT Thread Sizes
| Nominal Size | Threads/Inch | OD |
|---|---|---|
| 1/8" | 27 TPI | 0.405" |
| 1/4" | 18 TPI | 0.540" |
| 3/8" | 18 TPI | 0.675" |
| 1/2" | 14 TPI | 0.840" |
| 3/4" | 14 TPI | 1.050" |
| 1" | 11.5 TPI | 1.315" |
| 1-1/4" | 11.5 TPI | 1.660" |
| 1-1/2" | 11.5 TPI | 1.900" |
| 2" | 11.5 TPI | 2.375" |
NPT threads seal by wedging tapered threads together, deforming the crests and roots to create a seal. This means NPT connections are not reliably reusable. Every time you disassemble and reassemble, the seal quality degrades. They almost always need thread sealant (tape or paste). NPT is everywhere in plumbing and low-pressure hydraulics.
When to Use NPT
Plumbing, pneumatics, and low-pressure hydraulics.
NPT is the default thread in North American pipe systems.
Use it when the port is already tapped NPT and there is no adapter option.
Avoid NPT in high-vibration or high-pressure hydraulic circuits if you have a choice.
NPT Limitations
Not reliably reusable. Each assembly/disassembly degrades the seal.
Requires thread sealant every time.
Tapered threads generate hoop stress in the port, which can crack cast housings if overtightened.
How to Identify NPT Fittings
Threads are tapered, they get tighter as you screw them in. Thread pitch is coarser than JIC. Measure with a thread gauge to distinguish from BSPT (55-degree vs 60-degree thread angle).
NPT Measurement Tips
NPT sizes do not match the actual measured diameter of the thread. A 1/2 inch NPT fitting measures approximately 0.840 inches across the outside of the threads at the large end. This is because NPT sizes are based on the nominal inside diameter of the pipe, not the thread OD. Always use a thread pitch gauge and reference chart rather than measuring with calipers.
What NPT Is Confused With
NPTF (National Pipe Taper Fuel) is a dryseal variant of NPT designed to seal without thread sealant tape or compound. NPTF threads have tighter manufacturing tolerances that create a metal-to-metal interference fit. NPTF will mate with NPT threads but the seal quality depends on which side has the tighter tolerance. For hydraulic applications above 1000 psi, NPTF with sealant is preferred over standard NPT.
Data sourced from ANSI/ASME B1.20.1. Thread dimensions are nominal values. Always verify with a thread identification gauge before making connections.