NPT vs NPS: Tapered vs Straight American Pipe Threads Explained

The key difference: NPT has a 1 degree 47 minute taper per side. Full breakdown below.

NPT (National Pipe Taper) and NPS (National Pipe Straight) share the same 60-degree thread angle and threads-per-inch at each nominal size. The difference is that NPT threads are tapered (they get tighter as you thread in) while NPS threads are straight (parallel). NPT seals on the thread taper. NPS cannot seal on threads alone and requires an O-ring, gasket, or bonded washer to make a seal. They are NOT interchangeable even though the threads will partially engage.

Key Differences

NPT has a 1 degree 47 minute taper per side.

NPS has no taper, the threads are parallel (straight).

NPT seals by wedging tapered threads together, deforming the thread crests.

NPS threads only hold the fitting in place while an O-ring or gasket provides the seal.

NPT requires thread sealant.

NPS requires an O-ring or washer.

NPT connections degrade with each assembly/disassembly.

NPS connections are reusable because the seal element (O-ring) can be replaced.

When to Choose NPT

Fixed plumbing installations

Pneumatic systems

Low-pressure hydraulics

When the port is already tapped NPT

When cost and availability are priorities.

When to Choose NPS

Hydraulic systems where connections need to be assembled and disassembled repeatedly

When a reliable, reusable seal is needed

NPS with an O-ring (SAE ORB / SAE J1926) is the standard for modern hydraulic pump and valve ports

When overtightening risk exists (NPS does not generate the hoop stress that NPT does in cast housings).

Practical Notes

NPS threads at a given nominal size have the same OD and TPI as NPT at the large end of the taper. This means an NPT male will start threading into an NPS female (or vice versa) but will cross-thread or bind after a few turns. This partial engagement is what makes them dangerous to confuse. If threads engage easily for 2-3 turns then suddenly bind, you may be cross-threading NPT into NPS.

Common Mistake

Trying to seal an NPS connection without an O-ring or gasket. NPS threads are straight and cannot wedge tight like NPT. No amount of Teflon tape will make straight threads seal by thread interference. If the connection leaks and the threads are straight, you need the correct O-ring or bonded washer for that port.