NPT Thread Chart: Sizes, TPI and How to Identify

NPT (National Pipe Taper) is the tapered pipe thread used for threaded pipe connections across North America. It is defined in ASME B1.20.1. The thread has a 60° included angle and a 1-in-16 taper on diameter. Tightening creates interference between the mating thread flanks, and pressure-tight NPT joints require a suitable sealant. To identify one, match four things: nominal size, threads per inch (TPI), taper and thread angle.

The nominal size is a label, not a measurement. A 1/2 NPT connection has 14 TPI and is associated with pipe that has a 0.840-inch outside diameter. That value is the pipe OD, not a universal measurement across the tapered male thread. A caliper reading depends on where along the thread you measure and on the condition of the thread.

NPT Thread Chart: Sizes from 1/16 to 4 Inches

Find a size by its TPI and pipe OD. D is the outside diameter of the associated pipe, not a thread inspection diameter.

Source: TPI and D follow ASME B1.20.1. Metric values are conversions: pitch is 25.4 ÷ TPI rounded to 0.001 mm, and pipe OD is rounded to 0.1 mm.

Several sizes share the same TPI, so pitch alone cannot name a size. Both 1/4 and 3/8 NPT run at 18 TPI, and 1/2 and 3/4 NPT both run at 14. Add the pipe OD to tell them apart.

Reference Dimensions: E0 and L1

E0 is the basic pitch diameter at the small end of the male thread. L1 is the basic hand-tight engagement length between a male and a female thread. Both are reference values from ASME B1.20.1, and real parts carry tolerances.

E0 and L1 are not direct caliper acceptance limits. E0 is a pitch diameter, not the diameter across the crests, and thread acceptance requires the specified gages and inspection procedure.

How to Read an NPT Thread Designation

In 1/4–18 NPT, “1/4” is the nominal pipe size, “18” is the number of threads per inch, and “NPT” is the thread series. Listings often add male (external thread) or female (internal thread).

The “1/4” does not describe a 0.250-inch diameter. A 1/4 NPT thread is cut on pipe with a 0.540-inch outside diameter.

For general screw threads, see our thread pitch guide on converting pitch units. Converting TPI to millimetres changes the unit only; the thread standard stays the same.

Why an NPT Diameter Changes Along the Thread

NPT has a 60° included thread angle and a taper of 1 in 16 on diameter, which equals 3/4 inch per foot. The half-angle of the taper is about 1°47′. Every 16 mm of length along the axis changes the diameter by 1 mm. The ratio applies to diameter, not radius.

The small end of the male thread enters the fitting first, and the diameter grows toward the body. Calipers placed near the tip and near the body will read different values, and both readings can be correct.

Keep three diameters apart: the unthreaded pipe OD (D), the major diameter across the crests, and the pitch diameter (E0 at the small end). The pitch diameter defines the thread geometry. A pipe-OD chart cannot replace the specified thread dimensions or gaging.

NPT vs BSP, NPTF and NPSM

Several pipe threads look alike. The fastest way to separate them is thread angle and taper. NPT uses a 60° angle, while BSP threads use the 55° Whitworth form.

Pitch gives a second check. BSP sizes 1/8, 1/4 and 3/8 run at 28, 19 and 19 TPI, against 27, 18 and 18 for NPT. BSP sizes from 1 to 2 inches run at 11 TPI, against 11.5 for NPT. The 1/2 and 3/4 sizes share their TPI across both series, so the thread angle becomes the key difference. Matching TPI alone does not establish compatibility.

How to Check an Unknown NPT Thread

1. Record the part and the measurement location

Read any size marking and photograph the connection before cleaning it. On a male end, measure across undamaged full crests, away from the entry chamfer and incomplete threads, and note where the jaws sat.

2. Match the pitch

Hold an inch thread-pitch gauge across several full threads. The correct leaf meshes without visible gaps. For a manual measurement, count the spaces between crests, not the crests themselves. Measuring ten spaces and dividing by ten reduces reading error.

3. Confirm the thread series

Check the taper, the thread angle and any markings or equipment documents. A thread-form template or profile projector shows 60° against 55°. Then confirm the diameter and the applicable standard, because angle and TPI alone cannot name the series.

4. Use the specified gage for acceptance

Calipers and pitch gauges identify a thread; they do not accept one. Acceptance requires the thread gages and procedure in the applicable standard and drawing.

To measure the pipe itself, use the separate pipe-size measurement guide. It explains where to place the tools and how to tell pipe OD from bore diameter.

Worked Example: Checking a 14-TPI Candidate

Suppose a threaded steel nipple has an unthreaded section of about 0.840 inch across the outside, and its male thread matches a 14-TPI leaf. Two chart entries use 14 TPI: 1/2 NPT and 3/4 NPT. The 0.840-inch OD points to 1/2 NPT, since 3/4 NPT sits on 1.050-inch pipe.

The pitch is 25.4 ÷ 14 = 1.814 mm, rounded. Seven thread intervals span about 12.7 mm, which supports the pitch reading. Next, confirm the 60° angle and check the marking or drawing.

A diameter smaller than 0.840 inch near the tip is normal for a tapered thread and is no reason to reject the candidate.

Common NPT Thread Identification Mistakes

Three mistakes come up often when people identify NPT threads.

Ordering by measured diameter. A buyer measures 0.840 inch on a pipe and searches for a “0.840-inch fitting.” The correct search term is the nominal size, 1/2 NPT.

Mixing 11 TPI and 11.5 TPI. A 1-inch BSPT and a 1-inch NPT look almost identical. Over ten thread spaces, though, NPT measures 22.09 mm and BSPT measures 23.09 mm. That 1 mm gap is easy to catch with calipers and hard to see by eye.

Measuring on the chamfer. Jaws placed on the entry chamfer or a flattened crest give a low reading and point to the wrong size. Move to full, undamaged threads before measuring.

Next Step: Choose the Fitting

After the thread is identified, material, pressure rating and temperature limit come next. Those depend on the actual component and service conditions, not on the thread designation. Compare specifications in our stainless steel threaded pipe fittings range.

For an overview of elbows, tees, unions and reducers, see our guide to types of pipe fittings.

For other materials and connection types, browse our pipe fittings range.

FAQ

Can NPT and NPTF fittings be used together?

The threads can turn together, because NPTF (Dryseal, ASME B1.20.3) shares NPT’s angle, taper and TPI. That does not confirm the combination is suitable, and it does not carry over Dryseal sealing performance, which is designed for mating NPTF parts. If the component manufacturer permits the combination, follow its assembly and sealant instructions. Do not assume that a mixed joint provides Dryseal performance.

Do NPT threads need tape or sealant?

In most cases, yes. The taper wedges the threads together, but a small spiral leak path usually remains. PTFE tape or pipe-joint compound fills it. Choose a sealant rated for the fluid, temperature and material, and apply it according to the sealant and fitting instructions, usually to the male thread.

How tight should an NPT fitting be?

No single torque or turn count suits every NPT fitting. Assemble the connection with the system depressurized and follow the fitting manufacturer’s tightening instructions. Excess force can crack female ports, especially in cast or brass bodies, and stainless steel is prone to galling. Check for leakage afterward using the specified test procedure.

Can a worn NPT connection be identified reliably?

Only if enough original geometry remains or reliable documentation exists. Flattened crests, corrosion and earlier cross-threading make measurements misleading. If the result is uncertain, leave it unresolved instead of choosing the nearest chart entry.

Should I chase an unknown thread with an NPT tap to identify it?

No. Cutting alters the evidence and can ruin a connection made to another standard. Identify the original thread first, then decide whether repair is allowed for that component.

Technical Reference

ASME B1.20.1 — Pipe Threads, General Purpose, Inch covers dimensions and gaging for NPT and other pipe-thread series. ASME B1.20.3 covers Dryseal (NPTF) threads. Use the applicable edition and the component drawing for manufacture and inspection.

If you need help matching a connection, contact us with the marking, thread photographs, measured pitch and intended operating conditions.