NPT Fittings

NPT Thread Size Reference Table

Measure the outer diameter of the threads with calipers to identify your fitting size. The nominal size doesn't match any actual dimension - it's just a naming convention.

Nominal Pipe Size	Actual Pipe OD	Thread OD	Threads Per Inch
1/8"	0.405"	0.540"	27
1/4"	0.540"	0.675"	18
3/8"	0.675"	0.840"	18
1/2"	0.840"	1.050"	14
3/4"	1.050"	1.315"	14
1"	1.315"	1.660"	11.5
1-1/4"	1.660"	2.047"	11.5
1-1/2"	1.900"	2.297"	11.5
2"	2.375"	2.875"	11.5
2-1/2"	2.875"	3.500"	8
3"	3.500"	4.250"	8
4"	4.500"	5.563"	8

Note: Thread OD is measured at the outer diameter of the threads (the largest diameter). Pipe OD is the actual outside diameter of the pipe itself. To identify an unknown fitting, measure the thread OD with calipers and match it to this chart.

What is NPT?

NPT stands for National Pipe Thread (or National Pipe Taper), a U.S. standard for tapered threads used on threaded pipes and fittings. NPT threads are tapered at a 1° 47' angle (1.7899°), which creates a mechanical seal when the male and female threads are tightened together.

The taper means the threads get slightly smaller in diameter as you move away from the end of the fitting. This taper, combined with thread sealant (Teflon tape or pipe dope), creates a pressure-tight seal suitable for liquids and gases.

Key NPT Characteristics:

Tapered threads - 1° 47' angle creates interference fit
60° thread angle - Standard V-thread profile
Requires sealant - Teflon tape or pipe dope needed for leak-free seal
Tightens with force - Threads bind as you tighten, creating seal

How to Identify NPT Fittings

1

Measure Thread OD with Calipers

Use digital calipers or a thread gauge to measure the outer diameter of the threads at the widest point (closest to the fitting body). This is your thread OD.

2

Match to Reference Table

Compare your measurement to the "Thread OD" column in the table above. The closest match indicates your nominal pipe size. For example, if you measure 1.050", you have a 1/2" NPT fitting.

3

Verify Thread Pitch (Optional)

Count the threads per inch to confirm. Use a thread pitch gauge or count threads in a 1-inch section. Match this to the TPI column in the table.

4

Check for Taper

NPT threads are tapered - the diameter decreases as you move away from the fitting. If threads are parallel (same diameter throughout), you may have NPS (National Pipe Straight) instead.

Why Doesn't the Nominal Size Match Any Real Measurement?

This confuses everyone at first. A "1/2 inch NPT" fitting doesn't have any dimension that actually measures 1/2 inch. The nominal size is just a traditional naming convention dating back to when pipe sizes roughly corresponded to the inner diameter of the pipe.

Over time, manufacturing standards changed and wall thicknesses varied, but the naming stuck. Now the nominal size is simply a reference name - you need to know the actual thread OD to identify fittings correctly.

Pro Tip: Always measure the thread OD (outer diameter of the threads) to identify NPT fittings. Don't trust the nominal size marking if you're unsure - verify with calipers.

NPT vs. Other Pipe Thread Types

NPT (National Pipe Taper)

Tapered threads that create mechanical seal when tightened. Requires sealant. Most common in North America for plumbing and process piping.

Tapered 1° 47'
Requires Teflon tape or pipe dope
Pressure-tight seal

NPS (National Pipe Straight)

Parallel threads (not tapered). Used with O-rings or gaskets for sealing. Common in low-pressure mechanical applications.

Parallel threads (no taper)
Uses O-ring or gasket for seal
Not pressure-tight without gasket

BSP (British Standard Pipe)

Common outside North America. Similar taper angle but different thread pitch and angle (55° vs 60°). NOT compatible with NPT despite similar appearance.

55° thread angle (vs 60° for NPT)
Different pitch standards
Do not mix with NPT

⚠ Warning: Never mix NPT and BSP fittings. They look similar but have different thread angles and pitches. Mixing them will result in leaks and potential thread damage.

Common NPT Fitting Applications

NPT fittings are the standard in North American brewing, distilling, and food processing for pressure-rated connections:

💧 Pump Connections

Inlet and outlet ports on process pumps typically use NPT threads. Common sizes: 1/2", 3/4", 1", 1-1/2", 2"

🛡 Pressure Gauges

Most pressure gauges use 1/4" NPT bottom or back mount connections

🟡 Valves

Ball valves, gate valves, and check valves commonly available with NPT ends for threaded installation

🔌 Temperature Sensors

Thermowells and temperature probes often use 1/2" NPT threads for tank penetrations

🏭 Tank Ports & Bungs

Many fermentation tanks and vessels use NPT bung fittings for adding ports and accessories

📦 Hose Adapters

NPT to hose barb adapters for connecting flexible tubing to threaded ports

NPT Installation Best Practices

Always use thread sealant - Teflon tape (PTFE tape) or pipe dope is required for leak-free seal. Tape alone is usually sufficient for beverage applications
Apply tape correctly - Wrap clockwise (looking at the end of male threads) so it doesn't unwind when threading. 3-4 wraps is usually sufficient
Don't over-tighten - Hand-tight plus 1-2 turns with a wrench is usually enough. Over-tightening can crack fittings or strip threads
Support both sides - Use two wrenches when tightening - one to hold the fitting, one to turn. Don't put torque on connected equipment
Inspect threads before assembly - Damaged or cross-threaded fittings won't seal properly. Replace damaged components
Use proper wrench size - Oversized wrenches can round off hex flats. Use properly fitting wrenches or adjustable wrenches
Consider orientation - Remember NPT threads bind as you tighten. You can't adjust final orientation easily once tight

NPT Pipe Fitting Identification Guide