NPT Size Chart
NPT size chart: TPI, OD, and engagement length for all sizes.
NPT Size Chart
NPT (National Pipe Taper) threads are the standard for threaded pipe connections in North American plumbing, hydraulic, and pneumatic systems. The tapered thread geometry creates a seal as it tightens, but only if the taper, pitch, and OD are correct. This NPT size chart covers the critical dimensions for sizing and specifying NPT connections from 1/16" through 4" per ASME B1.20.1
NPT Thread Dimensions Chart
Pitch diameter and hand-tight engagement are the key dimensions for gaging NPT connections. The hand-tight engagement length (L1) is how far a thread gauge (or mating fitting) can be turned in by hand — this is the reference point for measuring thread engagement.
| Nom. Size | OD (in) | TPI | Pitch Dia. | Tap Drill | L1 Eng. (in) | Common Application |
|---|---|---|---|---|---|---|
| 1/16" | 0.3125" | 27 | 0.2611" | 0.27118" | 0.160" | Light instrument fittings |
| 1/8" | 0.405" | 27 | 0.3389" | 0.36351" | 0.180" | Grease fittings, small instrument ports |
| 1/4" | 0.540" | 18 | 0.4452" | 0.47739" | 0.200" | Common hydraulic and pneumatic fittings |
| 3/8" | 0.675" | 18 | 0.5782" | 0.61201" | 0.240" | Small pipe connections, valves |
| 1/2" | 0.840" | 14 | 0.7022" | 0.75843" | 0.320" | Standard pipe and fitting connections |
| 3/4" | 1.050" | 14 | 0.9116" | 0.96768" | 0.339" | Standard plumbing, pneumatics |
| 1" | 1.315" | 11.5 | 1.1441" | 1.21363" | 0.400" | General industrial pipe connections |
| 1-1/4" | 1.660" | 11.5 | 1.4876" | 1.55713" | 0.420" | Larger pipe fittings, manifolds |
| 1-1/2" | 1.900" | 11.5 | 1.7256" | 1.79609" | 0.420" | Large valves, industrial piping |
| 2" | 2.375" | 11.5 | 2.1995" | 2.26902" | 0.436" | Main process lines, compressors |
| 2-1/2" | 2.875" | 8 | 2.6196" | 2.71953" | 0.682" | Large industrial process piping |
| 3" | 3.500" | 8 | 3.2446" | 3.34139" | 0.766" | Large industrial piping |
| 4" | 4.500" | 8 | 4.2446" | 4.33441" | 0.844" | Very large industrial connections |
Understanding Thread Engagement: The L1 and L2 Lengths
To properly specify NPT connections, you must distinguish between Hand-Tight Engagement (L1) and Effective Thread Length (L2).
- L1 (Hand-Tight): This is the length of the thread that can be screwed in by hand. It is the distance from the end of the pipe to the basic notch on a ring gauge.
- L2 (Effective Thread): This is the total length of the "perfect" threads that are capable of sealing. It includes the L1 length plus the threads that will be engaged during the final wrench-tightening turns.
If your pipe is threaded too short (less than the L2 length), you will run out of threads before the taper has a chance to wedge properly, leading to a "bottomed out" fitting that leaks regardless of how much sealant you use.
How NPT Threads Work
NPT threads are tapered at 1" per foot (1/16" per inch on the radius), or 3/4° per side. As two NPT threads are tightened together, the taper causes the threads to wedge, creating mechanical interference. This interference is what creates the seal — but NPT threads alone do not make a pressure-tight seal without a sealant (Teflon tape, pipe dope, or anaerobic sealant).
The standard NPT assembly calls for 2–3 turns beyond hand-tight engagement for most fittings. Over-tightening can crack cast fittings or distort threaded ports in equipment.
NPT vs. NPS vs. NPTF
NPT (National Pipe Taper): Tapered external and internal threads. The standard for pressure connections. Requires sealant.
NPS (National Pipe Straight): Straight (non-tapered) threads. Used for mechanical joints, flanged connections, locknuts, not for pressure sealing on their own.
NPTF (National Pipe Taper Fuel / Dryseal): Same taper as NPT but with tighter thread form tolerances that allow metal-to-metal sealing without sealant. Required in hydraulic systems, fuel systems, and applications where sealant contamination is unacceptable.
NPT vs. BSPT
In global industrial environments, NPT is often confused with BSPT (British Standard Pipe Taper). They look almost identical to the naked eye and both use a 1:16 taper. However, they are not compatible:
- Thread Angle: NPT has a 60° inclusion angle (V-shaped), while BSPT has a 55° Whitworth angle (rounded peaks and valleys).
- Pitch (TPI): In many sizes, the threads per inch differ. For example, 1/2" NPT uses 14 TPI, while 1/2" BSPT uses 14 TPI (lucky match), but 1" NPT uses 11.5 TPI while 1" BSPT uses 11 TPI.
Never force an NPT fitting into a BSPT port. It may thread in a few turns, but the mismatched thread angles will prevent a pressure-tight seal and likely gall the threads permanently.
How to Identify an NPT Thread Size
NPT nominal size does not match the OD. A 1/2" NPT thread has a 0.840" OD. To identify an unknown NPT thread: measure the OD of the male thread or the ID of the female port (measured at the opening), then compare to the chart above. A thread pitch gauge will confirm TPI. Note that 1/4" NPT (18 TPI) and 1/8" NPT (27 TPI) have different TPI. TPI measurement is the quickest way to distinguish adjacent sizes with close ODs.
NPT Sealant and Assembly
- Teflon (PTFE) tape: Wrap 2–3 turns clockwise (when facing the thread end) over the male threads starting at the first thread. Leave the first thread exposed. Most common sealant for water, air, and light gas service.
- Pipe thread sealant (pipe dope): Applied to male threads only. Better for larger sizes and high-vibration applications. Select a sealant rated for your process fluid.
- Anaerobic thread sealant (e.g., Loctite 567): Excellent for hydraulics and pneumatics. Cures in the absence of air, fills thread voids, and locks against vibration loosening.
- Never use Teflon tape on NPTF (dryseal) connections. It defeats the metal-to-metal sealing action.
Product Compliance and Suitability
The statements contained in this guide are intended for general informational purposes only. Such statements do not constitute a product recommendation or representation as to the appropriateness, accuracy, completeness, correctness, or currentness of the information provided. Information provided in this guide does not replace the use by you of any manufacturer instructions, technical product manual, or other professional resource or adviser available to you. Always read, understand, and follow all manufacturer instructions. Portions of this article were generated in part by ChatGPT.
