Size charts
NPT Thread Size Chart (OD, TPI and Tap Drills)
In short
This NPT thread size chart lists threads per inch, real outside diameter and tap drills for every common pipe size. The key fact: the size name is not a measurement. A 1/8 NPT thread measures about 0.405 in (10.3 mm) across, 1/4 NPT about 0.540 in (13.7 mm), and 1/2 NPT about 0.840 in (21.3 mm). Count threads per inch, measure the OD, then match a row.
NPT thread size chart
| NPT size | Threads per inch | Pitch (mm) | Outside diameter (in) | Outside diameter (mm) | Tap drill: Allied Machine | Tap drill: Wikipedia table |
|---|---|---|---|---|---|---|
| 1/16 | 27 | 0.94 | 0.313 | 7.95 | C (0.242 in) ¹ | — |
| 1/8 | 27 | 0.94 | 0.405 | 10.29 | Q (0.332 in) | R (0.339 in) |
| 1/4 | 18 | 1.41 | 0.540 | 13.72 | 7/16 in (0.4375) | 7/16 in (0.4375) |
| 3/8 | 18 | 1.41 | 0.675 | 17.15 | 9/16 in (0.5625) | 37/64 in (0.5781) |
| 1/2 | 14 | 1.81 | 0.840 | 21.34 | 45/64 in (0.7031) | 23/32 in (0.7188) |
| 3/4 | 14 | 1.81 | 1.050 | 26.67 | 29/32 in (0.9062) | 59/64 in (0.9219) |
| 1 | 11.5 | 2.21 | 1.315 | 33.40 | 1-9/64 in (1.1406) | 1-5/32 in (1.1562) |
| 1-1/4 | 11.5 | 2.21 | 1.660 | 42.16 | 1-31/64 in (1.4844) ² | — |
| 1-1/2 | 11.5 | 2.21 | 1.900 | 48.26 | 1-23/32 in (1.7188) | 1-47/64 in (1.7344) ² |
| 2 | 11.5 | 2.21 | 2.375 | 60.33 | 2-7/32 in (2.2188) | 2-7/32 in (2.2188) |
| 2-1/2 | 8 | 3.18 | 2.875 | 73.03 | 2-5/8 in | 2-5/8 in |
| 3 | 8 | 3.18 | 3.500 | 88.90 | 3-1/4 in | 3-1/4 in |
| 3-1/2 | 8 | 3.18 | 4.000 | 101.60 | 3-3/4 in | 3-3/4 in |
| 4 | 8 | 3.18 | 4.500 | 114.30 | 4-1/4 in | 4-1/4 in |
All NPT threads taper 1 in on diameter per 16 in of length (3/4 in per foot), so the OD shrinks toward the end of a male fitting. Measure at the large end, near the fitting body.
¹ Tameson lists D (0.246 in) for 1/16 NPT. ² Tameson matches Allied for 1/8 through 1-1/4 and matches Wikipedia for 1-1/2.
Why NPT tap drill sizes don't agree
Published pipe tap drill sizes differ by about 1/64 in, and both are in use. Allied Machine, a drill maker, lists the smaller drill and notes it is for tapping without a taper reamer. The larger drill leaves less material for the tap to cut, so the tap turns easier. The smaller drill leaves fuller threads but takes more effort to tap.
Whichever you use, how deep you tap matters as much as the drill:
- Drill, then tap a few turns at a time.
- Back the tap out and try the actual fitting by hand.
- Stop when the fitting threads in by hand part way and still needs a wrench to finish.
1/4 NPT is the exception: every chart we checked says 7/16 in.
How to identify an NPT thread
- Count threads per inch with a thread gauge, or count the crests over 1/2 in and double it. NPT uses 27, 18, 14, 11.5 or 8 TPI.
- Measure the OD with calipers at the large end and match it to the table. A 0.54 in thread with 18 TPI is 1/4 NPT, not 1/2 in.
- Check for taper. A tapered thread is wider at the base than at the tip; a straight thread measures the same along its length.
- Check the angle if it could be BSP. NPT flanks are 60°; BSP flanks are 55°.
NPT vs BSP
The 1/2 and 3/4 sizes share a thread count, so a BSP fitting can start in an NPT port. Don't rely on it: the different thread angle leaves leak paths. BSP is the standard pipe thread outside North America, so equipment built for other markets may use it.
Common mistakes
- Measuring and calling it the wrong size. The 1/2 in thread you measured is probably 1/4 NPT (0.54 in OD). Use the table, not the ruler number.
- Mixing NPT and BSP. They look similar. Count threads and check the angle.
- Mixing NPT and straight threads. Straight threads such as NPS and JIC don't seal on the threads; JIC seals on its 37° flare. See the JIC thread size chart for hydraulic fittings.
- Over-tapping. Pipe taps cut a taper. Tap too deep and the fitting bottoms out loose. Test-fit as you go.
- Opening a pressurized line. Relieve pressure and drain the line before you loosen any fitting, especially on hydraulics or air.
Not sure what fitting you have? Snap it
Snap the fitting next to a ruler and Mechy helps you narrow down the thread type and size. For pipe threads, confirm the TPI with a thread gauge before you buy. Identify a part from a photo.
Questions
What size is a 1/4 NPT thread?
1/4 NPT has 18 threads per inch and an outside diameter of about 0.540 in (13.7 mm) at the large end. The 1/4 in is a nominal pipe size, not a measurement.
What size drill for a 1/8 NPT tap?
Published charts differ: Allied Machine and Tameson list a Q drill (0.332 in), while Wikipedia's NPT table lists R (0.339 in). Both are close; tap in steps and test-fit the fitting.
What size drill for a 1/4 NPT tap?
7/16 in (0.4375 in). This is the one pipe size where every chart we checked agrees.
What size is 1/2 NPT?
1/2 NPT has 14 threads per inch and an outside diameter of about 0.840 in (21.3 mm).
Is NPT the same as BSP?
No. NPT has a 60° thread angle; BSP has 55°. Some sizes share a thread count (1/2 and 3/4 are 14 TPI in both), but they don't seal reliably when mixed.
Sources
Updated
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