NPT Thread Size Table
| Nominal size | TPI | Outside dia. D (in) | Hand-tight L1 (mm) | Tap drill (in) |
|---|---|---|---|---|
| 1/16 | 27 | 0.28118 | 0.9407 | 0.2611 |
| 1/8 | 27 | 0.37360 | 0.9407 | 0.2639 |
| 1/4 | 18 | 0.49163 | 1.4111 | 0.4018 |
| 3/8 | 18 | 0.62701 | 1.4111 | 0.4078 |
| 1/2 | 14 | 0.77843 | 1.8143 | 0.5337 |
| 3/4 | 14 | 0.98887 | 1.8143 | 0.5457 |
| 1 | 11½ | 1.23863 | 2.2087 | 0.6828 |
| 1¼ | 11½ | 1.58338 | 2.2087 | 0.7068 |
| 1½ | 11½ | 1.82234 | 2.2087 | 0.7235 |
| 2 | 11½ | 2.29627 | 2.2087 | 0.7565 |
| 2½ | 8 | 2.76216 | 3.1750 | 1.1375 |
| 3 | 8 | 3.38850 | 3.1750 | 1.2000 |
| 4 | 8 | 4.38713 | 3.1750 | 1.3000 |
NPT - National Pipe Taper - is the standard American thread for sealing pipes and fittings, and its numbers mislead everyone once: a 1/2-inch NPT fitting measures 0.778 inches across the threads. The nominal size refers to the old inside diameter of the iron pipe the thread was designed for, not to anything you can put calipers on. The table below gives the real numbers - threads per inch, actual outside diameter, and the drill that taps the hole - for every size from 1/16" to 4".
Two structural facts explain the whole chart. First, the thread flanks taper at 1:16 (the diameter shrinks one inch per sixteen inches of length), which is what lets the threads wedge tight and seal - with pipe dope or tape, no gasket. Second, the thread count steps down as pipes get bigger: 27 TPI on the tiny sizes, 18, then 14, then 11-1/2, and 8 TPI from 2-1/2" up - coarse enough that each size is identified by its TPI almost as surely as by its name.
How to use
- Identify a fitting by measuring the thread outside diameter with calipers, then read down the OD column: about 0.373 in = 1/8 NPT, about 0.492 in = 1/4 NPT, about 0.778 in = 1/2 NPT. Confirm with the TPI if it is close (27 vs 18 is easy to feel with a fingernail).
- For tapping, drill the listed tap drill size first - the taper does the rest. The tap drill column gives the inch drill; letter and numbered drills land on these (a 1/8 NPT tap wants a letter Q drill, 11/32 oversize lands about right with honest margin for a taper tap).
- Assemble dry to count turns: hand-tight engagement is roughly 4.3 turns on the small sizes and 4.5 turns on 1/2", and wrench tightening adds about 2-3 turns more. If a fitting bottoms out in fewer turns than that, the thread is damaged or the part is NPS straight thread being forced into a taper.
Frequently asked questions
What size is a 1/8 NPT fitting, actually?
0.373 inches (9.49 mm) outside diameter at the thread crest, with 27 threads per inch. The "1/8" is a pipe-size nickname inherited from the iron-pipe era - the wall-thick pipe that carried 1/8-inch bore inside - not a measurement of the thread. This trips up everyone buying gauges, bulkhead fittings and sensors: a "1/8 NPT" port does not match anything that is literally 1/8 inch. The table's OD column exists precisely to translate nicknames into caliper numbers.
Do NPT threads need sealant, and what kind?
Yes - the taper wedges metal to metal, but the spiral gap at the crest and root still weeps, so NPT joints are assembled with pipe dope or PTFE tape (two to four wraps, applied in the tightening direction, leaving the first thread bare on fuel and gas lines so shavings cannot migrate). The sealant lubricates during tightening too, which matters for turn counts: with tape you stop about one turn earlier than dry. Mechanical strength comes from the threads; leak-tightness comes from the sealant - neither does the other's job.
Are NPT and NPS (or NPTF) interchangeable?
NPT and NPS share the same diameters, TPI and thread geometry - only the taper differs (NPT narrows at 1:16, NPS is straight) - so they engage fully but do not seal the same way: NPS needs an O-ring, gasket or locknut facing to seal, and tapering it into a taper seat stresses both parts. NPTF (dryseal) is a tapered thread with crest/root interference that crushes the leak path and assembles without sealant - it mates with NPT but the dryseal guarantee is lost. Rule of thumb: taper-to-taper with dope, straight-to-straight with a gasket, and never taper into straight for a sealed joint.
What drill size do I need for a 1/4 NPT tap?
0.4018 inches - a letter Y drill (0.404) is the closest standard, and 13/32 (0.40625) works with the tap doing slightly more cutting. Every size in the chart has a listed drill: the taper means the hole size sets how many usable threads you get, so too big a hole leaves the joint thin on engagement and too small makes the tap bind early. For pipe taps, drill to the listed decimal rather than guessing the nearest fraction.