Why a 1/2" pipe measures 0.84": actual OD, NPT and NPS tap drills, and BSPP / BSPT drills side by side.
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| 0.313" · 7.94 mm | 27 | D (0.246") | N/A | 28 | 7.723 mm | 6.8 / 6.6 mm | ||
| 0.405" · 10.29 mm | 27 | R (0.339") | U (62.4%) | 28 | 9.728 mm | 8.8 / 8.4 mm | ||
| 0.540" · 13.72 mm | 18 | 7/16" (0.4375") | 31/64" (62.6%) | 19 | 13.157 mm | 11.8 / 11.2 mm | ||
| 0.675" · 17.15 mm | 18 | 37/64" (0.5781") | 39/64" (73.8%) | 19 | 16.662 mm | 15.25 / 14.75 mm | ||
| 0.840" · 21.34 mm | 14 | 23/32" (0.7188") | 49/64" (65.1%) | 14 | 20.955 mm | 19 / 18.25 mm | ||
| 1.050" · 26.67 mm | 14 | 59/64" (0.9219") | 31/32" (71.1%) | 14 | 26.441 mm | 24.5 / 23.75 mm | ||
| 1.315" · 33.4 mm | 11.5 | 1-5/32" (1.1563") | 1-7/32" (69.2%) | 11 | 33.249 mm | 30.75 / 30 mm | ||
| 1.660" · 42.16 mm | 11.5 | 1-1/2" (1.5") | 1-9/16" (70.1%) | 11 | 41.91 mm | 39.5 / 38.5 mm | ||
| 1.900" · 48.26 mm | 11.5 | 1-47/64" (1.7344") | 1-51/64" (74.1%) | 11 | 47.803 mm | 45.25 / 44.5 mm | ||
| 2.375" · 60.33 mm | 11.5 | 2-7/32" (2.2188") | 2-17/64" (78.6%) | 11 | 59.614 mm | 57 / 56 mm |
05 / Field guide
Pipe sizes are trade names inherited from old iron pipe, where the number roughly matched the bore. Wall thicknesses changed and the outside diameter stayed fixed, so a 1/2" pipe actually measures 0.840" across.
NPT threads taper at 1 in 16 on diameter, so the joint tightens and seals as it is screwed together, with tape or sealant on the threads. NPS (straight) threads don’t taper; they seal with a gasket or O-ring. The tap drills differ because a straight thread needs full depth along the whole hole.
NPT taper = 3/4" per foot on diameter (1:16)
To identify a pipe, measure the outside diameter and count the threads. 0.840" OD with 14 TPI is 1/2" NPT. 1.050" OD with 14 TPI is 3/4". The sizer above draws the true OD circles so you can lay a fitting on the screen.
British Standard Pipe uses a 55° thread form and different thread counts at several sizes (for example 1/2" BSP and 1/2" NPT are both 14 TPI, but 1/4" BSP is 19 TPI against 18). They may start together but will not seal. BSPP is parallel, BSPT tapered.
Pipe taps cut a taper, so the depth you tap controls the fit. Tap until a fitting threads in hand-tight about three to four turns, then stop. Tapping too deep leaves a loose, leaking joint. For a cleaner thread in thick material, ream with a 3/4"-per-foot taper reamer first. Use plenty of cutting oil: pipe taps remove a lot of metal at once.
Questions
The size refers to the nominal bore of old schedule-40 iron pipe, not the outside diameter. A 1/4" pipe has a 0.540" OD; the thread is cut on that OD.
A 23/32" drill (0.719") for tapping without a taper reamer. With a reamer you can start slightly smaller.
The letter R drill (0.339"). 1/8" NPT has 27 threads per inch on a 0.405" OD.
Not reliably. The thread angles differ (60° against 55°) and some sizes differ in pitch. They may engage a turn or two and then bind or leak. Use an adapter.
For NPSM, the ANSI B1.20.1 tapping drill is 19.28 mm; the nearest standard drill is 49/64" (0.766"), giving about 65% thread.
Wrap PTFE tape clockwise (seen from the end) two to four turns, or use pipe dope rated for the fluid. Tighten hand-tight, then one to two turns with a wrench. Don’t use sealant on flared or compression fittings.
14 threads per inch, on a 1.050" outside diameter. 1/2" NPT is also 14 TPI; 1" to 2" NPT are 11.5 TPI.
A 7/16" drill (0.4375"). The pipe OD is 0.540" with 18 threads per inch.
A 1-5/32" drill (1.156"). 1" NPT has 11.5 threads per inch on a 1.315" outside diameter, so a 1" pipe is well over an inch across.
NPTF (dryseal) has the same taper and pitch but controlled crests and roots that seal metal to metal without sealant. NPT and NPTF parts will assemble, but only NPTF pairs are designed to seal dry.
A 37/64" drill (0.578"). 3/8" NPT has 18 threads per inch on a 0.675" outside diameter.
Nominal 1/8" pipe size with 27 threads per inch on a tapered thread. The pipe itself measures 0.405" across.