NPT Tap Drill Chart (1/8 to 1/2)
Published NPT tap drills do not agree. This chart lists what each source we read gives for 1/8-27 NPT, 1/4-18 NPT, 3/8-18 NPT and 1/2-14 NPT, with and without a taper reamer. The highlighted row, Allied Machine, is a tool maker's figure for tapping without a reamer.
| Source | Method it states | 1/8-27 NPT | 1/4-18 NPT | 3/8-18 NPT | 1/2-14 NPT |
|---|---|---|---|---|---|
| Allied Machine | Without a taper reamer | letter Q 0.3320 | 7/16 in 0.4375 | 9/16 in 0.5625 | 45/64 in 0.7031 |
| Chiggo | Direct tapping without reaming | letter Q 0.3320 | 7/16 in 0.4375 | 9/16 in 0.5625 | 45/64 in 0.7031 |
| Want | No method given | letter Q 0.3320 | 7/16 in 0.4375 | 9/16 in 0.5625 | 45/64 in 0.7031 |
| Emuge | Drilled, no reamer | 8.5 mm 0.3346 | 11.0 mm 0.4331 | 14.4 mm 0.5669 | 17.8 mm 0.7008 |
| Emuge | Drilled, then taper reamed | 8.3 mm 0.3268 | 10.75 mm 0.4232 | 14.15 mm 0.5571 | 17.45 mm 0.6870 |
| Engineering Toolbox's NPT table | No method given | letter R 0.3390 | 7/16 in 0.4375 | 37/64 in 0.5781 | 23/32 in 0.7188 |
| Engineering Toolbox's Briggs table | Direct tapping without reaming | 21/64 in 0.3281 | 27/64 in 0.4219 | 9/16 in 0.5625 | 11/16 in 0.6875 |
| Slugger Tool | No method given | letter R 0.3390 | 7/16 in 0.4375 | 37/64 in 0.5781 | 23/32 in 0.7188 |
Small figures are the drill diameter in inches. Click a size for the full page.
Why the sources disagree
NPT is tapered, so a hole can be drilled and tapped straight away, or drilled smaller and taper reamed first. Emuge gives a hole for each method, and the reamed one starts smaller. Engineering Toolbox prints two different sets on two of its pages. The other charts give one figure each, and only some say which method they assume.
Thread data
| Thread | Threads per inch | Pipe outside diameter | Hand-tight engagement | Effective thread | NPS drill (Allied) |
|---|---|---|---|---|---|
| 1/8-27 NPT | 27 | 0.405 in | 0.1615 in (4.36 turns) | 0.2639 in (7.13 turns) | letter S |
| 1/4-18 NPT | 18 | 0.540 in | 0.2278 in (4.10 turns) | 0.4018 in (7.23 turns) | 29/64 in |
| 3/8-18 NPT | 18 | 0.675 in | 0.2400 in (4.32 turns) | 0.4078 in (7.34 turns) | 19/32 in |
| 1/2-14 NPT | 14 | 0.840 in | 0.3200 in (4.48 turns) | 0.5337 in (7.47 turns) | 47/64 in |
The nominal size is not the thread diameter: a 1/8 pipe is 0.405 in across the outside. The taper is 1 in 16 on the diameter, 3/4 in per foot (Wikipedia).
FAQ
What is the tap drill size for 1/8 NPT?
Allied Machine gives letter Q (0.3320 in, 8.43 mm) for tapping without a taper reamer, and Chiggo and Want print the same drill. The charts we read run from 21/64 in to letter R, so check the 1/8 NPT page for each source.
What is the tap drill size for 1/4 NPT?
Allied Machine gives 7/16 in (0.4375 in, 11.11 mm) for tapping without a taper reamer, and Chiggo, Want, Engineering Toolbox's NPT table and Slugger Tool print the same drill. The charts we read run from 27/64 in to 7/16 in, so check the 1/4 NPT page for each source.
What is the tap drill size for 3/8 NPT?
Allied Machine gives 9/16 in (0.5625 in, 14.29 mm) for tapping without a taper reamer, and Chiggo, Want and Engineering Toolbox's Briggs table print the same drill. The charts we read run from 9/16 in to 37/64 in, so check the 3/8 NPT page for each source.
What is the tap drill size for 1/2 NPT?
Allied Machine gives 45/64 in (0.7031 in, 17.86 mm) for tapping without a taper reamer, and Chiggo and Want print the same drill. The charts we read run from 11/16 in to 23/32 in, so check the 1/2 NPT page for each source.
Is NPT the same as NPS?
No. NPT is tapered and NPS is straight, and the two do not interchange (Engineering Toolbox). NPS takes a bigger drill: Allied Machine gives S for 1/8 NPS against Q for 1/8 NPT.
Drills from Allied Machine, Chiggo, Want, Emuge, Engineering Toolbox's NPT table, Engineering Toolbox's Briggs table and Slugger Tool. Thread data from Wikipedia. Last reviewed October 12, 2026.