1/8 NPT Tap Drill Size
For 1/8-27 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 do not agree: their drills run from 21/64 in (0.3281 in) to letter R (0.3390 in).
If you ream the hole with a 1:16 taper reamer before tapping, drill smaller first. Emuge, a tap maker, lists 8.3 mm (0.3268 in) for that, against 8.5 mm without a reamer.
1/8-27 NPT tap drill: what each chart says
| Source | Method it states | Drill | Inches | mm |
|---|---|---|---|---|
| Allied Machine | Without a taper reamer | letter Q | 0.3320 | 8.43 |
| Chiggo | Direct tapping without reaming | letter Q | 0.3320 | 8.43 |
| Want | No method given | letter Q | 0.3320 | 8.43 |
| Emuge | Drilled, no reamer | 8.5 mm | 0.3346 | 8.50 |
| Emuge | Drilled, then taper reamed | 8.3 mm | 0.3268 | 8.30 |
| Engineering Toolbox's NPT table | No method given | letter R | 0.3390 | 8.61 |
| Engineering Toolbox's Briggs table | Direct tapping without reaming | 21/64 in | 0.3281 | 8.33 |
| Slugger Tool | No method given | letter R | 0.3390 | 8.61 |
Allied Machine, Chiggo and Want give letter Q. The others give 8.5 mm (Emuge), letter R (Engineering Toolbox's NPT table and Slugger Tool) and 21/64 in (Engineering Toolbox's Briggs table). Leaving out the reamed hole, the biggest and smallest drills differ by 0.0109 in (0.28 mm).
Why the charts disagree
NPT is a tapered thread. You can drill a straight hole and tap it, or drill smaller and open the hole with a taper reamer before tapping. Emuge gives a hole for each method, and its hole for tapping without a reamer is the larger one. Allied Machine says its drills are for tapping without a taper reamer.
Engineering Toolbox prints two different figures on two pages. Its Briggs pipe tap table, labelled "for direct tapping without reaming", gives 21/64 in. Its NPT thread table gives letter R and does not say which method it assumes.
Want lists other reasons charts differ, among them the thread engagement wanted and the tap's geometry. Apart from the reaming method, none of the charts says what its numbers assume.
Which drill to use
If your tap maker publishes a drill for its pipe tap, use that one. Without one, the two tool makers here are close for tapping without a reamer: letter Q from Allied (0.3320 in) and 8.5 mm from Emuge (0.3346 in), 0.0026 in apart. With a taper reamer, start from Emuge's 8.3 mm.
Whichever drill you pick, check the tapped hole with a fitting, a thread gauge or a leak test, as Want advises for pipe threads.
What 1/8-27 NPT means
- 1/8
- The nominal pipe size, not the thread diameter. A 1/8 pipe is 0.405 in (10.29 mm) across the outside.
- 27
- Threads per inch, so one thread every 0.0370 in (0.941 mm).
- NPT
- National Pipe Taper, defined by ASME B1.20.1. The thread narrows by 1 inch of diameter over 16 inches of length, which is 3/4 in per foot.
| Property | Value |
|---|---|
| Pipe outside diameter | 0.405 in (10.29 mm) |
| Threads per inch | 27 (pitch 0.0370 in, 0.941 mm) |
| Hand-tight engagement, L1 | 0.1615 in, about 4.36 turns |
| Effective thread length, L2 | 0.2639 in, about 7.13 turns |
| Taper | 1 in 16 on the diameter (3/4 in per foot) |
Thread data from the NPT table on Wikipedia, which cites Engineering Toolbox and piping-designer.com.
How much thread to cut
Hand-tight engagement is how far a fitting goes in by hand before you tighten it with a wrench. For 1/8-27 NPT it is 0.1615 in, about 4.36 turns, and the effective thread is 0.2639 in, about 7.13 turns (Wikipedia).
On the pipe end, Engineering Toolbox gives about 3/8 in of thread, roughly 10 threads.
NPT, NPTF and NPS
Allied Machine gives the same drill for NPTF, the dryseal version, as for NPT. NPT needs a thread sealant such as tape or paste, while NPTF is made to seal without it (Wikipedia).
Straight pipe thread (NPS) is a different thread and takes a bigger drill: letter S (0.3480 in) for 1/8 NPS, against letter Q for NPT (Allied). NPT and NPS threads do not interchange (Engineering Toolbox).
How to tap a 1/8-27 NPT hole
- Drill the hole. Without a reamer, letter Q (Allied) or 8.5 mm (Emuge).
- If you are reaming, drill 8.3 mm (Emuge), then open the hole with a 1:16 taper reamer. Chiggo says this lowers the tapping torque.
- Put cutting fluid on the tap (Wikipedia).
- Hold the tap in line with the hole.
- Turn the tap forward, then back it off now and then to break the chip (Wikipedia).
- Check the hole with a fitting, a thread gauge or a leak test (Want).
FAQ
What size drill do I use for 1/8 NPT?
For 1/8-27 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. Other charts give 8.5 mm (Emuge), letter R (Engineering Toolbox's NPT table and Slugger Tool) and 21/64 in (Engineering Toolbox's Briggs table), so the sources disagree. The table on this page shows each one.
What metric drill do I use for 1/8 NPT?
Emuge, a tap maker, lists 8.5 mm (0.3346 in) for tapping without a reamer and 8.3 mm (0.3268 in) for a hole you will taper ream.
Is 1/8 NPT 1/8 inch across?
No. 1/8 is the nominal pipe size. A 1/8 pipe is 0.405 in (10.29 mm) across the outside, and the thread has 27 threads per inch (Wikipedia).
Do I need a taper reamer for 1/8 NPT?
Not always. Chiggo says most field and DIY jobs drill and tap without one, and that reaming first with a 1:16 taper reamer lowers the tapping torque. If you ream, drill smaller first: Emuge gives 8.3 mm.
Why do charts give different drills for 1/8 NPT?
Part of it is method: a hole that will be taper reamed starts smaller than one that is tapped straight after drilling. Want also names the thread engagement wanted and the tap's geometry. Apart from the reaming method, the charts do not say what their numbers assume.
Related sizes
Drills from Allied Machine, Chiggo, Want, Emuge, Engineering Toolbox's NPT table, Engineering Toolbox's Briggs table and Slugger Tool. Thread data from Wikipedia. Tapping tips from Wikipedia: tap and die. Last reviewed October 12, 2026.