1/4 NPT Tap Drill Size
For 1/4-18 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 do not agree: their drills run from 27/64 in (0.4219 in) to 7/16 in (0.4375 in).
If you ream the hole with a 1:16 taper reamer before tapping, drill smaller first. Emuge, a tap maker, lists 10.75 mm (0.4232 in) for that, against 11.0 mm without a reamer.
1/4-18 NPT tap drill: what each chart says
| Source | Method it states | Drill | Inches | mm |
|---|---|---|---|---|
| Allied Machine | Without a taper reamer | 7/16 in | 0.4375 | 11.11 |
| Chiggo | Direct tapping without reaming | 7/16 in | 0.4375 | 11.11 |
| Want | No method given | 7/16 in | 0.4375 | 11.11 |
| Emuge | Drilled, no reamer | 11.0 mm | 0.4331 | 11.00 |
| Emuge | Drilled, then taper reamed | 10.75 mm | 0.4232 | 10.75 |
| Engineering Toolbox's NPT table | No method given | 7/16 in | 0.4375 | 11.11 |
| Engineering Toolbox's Briggs table | Direct tapping without reaming | 27/64 in | 0.4219 | 10.72 |
| Slugger Tool | No method given | 7/16 in | 0.4375 | 11.11 |
Allied Machine, Chiggo, Want, Engineering Toolbox's NPT table and Slugger Tool give 7/16 in. The others give 11.0 mm (Emuge) and 27/64 in (Engineering Toolbox's Briggs table). Leaving out the reamed hole, the biggest and smallest drills differ by 0.0156 in (0.40 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 27/64 in. Its NPT thread table gives 7/16 in 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: 7/16 in from Allied (0.4375 in) and 11.0 mm from Emuge (0.4331 in), 0.0044 in apart. With a taper reamer, start from Emuge's 10.75 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 1/4-18 NPT means
- 1/4
- The nominal pipe size, not the thread diameter. A 1/4 pipe is 0.540 in (13.72 mm) across the outside.
- 18
- Threads per inch, so one thread every 0.0556 in (1.411 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 | 0.540 in (13.72 mm) |
| Threads per inch | 18 (pitch 0.0556 in, 1.411 mm) |
| Hand-tight engagement, L1 | 0.2278 in, about 4.10 turns |
| Effective thread length, L2 | 0.4018 in, about 7.23 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 1/4-18 NPT it is 0.2278 in, about 4.10 turns, and the effective thread is 0.4018 in, about 7.23 turns (Wikipedia).
On the pipe end, Engineering Toolbox gives about 5/8 in of thread, roughly 11 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: 29/64 in (0.4531 in) for 1/4 NPS, against 7/16 in for NPT (Allied). NPT and NPS threads do not interchange (Engineering Toolbox).
How to tap a 1/4-18 NPT hole
- Drill the hole. Without a reamer, 7/16 in (Allied) or 11.0 mm (Emuge).
- If you are reaming, drill 10.75 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 1/4 NPT?
For 1/4-18 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. Other charts give 11.0 mm (Emuge) and 27/64 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 1/4 NPT?
Emuge, a tap maker, lists 11.0 mm (0.4331 in) for tapping without a reamer and 10.75 mm (0.4232 in) for a hole you will taper ream.
Is 1/4 NPT 1/4 inch across?
No. 1/4 is the nominal pipe size. A 1/4 pipe is 0.540 in (13.72 mm) across the outside, and the thread has 18 threads per inch (Wikipedia).
Do I need a taper reamer for 1/4 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 10.75 mm.
Why do charts give different drills for 1/4 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.