For the full breakdown, see our best 1/4 NPT Thread Tap guide.
The number in 1/4 NPT describes the pipe being sealed, not the hole you bore — so a 1/4-inch bit is far too small.
Why Isn’t The Hole A Quarter Inch?
1/4 names the pipe size, not the drill or thread outside diameter. A 1/4-18 NPT thread runs 18 threads per inch and measures about 0.540 in (13.72 mm) across its major diameter.
NPT threads are tapered, cut on a cone, and trace to ASME B1.20.1. The governing lengths, L1 at 0.2278 in and L2 at 0.4018 in, say how far the fitting travels and how much usable thread the hole must supply.
| Specification | 1/4-18 NPT Value |
|---|---|
| Thread designation | 1/4-18 NPT |
| Threads per inch | 18 TPI |
| Major diameter (outside) | 0.540 in / 13.72 mm |
| Drill for direct tapping | 7/16 in / 0.4375 in / 11.11 mm |
| Drill before taper reaming | 27/64 in / 0.4219 in / 10.72 mm |
| Pitch diameter (reference) | 0.47739 in / 12.13 mm |
| Hand-tight length L1 | 0.2278 in / 5.79 mm |
| Effective thread length L2 | 0.4018 in / 10.21 mm |
Drill Size For A 1/4 NPT Tap: Where The Numbers Come From
The 27/64 figure leaves material for the reamer — it is not a tighter direct-tapping size.
Published charts drift slightly by manufacturer and target thread percentage. Premsa Industries’ NPT thread chart makes the control explicit: the finished thread is judged by NPT gauge position, within about one turn, not by a turn count. Tap style (plug, taper, bottoming, hand, machine) shifts start and stop points, so the tap maker’s chart outranks a generic table.
Drilling And Tapping In The Right Order
The taper does the sealing; depth control keeps the fitting aimed.
- Confirm the thread form. Fitting and tap must both be 1/4-18 NPT — not NPS, BSPT, or BSPP, which do not share pitch, angle, or taper.
- Depth must clear the full thread and leave room for chips; one maker lists 17.5 mm as minimum tapping-hole depth for a 1/4-18 NPT machine tap, though that figure is tool-specific.
- Deburr both edges and clear chips — a burr at the entry tips a tap off-axis.
- Lubricate for the material, start the tap square, and reverse periodically to break chips.
- Stop on position, not count. Check the fitting’s orientation or run an NPT gauge.
- Test with the mating fitting or gauge. Binding in the first turn or bottoming out early means the hole or thread is wrong.
The thread is right when the fitting starts by hand, threads to roughly L1 (about 0.228 in; 5.79 mm) hand-tight, and still has turns left to reach final orientation.
Three mistakes wreck most of these holes: a 1/4-in bit, 27/64 without a reamer, and forcing a BSPT tap that threads in a turn or two. Tape or dope will not recover any of them.
FAQs
How Deep Should A 1/4 NPT Hole Be Drilled?
Deep enough for the full thread plus chip clearance. One manufacturer lists 17.5 mm (about 0.69 in) as minimum tapping-hole depth for a 1/4-18 NPT machine tap, but that number is tool-specific. Where the design allows, drill through — a through-hole clears chips better and removes any risk of bottoming the tap.
Can You Use A 1/4-Inch Bit And Force The Tap?
No. A 1/4-in hole is roughly 0.25 in across, while the 1/4-18 NPT thread runs about 0.540 in at its major diameter, so the tap has almost nothing to cut and will usually snap or tear the hole.
Does NPT Need Sealant If The Thread Is Cut Right?
Yes — NPT seals through tapered-thread interference, and that interference alone rarely holds pressure. Use pipe tape or a sealant rated for the fluid, temperature, pressure, and materials involved. Sealant is not a repair: it will not fix a wrong-size hole, a cross-threaded start, or a BSPT tap swapped in for NPT.
References & Sources
- Premsa Industries. “NPT Thread Chart” Source for the 18 TPI count, major diameter, drill sizes, and gauge-position tolerance.
- Milwaukee Tool. “1/4-18 NPT Straight Flute Plug Tap & 7/16 Drill Bit, 49-57-5591” Confirms the plug tap and 7/16-in bit are sold as a matched pair.
- Zoro. “NPT Size Chart” Supports the NPT dimension and sizing reference figures used above.