1/16 NPT Tap Drill Size

Tap drill, no reamer

C 0.242 in 6.15 mm

Use a C drill (0.242 in, 6.15 mm) for a 1/16 NPT tap, then run the tap to gauge depth.

Using a taper reamer first? Drill 15/64 (0.234 in) instead.

The drill size for a 1/16 NPT tap is C (0.242 in / 6.15 mm). If you are wondering what size drill bit a 1/16 NPT tap needs, that is the one for a plain drilled hole. 1/16 NPT is a tapered thread at 27 threads per inch, so unlike a straight thread there is no single percent of thread to quote: engagement changes with depth, which is the whole point of the taper.

Every drill for 1/16 NPT

Tap drills for 1/16 NPT, 27 threads per inch. Decimal sizes are reproduced exactly as the source charts print them.
Method	Drill	Decimal in	mm
Without a reamer Start here Tap straight into the drilled hole. This is the size tool makers agree on.	C	0.242	6.15
With a taper reamer Smaller, because the reamer opens the taper before the tap goes in.	15/64	0.234	5.94
Easier tapping The larger size on many older shop charts. It taps with noticeably less torque and leaves shallower thread engagement, so the joint holds less.	D	0.246	6.25

NPT is tapered

An NPT thread is cut on a 1 in 16 taper, which works out to 3/4 inch of diameter per foot of length. The thread gets larger as it goes, and so does the hole it is cut into, which is why the drill sizes above are not simply "major diameter minus something".

That taper is also why depth is the control, not turns. Keep tapping and the thread keeps growing; the fitting goes in further before it jams. Stop too early and the fitting bottoms out loose. The way to get it right is to work to a gauge: an NPT plug gauge tells you where the taper has reached, and standard practice is that the gauge face sits flush with the top of the hole, give or take a turn.

Without a gauge, test-assemble the actual fitting. Hand-tight plus one to three turns with a wrench is the usual target. If the fitting runs in much further than that before it tightens, the hole was tapped too deep.

Sealing

NPT seals by metal-to-metal interference on the thread flanks as the taper tightens. In practice that is not enough on its own for anything under pressure: the crests and roots leave a small spiral path. Use PTFE tape or a liquid thread sealant rated for the fluid and pressure you are running.

NPTF, also called Dryseal, is the variant machined to crush the crests and roots together and seal without a sealant. The tap drill sizes are the same; the thread form and the tolerances are tighter.

1/8 NPT Q · 0.332 in Next size up

All NPT sizes, 1/16 to 2 inch →

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1/16 NPT tap drill: common questions

What size drill do I need for a 1/16 NPT tap?

C (0.242 in, 6.15 mm) if you are tapping straight into a drilled hole. If you run a taper reamer first, use 15/64 (0.234 in) instead.

Is 1/16 NPT the same as a 1/16 inch hole?

No. NPT sizes are nominal pipe sizes, not hole sizes or thread diameters. 1/16 NPT needs a C drill, 0.242 in, which is nothing like 1/16 in. The name refers to the bore of the pipe the fitting suits.

How deep do I run an NPT tap?

To the depth that gives the right gauge standoff, not to a fixed number of turns. NPT threads are tapered, so the tap keeps cutting a bigger thread the further it goes and the fit tightens as it does. Check with a plug gauge, or test-assemble the fitting and aim for hand-tight plus one to three turns with a wrench.

Why is there more than one drill size listed?

Because published charts give different sizes for different methods. The without-reamer size is what the tap makers agree on for tapping a plain drilled hole. The with-reamer size is smaller because the reamer forms the taper first. The easier-tapping size is larger, reduces torque, and leaves a shallower thread.

Does a 1/16 NPT thread seal by itself?

Not reliably. NPT is an interference taper thread, so the flanks jam together and give some seal, but standard practice is to use thread sealant or PTFE tape on anything carrying pressure. NPTF (Dryseal) is the variant designed to seal without sealant.

Where these drill sizes come from

Without/with reamer: Kaufman (GTAP pipe taps sheet), Fastenal tap drill poster, Allied Machine NPT chart agree for 1/16-1-1/2. 'Easier tapping' = older shop charts (ipaco.biz, many wall charts). Checked 2026-10-27.