NPT Tap Drill Chart

Which of the three drills to use

Start with the without reamer column. It is the size the tap makers agree on, and it assumes you are doing what nearly everyone does: drilling a plain straight hole and tapping it. Every other column is a variation on that.

The with reamer column is smaller because a taper reamer has already opened the hole to roughly the thread's own taper before the tap arrives. There is less for the tap to remove, so the starting hole can be tighter. If you do not own a pipe reamer, this column is not for you.

The easier tapping column is the larger size printed on a lot of older wall charts. It is not wrong, but it is a trade: the tap turns with noticeably less effort, and in exchange the thread is shallower, so the fitting engages less metal and the joint holds less. On a low-pressure fitting in thick material that is a fine bargain. On something structural or high pressure it is not.

NPT sizes are not hole sizes

This catches people out constantly. A 1/2 NPT thread does not involve a half-inch hole anywhere: the drill is 45/64, 0.7031 in. The number is a nominal pipe size, a legacy name for the pipe the fitting is meant to join, and it stopped matching any real dimension a long time ago. Always look the drill up.

Tapered threads work differently

A straight thread like 1/4-20 has one diameter and one percent of thread. An NPT thread is cut on a 1-in-16 taper, three quarters of an inch of diameter per foot, so both grow the deeper the tap goes. That is why this chart has no percent-of-thread column: the number would be different at every depth.

It also changes how you stop. On a straight thread you tap through or to depth and that is that. On a tapered thread, depth is the fit. Too shallow and the fitting will not tighten; too deep and it runs in too far before it seals. Use a plug gauge if you have one, and a trial fitting if you do not.

Sealing

Standard NPT is not a dry seal. The flanks jam as the taper tightens, but the crest and root clearances leave a spiral leak path. PTFE tape or a liquid thread sealant rated for your fluid and pressure closes it. NPTF (Dryseal) is machined so the crests and roots crush together and seal without sealant; the drill sizes are the same.

One size where the sources disagree

2 NPT is the one size on this chart where the published charts do not agree. Allied Machine lists 2-7/32 without reamer; Kaufman/Fastenal list 2-3/16. The chart above gives 2-3/16, which is what the majority of sources list. For a pressure joint at this size, cut a test thread and check it with a ring gauge rather than trusting either figure.

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.

Lead with without_reamer (tool makers + ASME B1.20.1 suggestion for 1/8 = Q). Show with_reamer. Mention easier_tapping as the common chart size that taps easier but leaves shallower threads.