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Pipe Tap Drill Chart — NPT & NPTF

Commonly used drill sizes for tapered pipe taps in NPT and NPTF, for holes drilled only and for holes that will be taper reamed before tapping.

Ref CT-0052ASME B1.20.1ASME B1.20.3

Draft, open for review. Check every value against the governing standard and the manufacturer’s data before use.

What it is

Tapered pipe threads seal by wedging, so the hole must be sized for a taper, not a straight thread. NPT (ASME B1.20.1) is the general-purpose American pipe thread and is normally used with sealant or tape. NPTF (ASME B1.20.3) is the dryseal form, made to seal metal to metal at the crests and roots. Both use the same nominal sizes and threads per inch.

A pipe tap drill can be used two ways. Drilled only: the tap cuts the full taper from a straight hole. Taper reamed: a pipe reamer forms the taper first, so a slightly smaller drill is used and the tap does less work and cuts a cleaner thread.

How to read it

Find the nominal pipe size (not the outside diameter of the pipe). The nominal size of 1/8 NPT is far smaller than its actual thread diameter of about 0.405 in. Choose the column that matches how the hole will be made.

Chart

Nominal size	TPI	Drill, no reaming	Drill, with taper reamer
1/16	27	D (.246)	15/64 (.2344)
1/8	27	R (.339)	21/64 (.3281)
1/4	18	7/16 (.4375)	27/64 (.4219)
3/8	18	37/64 (.5781)	9/16 (.5625)
1/2	14	23/32 (.7188)	11/16 (.6875)
3/4	14	59/64 (.9219)	57/64 (.8906)
1	11-1/2	1-5/32 (1.1562)	1-1/8 (1.1250)
1-1/4	11-1/2	1-1/2 (1.5000)	1-15/32 (1.4688)
1-1/2	11-1/2	1-47/64 (1.7344)	1-23/32 (1.7188)
2	11-1/2	2-7/32 (2.2188)	2-3/16 (2.1875)

Key points

NPT and NPTF taps are commonly drilled with the same sizes. For dryseal work, taper reaming is the usual practice and the tap maker's chart governs.
Tap depth is checked with a thread plug gauge (L1 gauge), not by counting turns. Too deep and the fitting bottoms loose; too shallow and it will not make up.
BSPT (British tapered) and NPS (straight pipe) threads are different forms and need their own drill sizes.
Pipe taps take a large share of the hole wall. Use a tapping fluid and back out often in tough material.

Common mistakes

Picking the drill by the pipe's outside diameter instead of the nominal size.
Using the reamed-hole drill without reaming, which leaves too much material for the tap.
Using an NPT tap where a drawing calls for NPTF dryseal threads.
Running the tap to a fixed depth without gauging.

Governing standards: ASME B1.20.1 (pipe threads, general purpose, inch); ASME B1.20.3 (dryseal pipe threads, inch). Tap manufacturer's drill recommendations.

Reference only. Verify against the current governing code or standard, the manufacturer's data and the project documents before use. Apps for Distributors disclaims liability arising from reliance on this reference.

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