Pipe Thread Tap Drill Chart (NPT / BSPT / BSPP)

Pipe threads use different tap drills than machine-screw threads because the thread is tapered (3/4 inch taper per foot on NPT / BSPT). NPT and NPTF thread sealing relies on metal-on-metal interference at the crests; use thread sealant or PTFE tape. BSPP (also called "G") is a straight thread that seals on a gasket or O-ring, not by thread interference. Match thread standards strictly — NPT and BSPT look similar but have different angles (60° vs 55°) and are not interchangeable.

NPT — National Pipe Taper (ANSI B1.20.1)

Nominal	OD (in)	TPI	Tap Drill (in)	Decimal	Tap Drill (mm)
1/16"	0.3125	27	C (0.242)	0.242	6.15 mm
1/8"	0.4050	27	11/32	0.344	8.74 mm
1/4"	0.5400	18	7/16	0.438	11.11 mm
3/8"	0.6750	18	37/64	0.578	14.68 mm
1/2"	0.8400	14	23/32	0.719	18.26 mm
3/4"	1.0500	14	59/64	0.922	23.42 mm
1"	1.3150	11.5	1-5/32	1.156	29.37 mm
1-1/4"	1.6600	11.5	1-1/2	1.500	38.10 mm
1-1/2"	1.9000	11.5	1-47/64	1.734	44.05 mm
2"	2.3750	11.5	2-7/32	2.219	56.36 mm
2-1/2"	2.8750	8	2-5/8	2.625	66.68 mm
3"	3.5000	8	3-1/4	3.250	82.55 mm
3-1/2"	4.0000	8	3-3/4	3.750	95.25 mm
4"	4.5000	8	4-1/4	4.250	107.95 mm

NPTF — Dryseal (ANSI B1.20.3)

NPTF uses the same tap drill sizes as NPT but with tighter tolerance on thread crests and roots — seals without PTFE tape.
Requires NPTF-specific taps (dryseal); regular NPT taps will not produce a spec NPTF thread.
Drill the hole exactly as the NPT chart above, then reamer-ream for best finish before tapping.

BSPT — British Standard Pipe Taper (ISO 7 / R-series)

Nominal	OD (mm)	TPI	Tap Drill (mm)	Tap Drill (in)
R 1/16	7.723	28	6.5 mm	0.256 (≈ F)
R 1/8	9.728	28	8.8 mm	0.346 (≈ S)
R 1/4	13.157	19	11.8 mm	0.465 (≈ 15/32)
R 3/8	16.662	19	15.25 mm	0.600 (≈ 19/32)
R 1/2	20.955	14	19.0 mm	0.748 (≈ 3/4)
R 3/4	26.441	14	24.5 mm	0.965 (≈ 31/32)
R 1	33.249	11	30.75 mm	1.211 (≈ 1-7/32)
R 1-1/4	41.910	11	39.5 mm	1.555 (≈ 1-35/64)
R 1-1/2	47.803	11	45.25 mm	1.781 (≈ 1-25/32)
R 2	59.614	11	57.0 mm	2.244 (≈ 2-1/4)
R 2-1/2	75.184	11	72.5 mm	2.854 (≈ 2-27/32)
R 3	87.884	11	85.0 mm	3.346 (≈ 3-11/32)

BSPP / G — British Standard Pipe Parallel (ISO 228)

Nominal	OD (mm)	TPI	Tap Drill (mm)	Tap Drill (in)
G 1/16	7.723	28	6.8 mm	0.268
G 1/8	9.728	28	8.8 mm	0.346
G 1/4	13.157	19	11.8 mm	0.465
G 3/8	16.662	19	15.25 mm	0.600
G 1/2	20.955	14	19.0 mm	0.748
G 5/8	22.911	14	21.0 mm	0.827
G 3/4	26.441	14	24.5 mm	0.965
G 1	33.249	11	30.75 mm	1.211
G 1-1/4	41.910	11	39.5 mm	1.555
G 1-1/2	47.803	11	45.25 mm	1.781
G 2	59.614	11	57.0 mm	2.244

NPS — National Pipe Straight (ANSI B1.20.1)

NPS has the same TPI and OD as NPT but is parallel — used for mechanical connections (fittings, bushings), not sealing.
Use the NPT tap drill sizes; you may want to ream 0.005" oversize for slip fit.
NPSM (mechanical), NPSL (locknut), and NPSH (hose) are all variants with the same pitch but different tolerance classes.

Usage notes

Tap taper pipe threads with a taper tap only (pipe taps are self-starting and cut full profile). Never run a plug or bottoming tap after — you'll damage the seal.
Go one full turn past finger-tight after assembling NPT fittings, then 1½-2 turns more with a wrench. Over-tightening splits fittings.
PTFE tape: wrap 2-3 turns in the direction of thread rotation on the male thread. Leave the first 1-2 threads bare to prevent shavings getting into the system.
BSPP (G) threads seal on a face — add a bonded washer or O-ring on the male side. The thread itself does no sealing.
Never mix NPT and BSPT on the same joint; different thread angles (60° vs 55°) give only 3-4 contact threads which leaks fast.

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