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Sizes · Pipe (NPT / BSPT / BSPP)

NPT — American tapered pipe

1/4-18 NPT

1/4-18 NPT pipe thread: 18 TPI, major Ø 13.72 mm.

Tap-drill calculatorIdentify a thread
Tap drill
11.11 mm
13.72 mm major Ø

Profile

Thread form & key diameters

Major Ø 13.72 mmP 1.411 mm60°Root Ø 11.989 mm · Pitch Ø 12.803 mm1/4-18 NPT18 TPI · 60° form

Dimensions

Designation1/4-18 NPT
Major (outside) diameter13.72 mm · 0.5402"
Pitch1.411 mm · 18 TPI
Pitch diameter12.803 mm · 0.5041"
Minor diameter (external)11.989 mm · 0.472"
Minor diameter (internal)12.192 mm · 0.48"
Recommended tap drill11.11 mm

What tap drill do I need for 1/4-18 NPT?

1/4-18 NPT is a NPT pipe thread running at 18 threads per inch on a 13.72 mm major diameter. NPT is a 60° tapered thread that seals on the flanks; use a tapered reamer after drilling for a full-form thread.

All values are generated from the standard thread formulas (ISO 68-1 / ASME B1.1) and published standard tables — not hand-entered. See the full reference.

Nearby sizes

More npt — american tapered pipe threads

3/8-18 NPTTap 14.68 mmØ 17.15 mm · pitch 1.411 mm1/8-27 NPTTap 8.74 mmØ 10.29 mm · pitch 0.941 mm1/16-27 NPTTap 6.35 mmØ 7.94 mm · pitch 0.941 mm1/2-14 NPTTap 17.86 mmØ 21.34 mm · pitch 1.814 mm3/4-14 NPTTap 23.42 mmØ 26.67 mm · pitch 1.814 mm1-11.5 NPTTap 29.36 mmØ 33.4 mm · pitch 2.209 mm1-1/4-11.5 NPTTap 38.1 mmØ 42.16 mm · pitch 2.209 mm1-1/2-11.5 NPTTap 44.05 mmØ 48.26 mm · pitch 2.209 mm