NPT Thread Size Chart: Complete Dimensions from 1/16 to 2 Inch

The single most important thing to know before using this chart: the NPT size is a nominal designation, not a measurement. It traces back to the approximate bore of Schedule 40 steel pipe, so a 3/4 NPT fitting has a thread OD of 1.050 in — forty percent larger than its name. Identify any unknown fitting by measuring the OD and counting threads per inch, then matching both below. L1 is hand-tight engagement, L2 is total effective external thread length, and L3 is wrench-makeup length — always three pitches.

For straight unified and metric threads rather than tapered pipe threads, see our thread sizes chart. The tap drills above assume a tapered pipe tap; for the cleanest threads, ream the hole with a 1:16 taper reamer first.

Three series share the NPT name and cause regular confusion on drawings. NPT is the general-purpose tapered thread that seals with compound. NPTF (dryseal, ASME B1.20.3) uses the same basic dimensions but is controlled at root and crest so the threads crush into full metal-to-metal contact and seal dry. NPS (with its internal counterpart NPSC) is a straight, untapered 60-degree thread for mechanical joints — locknuts, hose couplings, swivel fittings — where a gasket or O-ring on a face does the sealing. An NPS male in an NPT female will not seal; an NPT male in an NPS port bottoms out loose.

Drawing callouts matter here. For fuel, hydraulic oil, or refrigerant service, specify NPTF explicitly — a shop will otherwise cut standard NPT, and the joint will depend on sealant in a system where sealant is a contaminant. For structural joints, NPS with a face seal decouples preload from sealing and is usually the better choice.

Producing an internal NPT thread is a three-step recipe. Drill with the taper-pipe tap drill from the chart, ream with a 1:16 taper pipe reamer, then tap with a tapered NPT pipe tap. Skipping the reamer is common on small sizes, but the tap then has to size and form the thread at once, which loads it hard and tears crests in stainless. Use cutting oil generously, and plan the tap depth so the L1 plug gauge lands within its ±1-turn window — that gauge check, not a depth measurement, is the acceptance test. Our guide to types of taps compares pipe taps with straight-flute, spiral-point, and form taps, and countersink holes explains why a small chamfer before tapping protects both the tap and the first thread.

On CNC equipment, thread milling is the preferred route for NPT from about 3/8 up: a tapered thread mill interpolates the 1:16 cone in a single helical path, one tool covers multiple sizes, and there is no risk of snapping an expensive large pipe tap in a blind port. External NPT threads are single-point turned with the taper programmed into the X path, or cut with a die head in production. Either way, verify with an L1 ring or plug gauge per B1.20.1 — face within one turn of the gauge notch. For context on how thread tolerances sit inside a broader print, see CNC machining tolerances. Typical production pitfalls: tapping too deep, bell-mouthed holes from a worn reamer, and over-torquing aluminum or brass, which seal at noticeably lower torque than steel.