ASME B1.20.1: NPT Pipe Threads (General Purpose, Inch): Scope, Dimensions & Conforming Fittings

Reviewed by the Crestflo Engineering Team

ASME B1.20.1 is the US national standard for "Pipe Threads, General Purpose (Inch)." It sets out the form, taper, pitch and gauging for NPT (National Pipe Taper), along with the related straight inch pipe threads: NPSC, NPSM, NPSL, NPSH and NPTR. Crestflo cuts and gauges NPT and NPTF end connections to ASME B1.20.1 / B1.20.3 on instrumentation tube fittings, valves and regulators, working in stainless and high-nickel alloys. Delivery runs 6 to 8 weeks, and every part ships with a heat and lot number.

Scope note: On this page, NPT refers to the ASME B1.20.1 tapered pipe thread used to make sealed threaded connections on instrumentation fittings and valves. It's not a garden-hose thread (GHT/NH), not a straight mechanical thread, and not a dryseal NPTF thread. ASME B1.20.1 is a thread dimensional and gauging standard. It governs the thread itself, not the pressure rating of the fitting it's cut on.

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Also known as: ASME B1.20.1 & NPT designations

Specifications, line lists and datasheets all name this standard differently. Whatever the label, the designations below point to the same general-purpose inch pipe-thread standard.

This thread also goes by the name "NPT" or "tapered pipe thread" on instrument-root, gauge and small-bore fittings. Its 60° form and self-wedging taper are why it gets called out here.

ASME B1.20.1 standard at a glance: attributes

ASME B1.20.1 tabulates exact pitch-diameter (E0/E1) and gauge-length (L1/L2/L3) values, and those tables are copyrighted and fixed by whatever edition is current. This page summarizes the thread geometry and basic dimensions instead of reproducing the licensed tables outright. Values follow the applicable standard, and they can be adjusted to your requirement.

What is ASME B1.20.1?

ASME B1.20.1 is the American national standard titled "Pipe Threads, General Purpose (Inch)." It covers the thread form, taper, pitch (threads per inch) and gauging of NPT, National Pipe Taper, along with the related straight pipe threads: NPSC, NPSM, NPSL, NPSH, NPTR. ASME publishes it. The standard carries the dual ANSI/ASME B1.20.1 designation, and the current edition is B1.20.1-2013 (R2018).

NPT dominates threaded end connections on US instrumentation and process piping. The taper is what makes it work: the thread narrows along the axis, so the male and female flanks wedge together as the joint is made up, and a thread sealant fills the small helical leak path to finish the seal. That's why a datasheet calling for a "1/2 NPT" port wants an ASME B1.20.1 tapered thread, not a straight machine thread.

The standard picks up where earlier ANSI pipe-thread documents left off, and it's still what fitting, valve and instrument makers cut their NPT ends to. It governs the dimensions and gauging of the thread. It says nothing about the pressure or temperature rating of the component the thread is machined on.

Who publishes ASME B1.20.1, and what is the current edition?

ASME writes and maintains the standard; ANSI accredits it, which is why you'll see it written as ANSI/ASME B1.20.1. The current edition is ASME B1.20.1-2013, reaffirmed as (R2018). Contract references vary quite a bit, though. Cite whatever edition your project specification names, and check it against the current ASME catalog. Don't just assume a fixed year.

NPT thread geometry: 60° form, 3/4-in per foot taper & TPI

NPT geometry is defined by four fixed characteristics under ASME B1.20.1:

Flank angle: a 60° symmetrical V thread form, with truncated (flattened) crests and roots so mating threads don't bottom out.

Taper: 3/4 in per foot on diameter, which works out to 1 in 16, or 1.7899° measured from the axis. This taper doesn't change across sizes, and it's what lets NPT threads wedge and seal.

Pitch and TPI: threads per inch shift with size, running 27, 18, 14 or 11½. Pitch gets coarser as the pipe gets bigger.

Direction and hand: external threads carry the designation MNPT (male), internal ones FNPT (female). Both are right-hand unless someone specifies otherwise.

Because the crests and roots get truncated, standard NPT leaves a small helical gap at the thread tips. That's why you need a sealant for a leak-tight joint. The dryseal NPTF thread closes that gap mechanically instead; see NPT vs NPTF.

NPT thread dimension chart (all sizes, 1/16"–2")

The chart below sets out the fixed basic dimensions for general-purpose NPT threads: nominal size, reference pipe outside diameter, threads per inch and thread pitch. These figures are what people are usually after when they search "NPT thread chart," "NPT thread dimensions" or "1/2 NPT thread dimensions."

Taper runs 3/4 in per foot for every size (that's a 1:16 ratio, or 1.7899° from the axis). Thread pitch is 1 divided by TPI. The pitch-diameter at the gauge plane (E0/E1), along with the hand-tight (L1), effective (L2) and vanish (L3) engagement lengths, comes straight from the licensed ASME B1.20.1 tables and gets applied at the gauging stage below. Figures follow the applicable standard and can be adjusted to fit specific requirements.

NPT thread engagement & make-up

An NPT joint comes together in two moves. Hand-tight engagement brings the threads together to the L1 reference plane, the point where a new male and female thread meet finger-tight. From there, roughly 2 to 3 additional wrench turns (size-dependent) carry the joint to a wrench-tight, sealed condition as the tapered flanks wedge against each other.

Since the thread self-wedges but the truncated crests and roots leave a small spiral clearance, standard NPT still needs a thread sealant, PTFE tape or pipe dope, to seal reliably. Over-torque won't do the job sealant does, and it risks splitting a female port. The practical field check for proper make-up is the number of turns past finger-tight.

How NPT threads are gauged (L1, L2, L3)

ASME B1.20.1 governs NPT threads, and it does so with two tools: taper ring gauges for external threads, taper plug gauges for internal ones. The gauge seats against the thread, and acceptance comes down to the standard's one-turn tolerance. If the gauge stands within one turn of the reference notch or face, the part is in spec.

Crestflo gauge-verifies NPT and NPTF ends on production lots, and this matters at three fixed reference planes along the taper: L1, the hand-tight engagement point; L2, the effective thread length; and L3, the vanish or overall thread. Pitch diameter gets checked at these planes, and make-up is measured there too. A thread cut to spec at these points will mate and seal with a compliant counterpart out in the field.

NPT vs NPTF (Dryseal): are they interchangeable?

NPTF and NPT assemble mechanically the same way, that's the answer to the most common NPT question. Both share a 60° form, matching TPI by size, and a 3/4-in per foot taper. Where they part ways is sealing.

NPTF is the dryseal pipe thread. It crush-seals on its own flanks, no tape needed. NPT, by contrast, relies on a sealant. The two will thread together fine, but a dryseal joint is only guaranteed when it's NPTF-to-NPTF. Want the full decision guide? See NPT vs NPTF. For parallel threads, check NPT vs BSP.

Related pipe threads: NPSC, NPSM, NPSL, NPSH & NPTR

ASME B1.20.1 also lays out the straight, non-tapered members of the inch pipe-thread family, and each one serves a specific joint.

• NPSC, straight thread for couplings.
• NPSM, straight mechanical thread for free-fitting mechanical joints.
• NPSL, straight thread for loose-fitting, locknut connections.
• NPSH, straight thread for hose couplings.
• NPTR, taper thread for railing (rigid mechanical) joints.

These sit alongside NPT in the same standard, but they seal or join differently, so a spec has to call out the exact member. Parallel BSP threads (BSPT/BSPP) belong to a separate ISO system entirely. You can compare them in the NPT vs BSP guide, and if you're not sure what you're looking at, the thread identification guide or the thread identifier tool will help you sort it out.

Crestflo instrumentation fittings & valves cut to ASME B1.20.1

Crestflo cuts and gauges NPT and NPTF end connections to ASME B1.20.1 / B1.20.3 on instrumentation tube and compression fittings, valves and regulators. Work is done in stainless and high-nickel/CRA alloys, and national, international and custom threads are available made to order. Every thread gets gauge-verified during production, and each part ships with a heat/lot number plus an EN 10204 3.1 material test certificate; 3.2 is available on request.

Crestflo makes datasheets and CAD models available to customers on request. A Swagelok® / Parker® cross-reference for NPT-threaded fittings can be obtained from the Engineering Team, request your part-number cross-reference. What Crestflo ships is finished instrumentation fittings, valves and tubing, never bar, sheet or mill product.

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Request NPT-threaded instrumentation fittings & valves

Send your requirement for a product, tube fitting, valve, or regulator, along with thread type (NPT or NPTF, MNPT/FNPT), size, material or grade, quantity, and any NACE sour-service target you're working to. The Crestflo Engineering Team will confirm thread, gauging and documentation for your line.

RFQ, tell us what you need:

• Name
• Company name
• Email
• Phone
• Requirement description (product, thread type & size, material, quantity, service conditions)

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Browse NPT tube fittings, request a sample, become a distributor, or contact Crestflo directly.

US contact: sales@crestflousa.com · +1 346 594 9005 · Houston, TX.

Ready stock covers standard items; specials are made to order, with delivery running 6-8 weeks. Origin is Indian, though US or European melt can be supplied on request, complete with full material traceability.

Why Crestflo for ASME B1.20.1 NPT threads

• The standard is published, not buried. NPT form, taper, TPI and the dimension chart sit here as HTML-native data, and the fittings are cut to that same B1.20.1 basis.
• A manufacturer, not a reseller. Crestflo machines and gauge-verifies its own NPT and NPTF ends in house, running CNC, VMC and EDM, with thread gauging and PMI on every production run.
• The alloy advantage. NPT and NPTF ends come in 316/316L, super-duplex, 6Mo, Alloy 20, Inconel, Monel and Hastelloy, built for sour, chloride and acid service.
• Interchange proof on fittings. Compression tube fittings are performance-qualified to ASTM F1387, so they drop in and interchange with the major brands.
• Traceability on every part. EN 10204 3.1 MTC (3.2 on request) and a heat/lot number on every part.
• Lead time that moves projects. Ready stock for standard items, made-to-order for specials, 6–8 week delivery.
• Honest origin. Indian origin; US / European melt available on request, with full material traceability.
• Certified quality system: ISO 9001 / 14001 / 45001 & PED certified.

Crestflo is the instrumentation division of a four-decade-old, Government-of-India-recognized export house, with a long Middle-East delivery record. Manufacturing pedigree stands behind a focused, US-serving instrumentation brand. We Engineer Confidence.

Frequently asked questions

Explore related standards, products & materials

Standards: Standards hub · ASME B16.11 · ASME B16.34 · ASTM F1387 · EN 10204 MTC

Products: NPT tube fittings · NPTF fittings · BSPT tube fittings · BSPP tube fittings · Instrumentation valves · Pressure regulators

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