Pipe threads: BSP and NPT

November 3, 2026  ·  Indumor

Pipe thread. A triangular thread, 55° Whitworth form in the British BSP family or 60° in the American NPT, parallel or tapered at 1:16, designated by the nominal size of the pipe rather than by any dimension you can take with a caliper. It joins pipe, fittings, valves, hydraulic and pneumatic adapters, gauges and plugs. In Argentina and Spain the BSP family goes by "gas thread".

That is the definition. Now the opinion: it is the thread that wastes the most time in a shop, because the name tells you nothing. A "1/2" is not half an inch anywhere, there are four letters (G, Rp, Rc and R) for what most people just call BSP, and the American family uses the same nominal sizes with another angle and another pitch. That is why the tables here sit side by side.

Why 1/2 does not measure 1/2

The nominal size comes from the bore of 19th-century wrought iron pipe. Walls got thinner over the years and the outside stayed fixed, because the fittings already existed. A 1/2 pipe is 21.3 mm outside, its BSP thread measures 20.955 mm over the crests and the NPT 21.336; wall thicknesses by schedule are in the pipe table. Threads per inch do not follow a formula either: BSP uses 28, 19, 14 and 11; NPT uses 27, 18, 14, 11.5 and 8. They agree only at 1/2 and 3/4, and that one coincidence ruins more joints than anything else.

BSP: G, Rp, R and the "gas" thread

The British family has two standards. ISO 228-1 defines the G thread, parallel on both the external and the internal side, which is not meant to seal on the thread: sealing comes from a flat gasket, a bonded washer or an O-ring bearing on the face. It is the thread of hydraulic hoses, pressure gauges and sump plugs. ISO 7-1 (EN 10226-1 in Europe; formerly DIN 2999 and BS 21, both withdrawn but still quoted in catalogs) defines the threads that do seal on the thread: the external R is tapered 1:16, the internal Rp is parallel and the internal Rc is tapered. An R fits either one and tightens on the taper; PTFE tape or paste fills the spiral leak path between crest and root. Japanese catalogs call these PT (R, Rc), PS (Rp) and PF (G).

The diameters are the same in both standards, so one table serves G, Rp and R. The form is the Whitworth of chapter 5, with rounded crests and a thread height of 0.640 times the pitch.

Min. useful thread: gauge length plus the ISO 7-1 fitting allowance, with the gauge length at nominal. 5/8, 7/8, 1 1/8, 1 3/4, 2 1/4, 2 3/4 and 3 1/2 exist only as G. Tap drill for a G tap. Rp seals on the thread and needs less: 8.6 at 1/8, 11.5 at 1/4, 15 at 3/8, 18.5 at 1/2, 24 at 3/4, 30.2 at 1″, 39 at 1 1/4, 45 at 1 1/2 and 56.5 at 2″. For Rc, a few tenths of a millimeter under Rp plus a taper reamer.

NPT: the American pipe thread

ASME B1.20.1 defines NPT with a 60° form, flat crests and roots, and a 1:16 taper on both the external and the internal thread. It seals on the thread, but a spiral clearance remains between the crests of one part and the roots of the other, and the standard itself calls for tape or pipe dope to close it. The NPTF variant, Dryseal, tightens that clearance so the crests crush into the roots and seal dry; it is the hydraulic version and should be assembled only a few times, since every make-up deforms it a little more.

Two lengths in the table matter. L₁ is the hand-tight engagement, about four and a half turns; L₂ is the effective thread, as far as the wrench takes it. There are about three turns between them, and that is the shop rule: hand-tight until it binds, then one and a half to three turns with the wrench. More than that cracks the fitting.

L₁: hand-tight engagement. L₂: effective thread, as far as the wrench takes it. Indicative tap drill, without a taper reamer (catalogs vary by a few tenths); from 1″ up, reaming is advisable.

The 1:16 taper and the gauge plane

Both R and NPT grow 1 mm in diameter for every 16 mm of length. That is a slope of 1/32 per side, and the half-angle comes from its tangent:

To turn an external taper thread you swing the compound to that angle (between centers, the tailstock is offset as in chapter 11). The table diameter is measured at the gauge plane, which sits one gauge length from the end: that is where the face of a hand-tight fitting lands. At the end the thread is smaller, and past the plane it keeps growing, because there must be thread left for the wrench turns (last column of the BSP table):

BSP versus NPT: which ones mate and which ones seal

By eye they look the same. In the table they do not. At 1/2 and 3/4 both have 14 tpi, and an NPT external goes into a BSP internal with little effort because the taper absorbs the diameter difference. The trouble is the angle: 55° against 60° puts the contact on the crests, sealing depends on the paste alone and the joint leaks as soon as pressure rises or something vibrates. At 1 inch they no longer mate (11 against 11.5 tpi binds after two turns), and from 2 1/2 up NPT drops to 8 tpi.

The marks you find on drawings and in catalogs:

Identifying a pipe thread

A caliper and a rule are enough almost every time. First the outside diameter over the crests, as close to the end as the caliper allows; on a taper thread it reads a few tenths of a millimeter under the table value. Then the threads in 25.4 mm, with a pitch gauge or by counting crests against a rule. When the pitch is the same in both families, the thread angle gauge or the shape of the crests decides. The full method for any thread is in chapter 7.

A 1/2 BSP nut and a 1/2 NPT nut in the lathe drawer, stamped so they cannot be mixed up, settle more arguments than any table: the one that goes on smoothly with no play is the one.

Frequently asked questions

What is the difference between BSP and NPT threads?

BSP is the British family, 55° (ISO 228-1 and ISO 7-1); NPT is the American one, 60° (ASME B1.20.1). They have different threads per inch except at 1/2″ and 3/4″, where both have 14, and the diameters differ by a few tenths of a millimeter. Do not mix them: the joint leaks under pressure.

What tap drill do I use for a 1/2 BSP thread?

19 mm for G 1/2 (minor diameter 18.631 mm) and 18.5 mm for Rp 1/2. For 1/2 NPT use 18.25 mm (23/32″). For a tapered Rc, a few tenths smaller plus a taper reamer.

Does BSP seal with PTFE tape?

R into Rp or Rc does: tape or paste fills the clearance between crests and roots and the taper tightens it. G never seals on the thread no matter how much tape you wrap: it needs a flat gasket, a bonded washer or an O-ring on the face.

Sources: ISO 228-1:2000 (G threads); ISO 7-1:1994 and EN 10226-1:2004 (Rp, R) and EN 10226-2 (Rc); ASME B1.20.1-2013 (NPT, NPSM); ASME B1.20.3 (NPTF Dryseal); DIN 2999 and BS 21 (withdrawn, quoted as legacy names); JIS B 0202 (PF) and JIS B 0203 (PT, PS).