NPT vs BSPT Pipe Threads
NPT and BSPT are different pipe-thread systems. NPT uses a 60° thread form; BSP uses 55°. Both NPT and BSPT are tapered, and some sizes share the same thread count. Do not order or install from nominal size or a partial hand fit alone. Identify the thread and the sealing connection together.
In this guide
Read this comparison first
Thread shape, taper, size and seal location must point to the same connection system.
| System | Recognition facts | Connection question |
|---|---|---|
| NPT | US tapered pipe thread; 60° form | Which specified thread sealant and installation procedure apply? |
| BSPT / ISO 7 family | 55° form; R identifies a tapered external thread | Which specified mating thread and jointing method apply? |
| BSPP / G, ISO 228 family | Parallel thread; 55° form | Which washer, O-ring, cone or other specified seat makes the seal? |
NPT means National Pipe Taper. BSPT means British Standard Pipe Taper; BSPP means British Standard Pipe Parallel. British threads are not metric merely because the equipment came from overseas. These pipe families use threads per inch.
Within ISO 7 terminology, R denotes the tapered external thread, Rc a tapered internal thread, and Rp a parallel internal thread. That Rp connection is not the same identification as a G/BSPP connection under ISO 228. Read the actual callout before reducing it to “British pipe.”
The fraction is a pipe size name
A nominal 1/4-inch pipe thread does not measure 0.250 inch across its outer crests.
This is where a bolt-sizing habit causes an expensive mistake. On a 1/4-20 machine bolt, 1/4 describes the nominal outside thread diameter. On a 1/4 NPT fitting, 1/4 is a nominal pipe size. Use the thread-system identification chart for the fitting.
Record the outside measurement of a male fitting and where it was taken. A tapered thread changes diameter along its length, so two measurements from different positions can differ without either tool being wrong. A female opening is another measurement again. A caliper reading is evidence to compare with the correct chart, not a standalone size declaration.
Keep schedule separate too. Pipe schedule describes a wall-thickness series for a given nominal pipe size; it is not a thread-count label.
Use pitch to narrow the choice, then check the form
A different thread count can reject a candidate. The same thread count cannot always confirm it.
| Nominal pipe size | NPT threads per inch | BSPT threads per inch | Practical consequence |
|---|---|---|---|
| 1/8 | 27 | 28 | Close by eye; use the proper gauge |
| 1/4 | 18 | 19 | Diameter resemblance does not cancel the pitch difference |
| 3/8 | 18 | 19 | Keep both diameter and pitch evidence |
| 1/2 | 14 | 14 | Count alone cannot identify the family |
| 3/4 | 14 | 14 | Count alone cannot identify the family |
Swagelok identifies the 1/2- and 3/4-inch NPT/BSPT cases as particularly close and notes that positive identification may require an optical comparator. Do not estimate 55° versus 60° from a phone photo of a worn thread. A proper gauge, drawing, product record, or qualified inspection can resolve what a ruler cannot.
A fitting starting in a port does not prove correct engagement or a sound pressure seal. Do not turn a partial fit into an acceptance test by adding wrench force.
Find the surface that actually stops the fluid
The thread may make the seal, or it may hold a separate sealing surface together.
Inspect the complete connection: the fitting, port, seat, shoulder, washer, O-ring, and old seal evidence. A parallel threaded connection may rely on a bonded washer, O-ring, cone, or other seat. The thread size alone does not identify that seat or the complete mating connection.
Worked example: a regulator port is called G 1/4 and the old fitting has a bonded washer beneath its shoulder. The useful request preserves G 1/4 and the washer/face arrangement. Buying a tapered 1/4 NPT fitting because the fraction matches loses both the thread and seal information.
A second example: a fitting measures close to the chart range for nominal 1/2 inch, has taper, and matches a 14 TPI leaf. That still leaves the NPT/BSPT question open. Record “tapered, 14 TPI, family not confirmed” rather than ordering one from habit.
Build a connection request that preserves uncertainty
Send both ends, both mating sides when available, and the service conditions.
| Evidence | What to send |
|---|---|
| Identity | Equipment make/model, fitting markings, drawing or original part number |
| Geometry | Male/female, measured diameter and position, pitch-gauge result, threaded length, taper evidence |
| Seal | Port face, cone/seat, shoulder, washer or O-ring photos and dimensions if known |
| Service | Fluid or gas, pressure, temperature, material and applicable site requirements |
| Procurement | Quantity, due date, proposed adapter if any, and details still unknown |
An adapter may connect two different systems, but each end and its rating must match the real service. Do not treat a catalog adapter as approval for a gas, steam, hydraulic, chemical, potable-water or compressed-air installation.
Only inspect a system made safe under its isolation and depressurization procedure. A damaged port, early binding, unidentified seal, or unknown rating is a reason to stop and refer the repair to the responsible qualified person. Extra tape or sealant cannot correct mismatched thread geometry.
Frequently Asked Questions
Can BSPT and NPT both be 14 TPI?
Yes. The common nominal 1/2- and 3/4-inch sizes share 14 TPI, while their thread form and connection requirements differ.
Is BSPP the same as a parallel ISO 7 internal thread?
Do not use those descriptions interchangeably. G/BSPP under ISO 228 and Rp under ISO 7 are different callouts with different connection context. Read the full specification.
Is NPTF just another name for NPT?
No. NPTF is a dryseal thread system with different crest/root requirements. Similar size notation does not establish interchangeable sealing behavior.
Sources and Review Boundaries
Published by GO4FORGE. Revised 2026-10-29. Editorial method. These notes help narrow sourcing evidence and selection risk; they do not replace the responsible site authority, OEM procedure, code requirement, or qualified engineering/safety review.
- GO4FORGE product knowledge files Internal product-knowledge references distilled into public decision guides.
- GO4FORGE RFQ evidence model Shop-floor evidence patterns for quotes, substitutions, and sourcing review.
- Swagelok: thread and end connection identification guide Thread-system identification, count comparisons and the close 1/2- and 3/4-inch cases. Identification values are not inspection limits.
- Parker: industrial tube fitting port dimensions ISO 7 Rp parallel internal ports and Rc tapered internal ports are distinct from the external R designation.
- GO4FORGE pipe fittings and service fittings knowledgebases Nominal sizing, seal-location evidence and pressure-service review boundaries.
- ASME B1.20.1: Pipe Threads, General Purpose (Inch)
- ASME B1.20.3: Dryseal Pipe Threads (Inch)
- ISO 7-1: Pipe threads where pressure-tight joints are made on the threads
- ISO 228-1: Pipe threads where pressure-tight joints are not made on the threads
- ASME B36.10 / B36.19 pipe dimension references
- SAE AS568 O-ring size reference
- ISO 3601 fluid-power O-ring reference
- Material compatibility data sheets from seal manufacturers