NPT has a 60-degree thread angle and tapers. BSP has a 55-degree thread angle and comes in parallel (BSPP/G) and taper (BSPT/R) variants. Metric threads have a 60-degree angle with pitch measured in millimetres. The fastest way to tell them apart: measure the major diameter and pitch, then check whether the thread tapers. Use a thread identification tool to cross-reference.
Three systems, overlapping sizes, maximum confusion
Industrial fittings, connectors, and enclosures use three different thread systems depending on region and application. North America uses NPT. Much of the world uses BSP. Continental European equipment often uses metric threads. And plenty of products show up with no markings, leaving you with a caliper and a thread gauge.
Getting the thread type wrong in a product catalog means customers order fittings that do not mate. A 1/2" NPT plug will not seal in a 1/2" BSP socket, even though both are nominally "half inch." The thread angles, pitch, and taper differ.
To identify a thread from measurements, look up your thread size in the cross-reference tool.
The key differences
| Feature | NPT | BSP (parallel/BSPP) | BSP (taper/BSPT) | Metric |
|---|---|---|---|---|
| Standard | ASME B1.20.1 | ISO 228 / BS EN 10226 | ISO 7 / BS EN 10226 | ISO 261 |
| Thread angle | 60 degrees | 55 degrees | 55 degrees | 60 degrees |
| Taper | Yes (1:16) | No (parallel) | Yes (1:16) | No (parallel) |
| Pitch measure | TPI (threads per inch) | TPI (threads per inch) | TPI (threads per inch) | mm (pitch in millimetres) |
| Size designation | Nominal pipe size (NPS) | Nominal pipe size (G/R) | Nominal pipe size (R) | Diameter x pitch (e.g., M20x1.5) |
| Sealing | Thread tape/compound | O-ring/washer on face | Thread tape/compound | Washer or O-ring |
| Region | North America | Global (UK origin) | Global (UK origin) | Continental Europe, Asia |
Thread angle: 55 vs 60 degrees
The thread angle is the most fundamental difference between BSP and NPT/Metric.
BSP threads use a 55-degree Whitworth profile. The crests and roots of the thread form a 55-degree angle. This is true for both parallel (BSPP) and taper (BSPT) variants.
NPT and Metric threads both use a 60-degree angle. This means NPT and Metric threads are closer to each other in profile than either is to BSP. However, NPT tapers while Metric is parallel, and they use completely different size designations.
You can identify the thread angle with a thread gauge or profile gauge. Hold the gauge against the thread. A BSP gauge fits BSP threads. An NPT gauge fits NPT and some Metric threads. If neither fits perfectly, measure the angle with a thread comparator.
Taper vs parallel
Tapered threads (NPT and BSPT) seal by wedging the male thread into the female thread. As you tighten, the taper creates metal-to-metal contact that (with tape or compound) forms a pressure-tight seal. You can identify a taper by measuring the diameter at two points along the thread. If the diameter decreases toward the end of the fitting, it is tapered.
Parallel threads (BSPP and Metric) have the same diameter along the full thread length. They do not seal on the thread itself. Sealing comes from an O-ring, washer, or face seal at the joint. Parallel threads engage fully and can be tightened to a defined torque without over-compression.
Quick taper test: try to rock a straight edge along the thread. If the thread surface curves away from the edge, it is tapered.
Size designations
Each system names its sizes differently, which creates confusion when the numbers are similar.
NPT uses nominal pipe size (NPS) in inches. NPS does not equal the actual thread diameter. A 1/2" NPT thread has a major diameter of approximately 0.840 inches (21.3mm), not 0.5 inches.
BSP also uses nominal pipe size in inches, but the dimensions differ from NPT. A 1/2" BSP thread has a major diameter of approximately 0.825 inches (20.95mm). Close to NPT, but not interchangeable.
Metric uses the actual diameter and pitch: M20x1.5 means 20mm major diameter with 1.5mm pitch. No nominal sizing confusion.
This is where cross-referencing matters. A fitting marked "1/2" could be NPT or BSP. Measuring the actual diameter and pitch resolves the ambiguity. Look up your thread from diameter and pitch to determine the exact standard.
Identification by measurement
Step 1: Measure the major diameter
Use calipers to measure the outside diameter of the male thread (or the inside diameter of the female thread). Measure at the first full thread, not at the entry chamfer.
Step 2: Measure the pitch
For inch threads (NPT/BSP): Count the number of threads in one inch of length using a thread pitch gauge or ruler. This gives you TPI (threads per inch).
For metric threads: Measure the distance between two adjacent thread crests in millimetres. Or use a metric pitch gauge.
Step 3: Check for taper
Measure the major diameter at two points separated by about 15mm. If the diameter changes, the thread tapers. If it stays the same, it is parallel.
Step 4: Cross-reference
With diameter, pitch (TPI or mm), and taper/parallel, you can identify the thread:
| Diameter ~21.3mm, 14 TPI, tapered | = 1/2" NPT |
|---|---|
| Diameter ~20.95mm, 14 TPI, parallel | = 1/2" BSP parallel (G1/2) |
| Diameter ~20.95mm, 14 TPI, tapered | = 1/2" BSP taper (R1/2) |
| Diameter ~20.0mm, 1.5mm pitch, parallel | = M20x1.5 |
Note that 1/2" NPT and 1/2" BSP both have 14 TPI. The diameter difference (21.3mm vs 20.95mm) is only 0.35mm. This is why caliper measurement alone is sometimes ambiguous, and a thread gauge set is the definitive tool.
Worked example: identifying a cable gland thread
You receive a batch of cable glands from a supplier with no markings. You need to determine the thread type to list them correctly.
Measurements:
- Major diameter: 20.0mm
- Pitch: 1.5mm (measured with metric pitch gauge)
- No taper (diameter constant along thread length)
Cross-reference: 20mm diameter, 1.5mm pitch, parallel = M20x1.5
This is a standard metric cable gland thread, very common in European electrical enclosures. The thread mates with an M20 tapped hole in a junction box or control panel.
After identifying the thread, verify the match in the cross-reference tool. Enter the diameter and pitch to confirm the standard and find nearest equivalents in other systems.
When each system appears
NPT dominates in:
- US and Canadian plumbing and gas systems
- US-manufactured process control equipment
- Hydraulic fittings in North American machinery
- Oil and gas industry (globally, by convention)
BSP dominates in:
- UK, Australia, and former Commonwealth countries
- European plumbing (some countries)
- Japanese industrial equipment (JIS pipe threads are BSP-compatible)
- Fire protection systems (many countries)
Metric dominates in:
- European electrical enclosures (cable glands, conduit fittings)
- Automotive fasteners globally
- General-purpose fasteners in metric-system countries
- Machine tool fittings
Catalog data requirements
When listing threaded products in a catalog, include:
- Thread system (NPT / BSP / Metric / UNF / UNC)
- Nominal size (1/2" / M20)
- Pitch (14 TPI / 1.5mm)
- Type (taper / parallel)
- Standard reference (ASME B1.20.1 / ISO 228 / ISO 261)
Omitting the thread system forces the customer to guess. Listing "1/2 inch" without specifying NPT or BSP is ambiguous. Listing "M20" without the pitch is incomplete (M20x1.5 and M20x2.5 are different threads with different applications).
For a deeper walkthrough of the measurement and identification process, see how to identify a thread from diameter and pitch.