In global engineering projects, petrochemicals, gas transmission and distribution, fire protection, and HVAC installation, “inconsistent size system” is often one of the problems that are most likely to cause on-site mismatching, leakage,e and even safety accidents. Especially when the design drawings come from different countries, or when the procurement process introduces multi-regional supply chains, the differences between the metric and imperial (Imperial / Inch) pipe fitting systems must be clarified in advance. This article will systematically sort out the size identification, thread form, adaptation method, and how to correctly select in cross-system projects. At the same time, it will introduce international supporting product solutions such as NPT thread fittings and BSPT threaded fittings provided by our company.
1. Basic concepts: metric size vs. imperial nominal size
The metric system is usually expressed in millimeters (mm), and DN (nominal diameter) will appear at the same time on many industrial occasions. In plastic, copper, stainless steel, and some international standard pipes, the nominal size is often close to or directly equal to the outer diameter (O/D) of the pipe, so the measurement is intuitive.
The imperial system is often expressed in NPS (Nominal Pipe Size) or “× inches”. But the “½”, 1″, and 2″ here are not the actual outer diameters that can be directly measured in most cases, but the “nominal diameters” used historically. With the emergence of wall thickness series, the actual inner diameters continue to change, and the nominal size gradually becomes the “series number”. This is why the ½” pipe is not 12.7mm outer diameter. The actual ½” NPS steel pipe outer diameter is about 21.3mm, and the 1″ NPS outer diameter is about 33.4-33.7mm. The size table must be checked when designing or modifying.
2. Why can’t metric pipes be directly inserted into imperial pipe fittings?
Many engineers think that “20mm ≈ ½″”, but they can’t screw it on or the seal fails on site. The reasons are:
- The outer diameter of a metric 20mm pipe is about 20mm;
- The outer diameter of a ½″ NPS steel pipe is about 21.3mm;
- The thread taper, pitch, and thread angle are completely different.
- The sealing surface design of the pipe fittings does not match, which is prone to leakage.
When connecting across the system, a special conversion joint (metric-to-imperial adaptor) must be used. Or flange/transition joint must be used to re-match the interface.
3. Thread system: NPT and BSPT are the biggest “pitfalls”
Finding out the size is only the first step; what makes engineers “turn over” is the incompatibility of thread standards.
①. NPT thread fitting (American standard taper pipe thread)
- Full name: National Pipe Taper (ASME B1.20.1).
- Thread angle 60°.
- The thread is tapered (1:16), and the sealing is achieved by interference fit + sealing material (PTFE raw tape, sealing paste).
- Widely used in North American oil and gas, chemical, gas, and industrial equipment pipelines.
- When measuring the size, the position of the mid-diameter taper line (L1 gauge) is usually referenced.
②. BSPT threaded fittings (British standard taper pipe thread)
- Corresponds to ISO 7-1 / EN 10226 / BS 21 series historical system.
- Thread angle 55° Whitworth.
- Also contains a taper (1:16), but the thread profile is different from NPT; the sealing characteristics and regulatory length are also different.
- Widely used in some markets in Europe, the Commonwealth, and Asia (fire protection, gas, and general mechanical pipelines).
Warning: NPT and BSPT cannot be directly mixed. It can be screwed together in a small number of scenarios, but the sealing surface does not match, and the risk of residual leakage is extremely high. It is more dangerous under pressure cycling or temperature fluctuations.
4. Common comparison of size, thread, and marking
| Item | Metric examples | Imperial examples | Description |
| Nominal method | 20mm, 25mm, DN20 | ½″, 1″, NPS 1 | The British system requires a table to know the actual outer diameter |
| Thread type | ISO metric thread (rarely used for pipe sealing) | NPT, BSPT, BSPP | Pipeline sealing is mainly based on tapered threads |
| Thread angle | 60° (metric thread) | 60° (NPT); 55° (BSPT) | Different thread types cannot be mixed |
| Sealing method | Flange, welding, O-ring | Interference cone seal + sealant | Different systems have different sealing strategies |
5. How to match cross-system projects?
Steps recommended:
- Confirm the source of the drawing: Europe and the United States → possibly imperial; Made in Asia → mostly metric.
- Measure the actual outer diameter (caliper), don’t believe the nominal value.
- Check the thread tooth type (pitch gauge, tooth type gauge): 60° or 55°?
- Pressure and media requirements determine the sealing method: dry seal, sealant, metal face seal?
- If necessary, use metric-imperial conversion joints or switch to flange connection.
6. International threaded pipe fitting solutions provided by our company
Jianzhi has been focusing on the production and export of threaded pipe fittings for many years, serving the gas, oil, fire protection, HVAC and general industrial supporting markets. We understand the pain point of “inconsistent metric/imperial/thread standards” in global projects, so we focus on developing the following series:
- NPT thread fitting series: suitable for North American and multi-national process lines, with a tooth angle of 60° and strict control of taper tolerance; options include black pipe fittings, galvanized malleable cast iron, and corrosion-resistant materials.
- BSPT threaded fittings series: in line with ISO 7-1/EN 10226 requirements, 55° tooth profile, ensuring compatibility with common systems in Europe and the Commonwealth; multiple specifications of elbows, tees, unions, and internal and external thread adapters are complete.
- Cross-system conversion parts: for metric pipe outer diameter → imperial NPT/BSPT thread interface, support OEM/ODM; welcome to provide a project size table.
- Inspection and traceability: 100% sampling inspection of thread gauges; batch report of zinc coating thickness/adhesion can be provided; can be implemented according to ASME B16.3, ISO49, customer company standards.
If you encounter problems with mixed metric/imperial sizes, NPT and BSPT conversion, or gas system sealing in your project, please contact us for technical support and quotation.
7. Practical case: 20mm pipe vs ½″ NPT fittings
a It was measured that 20mm O/D plastic pipe cannot directly enter ½″ NPT metal internal thread;
Even if the outer diameter is close, it cannot be sealed due to different thread types.
Correct approach: use 20mm ferrule or welding adapter → NPT thread adapter;
For gas, pressure, or fire protection systems, a pressure test must be performed before commissioning.
8. Quick checklist (on-site check before installation)
| Inspection items | Question | How to confirm |
| Dimension system | According to the standard, NPT uses PTFE; BSPT uses common thread sealant | Check the material list; measure the outer diameter with a caliper |
| Thread type | 60° or 55°? | Observe with a thread gauge/magnifying glass |
| Sealing method | Dry seal or sealant? | Low pressure water or high-pressure gas? |
| Pressure level | Low-pressure water or high-pressure gas? | Check the design pressure and material grade |
| Material compatibility | Carbon steel, malleable cast iron, galvanized? | Select based on the medium and environment |
Conclusion
Metric dimensions are usually based on actual outer diameters; imperial dimensions are mostly nominal dimensions (need to check the table).
Thread incompatibility is the biggest risk point: NPT (60°) ≠ BSPT (55°).
Mixed-use projects must use conversion joints and perform pressure tests.
For international projects, it is recommended to check the dimension system with the supplier in advance to avoid installation rework.