Different industries and regions use different thread standards. This guide summarises the six most common systems — their origin, pitch system, and typical applications — so you can identify an unknown thread and pick the right tap, die or fitting.
Thread standards comparison
| Standard | Origin | Pitch system | Profile angle | Typical use |
|---|---|---|---|---|
| Metric (M) | ISO 261 / DIN | Pitch in mm | 60° | General engineering worldwide; the most common standard for fasteners and machine parts |
| UNC | ASME B1.1 (USA) | TPI (coarse) | 60° | General-purpose fasteners in the US, aerospace, automotive. Equivalent to metric coarse |
| UNF | ASME B1.1 (USA) | TPI (fine) | 60° | Fine-thread applications: precision instruments, thin-walled parts, vibration environments |
| G / BSPP | ISO 228 (UK/EU) | TPI, cylindrical | 55° | Hydraulic and pneumatic fittings, fluid connectors. Parallel (non-sealing) pipe thread |
| R / BSPT | ISO 7 (UK/EU) | TPI, tapered 1:16 | 55° | Water, gas and steam pipework in Europe and Asia. Self-sealing taper thread |
| NPT | ASME B1.20.1 (USA) | TPI, tapered 1:16 | 60° | Pipe fittings, valves and plumbing in North America. Self-sealing taper thread |
Metric threads (ISO 261)
The ISO metric thread is designated by the letter M followed by the nominal diameter in mm and, optionally, the pitch: M10 (coarse, 1.5 mm pitch) or M10×1.25 (fine). Coarse pitch is assumed when no pitch is stated. The thread profile angle is 60° and the root/crest are rounded per ISO 68-1. Used everywhere from M1 watch screws to M100 anchor bolts.
UNC and UNF (ASME B1.1)
The Unified National thread family uses the same 60° profile as metric but expresses pitch as threads per inch (TPI). UNC (Unified National Coarse) is the standard for general fasteners; UNF (fine) has more threads per inch and is used where greater holding strength or fine adjustment is needed. A UNC bolt is not interchangeable with a metric bolt of similar size.
G / BSPP (ISO 228)
The British Standard Pipe Parallel thread (G) has a 55° Whitworth profile and a cylindrical (non-tapering) form. Because it does not seal on its own, G fittings require a bonded seal (O-ring face seal or soft washer) or sealant. Sizes are named by nominal bore, not actual diameter: G 1/2 has an actual OD of about 20.96 mm.
R / BSPT (ISO 7)
The British Standard Pipe Taper thread (Rp for parallel internal, Rc for tapered internal, R for external taper) uses the same 55° profile as G but tapers 1:16 along the axis. The taper creates a mechanical seal when assembled tightly; PTFE tape or thread sealant completes the pressure seal. Common in European and Asian hydraulic and gas systems.
NPT (ASME B1.20.1)
The American National Pipe Taper thread has a 60° profile and a 1:16 taper (same angle as BSPT, but a different profile angle, so they are not interchangeable). NPT is the standard for pipe fittings, valves and instrumentation in North America. NPTF (Dryseal) is a variant that seals on the thread form alone without sealant.
How to identify an unknown thread
- Measure the outer diameter with calipers. If it matches a metric M designation (e.g. 10.0 mm ≈ M10) — likely metric.
- Count threads per inch with a thread gauge or ruler. Whole-number TPI → probably UNC/UNF or pipe.
- Check the thread angle: 60° = metric/UN/NPT; 55° = BSP.
- Check taper: parallel threads feel uniform along the shank; tapered threads visibly narrow towards the end.
- Use the Thread Identifier & Reference app for quick cross-reference across 27 standards.