Thread Types Explained
UNC, UNF, metric coarse and fine, and BSW thread types—when to use each and how to identify them.
Common thread types
Thread standards define diameter, pitch, and profile. UNC (Unified National Coarse) and UNF (Unified National Fine) are imperial. Metric uses ISO designations (e.g. M8 x 1.25). BSW (British Standard Whitworth) is another imperial system. Use our thread identifier and pitch to TPI converter to match measured values to these standards.
Thread type comparison chart
| Type | System | Typical use |
|---|---|---|
| UNC | Imperial (US) | General purpose, coarse |
| UNF | Imperial (US) | Automotive, fine thread |
| Metric (coarse) | ISO | Default metric, most applications |
| Metric (fine) | ISO | Thin sections, precision |
| BSW | British | Legacy UK equipment |
Coarse vs fine thread
Coarse threads have larger pitch (fewer threads per inch or larger mm pitch). They're stronger in soft materials and quicker to assemble. Fine threads give more threads per inch (or smaller mm pitch), better in thin sections and where precision or fine adjustment matters. Check screw size chart and tap drill chart for correct hole sizes.
FAQ
What is UNC vs UNF thread?
UNC is coarse (fewer TPI); UNF is fine (more TPI). UNF is common in automotive and precision applications.
What thread type does metric use?
Metric uses ISO threads: diameter and pitch in mm (e.g. M8 x 1.25). Coarse is default; fine is specified when needed.
When do I use coarse vs fine thread?
Coarse for general use and soft materials; fine for thin sections, higher strength in shear, or fine adjustment.