Fasteners are an essential component of modern manufacturing and assembly, and understanding the different thread standards is crucial for engineers, buyers, and production teams working in global supply chains.
In this post, we’re presenting a handy thread size conversion chart that helps translate metric screw sizes to their approximate UNC, UNF, BSW (British Standard Whitworth), and BSF (British Standard Fine) counterparts.
This is especially useful when:
- Substituting fasteners across systems (e.g., metric vs imperial).
- Working with international suppliers or customers.
- Repairing older machines with different thread standards.
- Avoiding delays due to unavailability of exact matches.
⚠ Note: These conversions are approximate and are not always direct replacements. Always double-check fitment, pitch, and mechanical requirements before substituting fasteners.
📊 Thread Size Conversion Table
| Metric Coarse | Metric Fine | BSF/BSW | UNC | UNF |
|---|---|---|---|---|
| M2 × 0.4 | – | – | 1-64 | 1-72 |
| M2.5 × 0.45 | – | – | 3-48 | 3-56 |
| M3 × 0.5 | – | – | 4-40 | 4-48 |
| M3.5 × 0.6 | – | – | 6-32 | 6-40 |
| M4 × 0.7 | – | – | 8-32 | 8-36 |
| M5 × 0.8 | – | – | 10-24 | 10-32 |
| M6 × 1 | M6 × 0.75 | 1/4″ | 1/4-20 | 1/4-28 |
| M8 × 1.25 | M8 × 1 | 5/16″ | 5/16-18 | 5/16-24 |
| M10 × 1.5 | M10 × 1.25 | 3/8″ | 3/8-16 | 3/8-24 |
| M12 × 1.75 | M12 × 1.25 | 1/2″ | 1/2-13 | 1/2-20 |
| M14 × 2 | M14 × 1.5 | 9/16″ | 9/16-12 | 9/16-18 |
| M16 × 2 | M16 × 1.5 | 5/8″ | 5/8-11 | 5/8-18 |
| M20 × 2.5 | M20 × 1.5 | 3/4″ | 3/4-10 | 3/4-16 |
| M24 × 3 | M24 × 2 | 1″ | 1-8 | 1-12 |
| M30 × 3.5 | M30 × 2 | 1-1/8″ | 1 1/4-7 | 1 1/8-12 |
| M36 × 4 | M36 × 3 | 1-3/8″ | 1 1/2-6 | 1 1/4-12 |
🧠 Tips When Converting Thread Sizes
- Pitch matters: Even if the diameters are close, thread pitch differences can cause poor engagement or damage.
- Tapping vs. threading: When replacing metric with imperial, confirm if internal threads need to be re-tapped.
- Strength ratings: UNC/UNF fasteners may have different tensile strengths than metric bolts of the same diameter.
- Tolerance & fit: Always verify the mechanical and dimensional fit with calipers or gauges.
📌 Final Thoughts
As global sourcing and servicing become the norm, having a reliable fastener thread conversion chart can be a lifesaver. Bookmark this guide or print it out for your workshop or engineering team. If you’re a buyer, fabricator, or OEM working with fasteners, consider maintaining a ready reckoner for such cross-references in your ERP or inventory system.
Need help sourcing equivalent fasteners or verifying thread sizes for your application? Contact Horizon Inc. – your reliable partner for industrial fasteners and fastening solutions.