UNC Thread Specifications
This table gives the basic thread dimensions for UNC (Unified National Coarse) threads per ASME B1.1. These are the numbers you need for thread design, tolerance calculation, and thread gauge verification.
| Size | TPI | Major Ø (in) | Pitch Ø (in) | Minor Ø (in) | Tensile Stress Area (in²) |
|---|---|---|---|---|---|
| #1-64 | 64 | 0.0730 | 0.0629 | 0.0527 | 0.00263 |
| #2-56 | 56 | 0.0860 | 0.0744 | 0.0628 | 0.00370 |
| #4-40 | 40 | 0.1120 | 0.0958 | 0.0795 | 0.00604 |
| #6-32 | 32 | 0.1380 | 0.1177 | 0.0974 | 0.00909 |
| #8-32 | 32 | 0.1640 | 0.1437 | 0.1234 | 0.01400 |
| #10-24 | 24 | 0.1900 | 0.1629 | 0.1359 | 0.01750 |
| 1/4-20 | 20 | 0.2500 | 0.2175 | 0.1850 | 0.0318 |
| 5/16-18 | 18 | 0.3125 | 0.2764 | 0.2403 | 0.0524 |
| 3/8-16 | 16 | 0.3750 | 0.3344 | 0.2938 | 0.0775 |
| 7/16-14 | 14 | 0.4375 | 0.3911 | 0.3447 | 0.1063 |
| 1/2-13 | 13 | 0.5000 | 0.4500 | 0.4001 | 0.1419 |
| 9/16-12 | 12 | 0.5625 | 0.5084 | 0.4542 | 0.182 |
| 5/8-11 | 11 | 0.6250 | 0.5660 | 0.5069 | 0.226 |
| 3/4-10 | 10 | 0.7500 | 0.6850 | 0.6201 | 0.334 |
| 7/8-9 | 9 | 0.8750 | 0.8028 | 0.7307 | 0.462 |
| 1-8 | 8 | 1.0000 | 0.9188 | 0.8376 | 0.606 |
| 1-1/4-7 | 7 | 1.2500 | 1.1572 | 1.0644 | 0.969 |
| 1-1/2-6 | 6 | 1.5000 | 1.3917 | 1.2835 | 1.405 |
How to Use This Table
- Tensile stress area — multiply by material’s tensile strength (in psi) to get the bolt’s proof load. Example: 1/2-13 Grade 8 bolt = 0.1419 in² × 150,000 psi = 21,285 lbs.
- Pitch diameter — this is what thread gauges measure. It’s the functional diameter that determines fit class.
- Minor diameter — the weakest cross-section of an external thread and the tap drill target for internal threads.
Related: UNC Tap Drill Sizes | UNF Thread Dimensions | Hex Bolt Grades & Properties