Unified thread dimensions
Major diameter, minor diameter, and tensile stress area for Unified coarse (UNC) threads. The tensile stress area is the figure to use when calculating how much load a bolt or threaded rod can carry.
UNC thread dimensions
| Size | TPI | Major dia (in) | Minor dia (in) | Stress area (in²) |
|---|---|---|---|---|
| #4-40 | 40 | 0.1120 | 0.0813 | 0.00604 |
| #6-32 | 32 | 0.1380 | 0.0997 | 0.00909 |
| #8-32 | 32 | 0.1640 | 0.1257 | 0.0140 |
| #10-24 | 24 | 0.1900 | 0.1389 | 0.0175 |
| 1/4-20 | 20 | 0.2500 | 0.1887 | 0.0318 |
| 5/16-18 | 18 | 0.3125 | 0.2443 | 0.0524 |
| 3/8-16 | 16 | 0.3750 | 0.2983 | 0.0775 |
| 7/16-14 | 14 | 0.4375 | 0.3499 | 0.1063 |
| 1/2-13 | 13 | 0.5000 | 0.4056 | 0.1419 |
| 5/8-11 | 11 | 0.6250 | 0.5135 | 0.226 |
| 3/4-10 | 10 | 0.7500 | 0.6273 | 0.334 |
| 7/8-9 | 9 | 0.8750 | 0.7387 | 0.462 |
| 1-8 | 8 | 1.0000 | 0.8466 | 0.606 |
Major diameter is the nominal size measured across the thread crests; minor diameter is across the roots. The tensile stress area is an effective area between the two, and multiplying it by the bolt proof or yield strength gives the load the fastener can carry. Use stress area, not nominal diameter, for strength calculations.
Need the tap drill or the fine-thread version?
See the Tap Drill Calculator for hole sizes, or the Thread Pitch Chart for UNF fine threads.
Why stress area matters
A threaded section is weaker than the solid rod it is cut from, because the threads remove material. The tensile stress area accounts for this by defining an effective diameter partway between the major and minor diameters. Bolt strength tables and torque formulas all use stress area, so picking it instead of the nominal size avoids overestimating capacity.
Major and minor diameters
The major diameter is the bolt nominal size, set by the crests of the thread. The minor diameter, across the thread roots, is what the tap drill produces in the mating part. The difference between them is the thread depth, and the average defines the pitch diameter where the threads actually mate.
FAQ
What is the tensile stress area of a 1/2-13 bolt?
About 0.1419 square inches. Multiply it by the bolt proof strength to find the working load.
Should I use major diameter or stress area for strength?
Use the tensile stress area. It is smaller than the area from the major diameter and reflects how the threads reduce the load-bearing cross-section.
What is the minor diameter of a thread?
The diameter across the thread roots, the smallest part of the threaded section. It roughly matches the tap drill size for the tapped hole.