The torque equation
Proof strength is 120 ksi for Grade 8, 85 ksi for Grade 5 (1/4–1 in) and 55 ksi for Grade 2 (1/4–3/4 in, 33 ksi above).
Worked example
A 1/2-13 Grade 5 bolt, dry, at 75% of proof: As = 0.1419 in², F = 0.75 × 85,000 × 0.1419 = 9,046 lbf, T = 0.20 × 0.5 × 9,046 = 905 in·lbf, or 75 ft·lbf.
Why the nut factor matters so much
Roughly 90% of applied torque goes into overcoming friction under the head and in the threads. Only about 10% stretches the bolt. Changing from dry to oiled threads drops K from about 0.20 to 0.15, which means the same torque produces 33% more clamp load. Torquing a lubricated bolt to a dry spec can yield it.
| Condition | Typical K |
|---|---|
| Plain steel, dry | 0.20 |
| Zinc plated | 0.20–0.22 |
| Lightly oiled | 0.15–0.18 |
| Moly or anti-seize | 0.10–0.12 |
Frequently asked questions
- What is the torque for a 1/2 inch Grade 8 bolt?
- About 106 ft·lbf dry at 75% of proof load (K = 0.20), or about 80 ft·lbf lubricated (K = 0.15).
- Should I use the published torque chart or this calculator?
- If the equipment manufacturer gives a torque, use it. This calculator is for joints without a spec, and it lets you adjust for your actual thread condition.
- Does this work for structural A325 or A490 bolts?
- No. Structural bolts are installed by turn-of-nut, calibrated wrench or tension-indicating methods under the RCSC specification, not by a torque chart.
Quick answers
- 1/2-13 Grade 5 bolt torque
- 3/8-16 Grade 5 bolt torque
- 5/8-11 Grade 5 bolt torque
- 1/2-13 Grade 8 bolt torque
- 3/4-10 Grade 5 bolt torque
- 3/8-16 Grade 8 bolt torque
Results are estimates for planning and education. Verify against the current code edition your jurisdiction has adopted, manufacturer data and a licensed professional before you build or install.