Bolt Torque Calculator
Find the tightening torque for a bolt from its size, grade, thread and lubrication — using T = K × F × D — with the resulting clamp load, the proof-load margin, and a realistic torque range, because friction scatter is real.
How to Calculate Bolt Torque
This bolt torque calculator works from the standard fastener equation used across SAE and metric design:
where T is tightening torque, K is the nut factor (a friction term), F is the clamp load or preload you want in the joint, and D is the nominal bolt diameter. The trick is that you rarely know F directly — so this clamp load calculator derives it from the bolt’s proof strength and tensile stress area, then multiplies through to give torque in in-lb, ft-lb and N·m at once.
Clamp load comes from the bolt’s grade. Proof load = proof strength × tensile stress area, and a sensible preload is a fraction of that. This is what makes a bolt preload calculator far more useful than a fixed torque chart: change the grade, thread or lubrication and the numbers move with you, for both SAE bolt torque and metric bolt torque.
What Is the Nut Factor (K)?
The nut factor K rolls all the friction in a joint — under the head, in the threads, and any plating or lubricant — into a single multiplier. It is the single biggest source of scatter in torque control, which is why this fastener torque calculator reports a torque range, not just one number. Typical values:
| Condition | Typical K |
|---|---|
| Dry steel | 0.20 |
| Zinc plated | 0.18 – 0.22 |
| Light oil | 0.15 |
| Anti-seize | 0.11 – 0.14 |
Lower friction means more clamp load for the same torque — anti-seize can put roughly 40% more tension in a bolt than a dry assembly at the same wrench setting, which is exactly how fasteners get over-tightened.
How Much Preload Should a Bolt Have?
For a reusable structural joint, target a preload of 60-75% of the bolt’s proof load. That keeps the joint tight against vibration and fatigue while leaving margin below yield. The calculator defaults to 65% and flags you if you push past 75% (near proof) or above 90% (risk of yielding).
Worked example
A 1/2-13 Grade 5 bolt, dry (K = 0.20), at 65% preload:
Tensile stress area = 0.1419 in² · proof load = 85,000 × 0.1419 ≈ 12,060 lb
Clamp load F = 0.65 × 12,060 ≈ 7,840 lb
T = 0.20 × 7,840 × 0.5 = 784 in-lb ≈ 65 ft-lb
Bolt Torque Reference Table
Approximate dry torque at 65% preload (your results above are exact for the inputs you choose):
| SAE bolt (Grade 5) | ft-lb | Metric (Class 8.8) | N·m |
|---|---|---|---|
| 1/4-20 | 7 | M8 | 23 |
| 3/8-16 | 27 | M10 | 45 |
| 1/2-13 | 65 | M12 | 79 |
| 5/8-11 | 130 | M16 | 196 |
| 3/4-10 | 231 | M20 | 383 |
Frequently Asked Questions
Why give a torque range instead of one value?
Because friction varies. The same wrench setting on a dry, oiled or plated bolt produces noticeably different clamp loads, so the range shows the realistic spread for your chosen condition.
Does lubrication change the torque?
Yes. A lubricated or anti-seize bolt needs less torque to reach the same clamp load. Use the matching lubrication setting, or your bolt will be over-tightened.
What is tensile stress area?
It is the effective cross-section that carries tension in a threaded bolt — smaller than the nominal area because of the threads. Proof and clamp loads are based on it.
Should I torque to a percentage of proof or yield?
Proof load is the standard reference for preload because it is the load a bolt can take with no permanent set. 60-75% of proof is the usual working target.
Related calculators
- Weld Strength Calculator — check the welds in the same connection.
- Factor of Safety Calculator — set the margin for your load case.
- Stress Calculator — check stress in the clamped members.
- Bolt Load Calculator — the strength side: tension, shear and combined capacity.
- Steel Weight & Material Weight calculators.