Bolt Torque Calculator
This bolt torque calculator finds the tightening torque needed to reach a target preload in a bolt, using the bolt torque formula T = K × F × d. Enter the nut factor, preload, and diameter, and the bolt torque calculator returns the torque in newton-meters, pound-feet, and pound-inches. Enter a torque instead and it works the other way, as a bolt tension calculator that returns the clamp load in N, kN, and lbf.
Results
The Bolt Torque Calculator is part of our Machinery collection. Browse more machinery to size, convert, and check your work alongside this tool.
What the bolt torque calculator does
The bolt torque calculator finds the wrench torque that produces the clamping force, or preload, a joint needs. A bolt works by clamping parts together, and this bolt torque calculator links the torque you apply to the tension you create using the bolt torque formula. It doubles as a bolt clamp force calculator, turning a target preload into a torque you can set.
Bolt torque formula
The bolt torque calculator uses the short-form bolt torque formula, also called the torque-tension equation:
K is the nut factor, F the preload, and d the bolt diameter. The nut factor bundles thread and head friction, which is why this bolt torque equation is sensitive to lubrication in any bolt preload calculation.
Bolt torque calculator example
A 12 mm bolt to a 10,000 N preload with a 0.2 nut factor, through the bolt torque formula:
That is about 17.7 lb·ft from the bolt torque calculation.
Reverse check: tighten the same 12 mm bolt to 24 N·m with K = 0.2 and the bolt clamp load calculator returns 24 ÷ (0.2 × 0.012) = 10,000 N, or 2,248 lbf, the preload you started from.
How to use the bolt torque calculator
To find the tightening torque, enter the nut factor K, the target preload in newtons, and the nominal bolt diameter in millimeters. The bolt torque calculator returns the torque in N·m, lb·ft, and lb·in, plus the preload in pounds force. To find the clamp load a wrench setting produces, enter the applied torque in the last field: the calculator switches to a torque to tension calculation and returns the bolt clamp load. Use K = 0.2 for plain dry steel, about 0.15 to 0.18 for zinc plated or lightly oiled bolts, and about 0.1 to 0.15 with anti-seize or moly paste.
Reading the bolt torque result
Apply the torque from the bolt torque calculator with a calibrated wrench. Most of the effort fights friction, not tension, so the nut factor dominates the result. Lubricate a bolt and the same torque gives more preload, which is why the bolt clamp force calculator needs the right K for your condition.
When to use the bolt torque calculator
- Setting a torque wrench for a target preload
- Running a bolt preload calculation for a joint
- Comparing dry and lubricated tightening torque
- Finding clamp force with the bolt clamp force calculator
Common bolt torque mistakes
- Wrong nut factor. Using dry K on a lubricated bolt over-tightens it; match K in the bolt torque calculator.
- Ignoring friction. Most torque fights friction, so condition matters more than people think.
- Uncalibrated wrench. The bolt torque formula result is only as good as the tool applying it.
Tips for accurate bolt torque results
- Pick the nut factor that matches the bolt finish and lubrication.
- Use a calibrated torque wrench for the bolt torque calculator figure.
- For critical joints, back up torque with angle or tension methods.
Why the nut factor rules the bolt torque formula
The surprising truth the bolt torque calculator teaches is that most of your effort never becomes clamping force. In a typical joint, around 90 percent of the tightening torque overcomes friction under the head and in the threads, and only about 10 percent stretches the bolt to create preload. This is why the nut factor dominates the bolt torque formula: a small change in friction shifts how much of your torque turns into tension, which any careful bolt preload calculation must respect.
Torque as a proxy for preload
What a joint really needs is preload, the clamping tension, but preload is hard to measure on the line, so torque stands in for it. The bolt torque calculator makes that substitution usable by converting a target preload into a torque a wrench can set, and the bolt clamp force calculator reverses it. The weakness is friction scatter, which is why critical joints use angle tightening or tension indicators. For everyday assembly, the bolt torque formula with the right nut factor gets preload close enough to keep joints tight and safe.
Reference Table
| Condition | Nut factor K |
|---|---|
| Plain steel, dry | ~0.20 |
| Zinc plated | ~0.22 |
| Lubricated | ~0.15 |
Bolt Torque Calculator FAQ
What is the bolt torque formula?
Torque equals nut factor times preload times bolt diameter (T = KFd). The bolt torque calculator applies this bolt torque formula and shows the result in newton-meters and pound-feet.
How do I calculate bolt clamp load from torque?
Divide the torque by the nut factor times the diameter in meters: F = T ÷ (K × d). Enter the torque in the applied torque field and the bolt torque calculator returns the clamp load in N, kN, and lbf. 24 N·m on a 12 mm bolt with K = 0.2 gives 10,000 N.
What is the nut factor?
It is a coefficient bundling thread and head friction, about 0.2 for plain steel. The bolt torque calculator uses it because most tightening torque overcomes friction, not tension.
Does lubrication change bolt torque?
Yes. Lubrication lowers the nut factor, so the same torque gives more preload. Set the right K in the bolt torque calculator for dry, plated, or lubricated bolts.