What this calculator does
Tightening a bolt to a torque figure is really a proxy for achieving a target clamping force, the preload that holds two parts together. This bolt torque calculator uses the standard relationship T = K x D x F, where T is torque, K is a friction-dependent nut factor, D is the bolt diameter and F is the desired clamping force.
The K-factor is the part most people get wrong, because it is treated as a constant when it actually depends on lubrication, plating and thread condition. A dry, plain steel bolt sits around K = 0.2, while a lubricated or plated bolt can be noticeably lower. That is why the K-factor here is a free input rather than a fixed number: the right value depends on the actual bolt in hand, not a rule of thumb baked into the formula.
The formula
Multiply the K-factor by the bolt diameter (converted to metres) and by the desired clamping force in newtons. The result is the torque in newton-metres needed to reach that clamping force with that bolt and that friction condition.
| Term | Meaning |
|---|---|
| T | Torque: the tightening force applied to the bolt head or nut, in newton-metres. |
| K | The nut factor, a dimensionless number describing friction under the bolt head, in the threads and between mating surfaces. |
| D | The nominal diameter of the bolt. |
| F | The desired clamping force, also called preload, holding the joint together. |
The inputs explained
| Field | What to enter |
|---|---|
| Bolt diameter (mm) | The nominal bolt diameter, for example 10 for an M10 bolt. |
| Desired clamping force (preload) (N) | The clamping force you want the bolt to achieve once tightened. |
| K-factor (nut factor) | The nut factor. About 0.2 suits dry, non-plated steel. Lower it for lubricated or plated threads, higher for rougher or dry-galled surfaces. |
When to use it
Setting a torque wrench for an engineering joint
A specification usually gives a target clamping force or preload rather than a torque figure directly. Converting it to torque with the actual K-factor for the fastener condition gives the number to dial into the wrench.
Adjusting for a lubricated bolt
A manufacturer torque spec assumes a particular friction condition. If the bolts are being installed with thread lubricant instead of dry, lowering the K-factor and recalculating shows the reduced torque actually needed for the same clamp load.
Comparing bolt sizes for the same clamp load
A larger diameter bolt reaches the same clamping force at a different torque than a smaller one, since torque scales directly with diameter in this formula.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
Torque needed across bolt diameters for the same clamping force
The same target clamp load applied across a range of common bolt diameters.
| Bolt diameter | Torque required | In pound-feet |
|---|---|---|
| M6 | 18.00 N·m | 13.28 lb-ft |
| M8 | 24.00 N·m | 17.70 lb-ft |
| M10 | 30.00 N·m | 22.13 lb-ft |
| M12 | 36.00 N·m | 26.55 lb-ft |
| M16 | 48.00 N·m | 35.40 lb-ft |
| M20 | 60.00 N·m | 44.25 lb-ft |
Torque needed across K-factors for an M10 bolt
The same bolt and clamp load, shown across a range of K-factors from well-lubricated to dry and rough.
| K-factor | Torque required | In pound-feet |
|---|---|---|
| 0.12 | 18.00 N·m | 13.28 lb-ft |
| 0.15 | 22.50 N·m | 16.60 lb-ft |
| 0.18 | 27.00 N·m | 19.91 lb-ft |
| 0.20 | 30.00 N·m | 22.13 lb-ft |
| 0.24 | 36.00 N·m | 26.55 lb-ft |
| 0.30 | 45.00 N·m | 33.19 lb-ft |
Questions
Why is K-factor not just a fixed number?
Because friction under the bolt head, in the threads and between the mating surfaces varies with lubrication, plating and surface condition. Using the wrong K-factor for the actual bolt can leave a joint significantly under or over its intended clamping force even at the "correct" torque.
What K-factor should I use if I am not sure?
About 0.2 is the commonly cited figure for dry, non-plated steel bolts and is a reasonable default. For anything safety-critical, use the value from the fastener manufacturer or a torque-tension test rather than a generic assumption.
Is torque a direct measurement of clamping force?
No, it is an indirect proxy. Two identical-looking bolts tightened to the same torque can end up at different clamping forces if their friction conditions differ, which is exactly what the K-factor is accounting for.
How is this different from the general torque calculator?
The general torque calculator finds turning force from an applied force and lever arm. This one works the other way for threaded fasteners specifically, starting from a target clamping force and working out the tightening torque needed to reach it.
For turning force and rotational power more generally, see the torque calculator.