Bolt Torque Calculator
Calculate the installation torque needed to reach a target bolt clamp force, using the standard torque-tension formula.
How this calculator works
Torque is the practical way to control how tightly a bolt clamps two parts together, since clamp force itself is hard to measure directly. This calculator uses the standard torque-tension formula to work out how much torque, in foot-pounds, is needed to reach a target clamp force for a given bolt diameter.
The formula factors in the nut factor K, which accounts for friction under the bolt head and in the threads. Because the raw result comes out in inch-pounds, it’s divided by 12 to convert to the more commonly used foot-pounds.
Formula: Torque = nut factor K × bolt diameter × target clamp force, converted from inch-pounds to foot-pounds by dividing by 12. K (the nut factor, accounting for thread and under-head friction) is commonly about 0.2 for lubricated or plated steel fasteners, higher (0.2-0.3) for dry/unlubricated.
Worked example
A 0.75-inch diameter bolt targeting 20,000 lb clamp force, with K = 0.2:
- Nut factor K: 0.2
- Bolt diameter: 0.75 in
- Target clamp force: 20,000 lb
- Torque: 0.2 × 0.75 × 20,000 ÷ 12 = 250 ft-lb
Notes
The nut factor K is the biggest source of uncertainty in this formula. The same applied torque can produce a very different actual clamp force depending on lubrication, plating, and thread condition, since K is standing in for friction losses that vary from one fastener and joint to the next.
That uncertainty is why critical joints, such as structural steel connections and pressure vessels, often skip torque-only tightening in favor of direct tension indicators or turn-of-nut methods, which measure the actual bolt stretch or force rather than relying on an assumed friction value.
Always use the fastener manufacturer’s specified torque value when one is provided, rather than calculating from an assumed K. A published spec reflects the real fastener and coating in use, which is more reliable than a general planning estimate.
How to use
Enter the nut factor K, the bolt diameter, and your target clamp force. The calculator returns the torque in foot-pounds needed to reach that clamp force, which you can use as a planning estimate before checking the fastener manufacturer’s actual spec.
This is a planning estimate using standard mechanical engineering formulas, not a substitute for engineering design. Confirm actual fastener sizing and torque specs with the fastener manufacturer or a structural engineer.
Frequently asked questions
What is clamp force and why does torque control it?
Clamp force is the squeezing force a bolt applies to hold two parts together once it's tightened. You can't measure clamp force directly with a simple tool, so torque is used as a stand-in — turning the bolt stretches it slightly, and that stretch creates the clamp force. Torque is just the easiest way to get there indirectly.
What is the nut factor K and where does it come from?
The nut factor K represents how much of your applied torque is lost to friction under the bolt head and in the threads, rather than converting into clamp force. It typically runs around 0.2 for lubricated or plated steel fasteners, and higher, around 0.2 to 0.3, for dry, unlubricated ones. It's an empirical value, not a fixed constant.
Why is the same torque sometimes not enough, and sometimes too much?
Because the nut factor K varies with lubrication, plating, and thread condition, the same torque reading can produce very different actual clamp forces on different bolts. A dry, rusty bolt eats more torque as friction, leaving less for clamp force, while a well-lubricated one converts torque to clamp force more efficiently at the same setting.
Should I always calculate torque with this formula?
Use the fastener manufacturer's specified torque value whenever one is provided, rather than calculating from an assumed K. This formula is useful for planning and estimating, but a published spec accounts for the real fastener, coating, and application, which is more reliable than an assumed nut factor.
Estimates only. Verify quantities with your supplier before purchasing.