Bolt Torque Calculator
Find the tightening torque for a bolt preload.
Default 0.2 (dry steel)
Results update as you type.
How it works
Tightening a bolt to the right torque is essential for a reliable joint, and this calculator finds the torque needed to reach a target clamp force. You enter the bolt's nominal diameter, the desired clamp force or preload, and a friction factor, and it returns the required tightening torque in newton-meters. The relationship uses the widely applied short-form equation: torque equals the friction factor times the diameter times the clamp force. The clamp force, or preload, is the tension the bolt holds that squeezes the joint together, and it is what actually keeps a joint from loosening or failing, not the torque itself. Torque is simply the practical way to achieve that preload with a wrench. The friction, or K, factor is crucial and often underappreciated: most of the applied torque is consumed overcoming friction in the threads and under the bolt head, and only a small fraction stretches the bolt to create preload. A common value is 0.2 for dry, plain steel, dropping to around 0.15 or lower with lubrication, which is why lubricated bolts need less torque for the same preload. Because K varies with materials, coatings, and lubrication, treat the result as a guide and follow manufacturer specifications for critical joints. Enter the diameter, clamp force, and friction factor to compute bolt torque.
T = K · D · F
The tightening torque equals a friction factor K, times the bolt's nominal diameter in meters, times the target clamp force, or preload, in newtons. The K factor bundles thread and head friction into a single empirical value, typically around 0.2 for dry steel and lower when lubricated. Most of the torque overcomes friction, not the stretching of the bolt.
Source:Mechanical engineering
We review all our tools from time to time to ensure accuracy.
Worked example
How to use this calculator
- 1
Enter your values
Enter bolt diameter and target clamp force.
- 2
Adjust the options
Adjust friction factor (k) if they apply — sensible defaults are already set.
- 3
Read your result
Tightening torque updates instantly as you type — no button to press.
Frequently asked questions
How is bolt torque calculated?
Use the short-form equation: torque equals the friction factor K, times the bolt diameter in meters, times the target clamp force in newtons. For a 12-millimeter bolt at 10,000 newtons preload with K of 0.2, the torque is 24 newton-meters. The calculator applies this formula when you enter the diameter, clamp force, and friction factor.
What is the K factor?
The K, or nut, factor is an empirical value that combines all the friction in the threads and under the bolt head into one number. It typically ranges from about 0.2 for dry plain steel down to 0.15 or lower when lubricated or coated. K strongly affects the required torque, so using the right value for your conditions is important.
What is the difference between torque and preload?
Preload, or clamp force, is the tension the tightened bolt holds, squeezing the joint together, and it is what actually keeps the joint secure. Torque is just the twisting effort applied to achieve that preload. Most of the torque overcomes friction rather than creating tension, which is why the K factor matters so much.
Why do lubricated bolts need less torque?
Lubrication lowers the friction in the threads and under the head, so more of the applied torque goes into stretching the bolt and creating preload rather than being wasted on friction. This lowers the K factor, meaning less torque achieves the same clamp force. Applying dry torque specs to a lubricated bolt can overload and even break it.
Should I rely on this for critical joints?
Treat this as a useful estimate and educational guide. The K factor varies with materials, coatings, surface finish, and lubrication, so real preload can differ significantly from the calculation. For safety-critical or engineered joints, always follow the manufacturer's torque specification or use direct tension-control methods rather than a general formula.