Bolt torque calculator
Estimate the tightening torque needed to reach a target bolt preload, using the torque-coefficient method.
How it is calculated
Tightening torque T = K × F × d, where F is the preload force, d the nominal diameter and K the torque coefficient. Use d in metres and F in newtons to get N·m.
Worked example
M10 bolt, preload 20 kN, K = 0.2: T = 0.2 × 20,000 N × 0.010 m = 40 N·m (about 29.5 lbf·ft).
Steps
- Enter the nominal bolt diameter in mm.
- Enter the target preload from the joint design or bolt proof load.
- Enter K for your lubrication condition, then read the torque.
Frequently asked questions
What K value should I use?
About 0.2 for dry, plain steel; lower values apply with lubricated threads. Use the value from your fastener supplier or the joint specification.
Why is the torque only an estimate?
K varies with friction, plating and lubricant, so scatter of ±25 % or more is normal. Use a torque-angle or a tension-controlled method for critical joints.
Is this for metric and imperial bolts?
Yes. Enter the nominal diameter in mm and the preload in N; the result is shown in N·m and lbf·ft.
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References
Torque-preload relation T = K F d as used in VDI 2230 and ISO 898-1 (bolt property classes). Verify with the joint specification.
Estimates for information only. Verify critical values with a qualified engineer or your supplier standards.