Engineering Tools

Bolt Torque & Preload Calculator

Pick a metric bolt size and property class, choose how much of proof load to target and the friction condition at the threads, and get the clamping preload and tightening torque — using the short-form torque equation T = K · F · d.

Joint inputs

Standard metric coarse threads. Pick “Custom” to enter your own diameter, stress area, or proof strength.

Bolt size

Property class

Friction condition (K)

Target preload (% of proof)

Reference

How the numbers are derived

Preload comes from the bolt's proof strength and tensile stress area; torque converts that preload through the friction in the threads and under the head.

Equations & data

F	Target preload = %proof × S_p × A_s. Proof strength S_p: class 8.8 ≈ 580 MPa (≤M16) / 600 MPa (>M16), 10.9 ≈ 830 MPa, 12.9 ≈ 970 MPa.
A_s	Tensile stress area of the thread (ISO 898-1), smaller than the nominal shank area.
K	Nut factor — an empirical lump of thread and under-head friction. 0.20 dry steel is the common default; lubrication can halve it, which doubles preload at the same torque.
T	Tightening torque = K · F · d, with d the nominal diameter.

Watch-outs

K is the biggest uncertainty. If the joint is critical, measure it for your actual fastener, finish, and lube — don't trust a table.
Same torque + lubricated threads = much higher preload. Over-torquing a lubed 12.9 bolt can yield it.
Soft or gasketed joints relax; plan a re-torque or use lower preload targets.
These values assume room temperature, steel-on-steel joints, and standard coarse threads.
For safety-critical joints follow VDI 2230 or your industry standard — this tool is a sizing aid, not a substitute.

Designing a joint that can't come loose?

Fastened joints fail at the system level — embedment, vibration, thermal cycling, torque strategy on the line. If your product depends on bolted joints staying tight, bring the design to a consultation.

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