Bolt Torque & Preload Calculator
Ninety percent of your torque is fighting friction.
Ninety percent of your torque is fighting friction.
Tightening torque
88 N·m
36.7 kN clamp force — 75% of a 48.9 kN proof load
The relationship is T = K · F · d, where K is the nut factor. Everything difficult about bolted joints lives inside that K, because it bundles two separate frictions, under the bearing face and in the threads, into one empirical number. Lubrication is the trap. A well-oiled thread has a nut factor around 0.15 against 0.20 dry, so applying a dry torque figure to a lubricated bolt produces about a third more preload than intended, which is enough to yield or snap it. Preloading to 75% of proof load is the usual target for a reusable joint. It sounds high and it is deliberate: a properly preloaded bolt sees almost no fatigue loading, because the joint carries the varying load rather than the fastener. Under-tightening, not over-tightening, is what causes most bolted joint failures.
How the Bolt Torque & Preload Calculator works
Preload and tightening torque for a metric bolt at a chosen fraction of proof load, with the nut factor made explicit. Torque is a poor proxy for preload, and this page shows exactly why.
Also known as: torque for an m12 bolt · bolt preload calculator · should i lubricate before torquing · how tight is too tight
Where to go next
The Bolt Torque & Preload question rarely arrives on its own. These are the ones that usually come with it:
- Tap Drill Calculator — 75% engagement is optimal, not a compromise.
- Thread Pitch Calculator — Pitch and lead differ the moment you add a start.
- Fillet Weld Calculator — The throat carries the load, not the leg.
- Speed Distance Time Calculator — Any one of the three from the other two, in four units.
Frequently asked questions
How do I calculate bolt torque?
T = K × F × d, where K is the nut factor, F the target preload and d the nominal diameter. An M12 class 8.8 at 75% of proof takes about 88 N·m dry.
Why is torque a poor way to control preload?
Because roughly 90% of it is consumed by friction under the head and in the threads. The preload actually achieved varies by ±25% or worse with finish, lubrication and reuse.
Does lubrication change the torque?
Substantially. A well-oiled thread has a nut factor around 0.15 against 0.20 dry, so applying a dry torque figure to a lubricated bolt produces about a third more preload — enough to yield it.
Why preload to 75% of proof load?
Because a properly preloaded bolt sees almost no fatigue loading — the joint carries the varying load rather than the fastener. Under-tightening causes far more bolted joint failures than over-tightening.
How do critical joints get tightened?
By angle after snug, or by measuring bolt stretch directly. Both bypass friction entirely, which is why engine and flange bolts are specified that way rather than by torque alone.
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Related calculators
Tap Drill Calculator
75% engagement is optimal, not a compromise.
OpenThread Pitch Calculator
Pitch and lead differ the moment you add a start.
OpenFillet Weld Calculator
The throat carries the load, not the leg.
OpenSpeed Distance Time Calculator
Any one of the three from the other two, in four units.
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