Bolt Torque Calculator
Tightening torque from size, grade and lubrication — with the preload it produces.
Static, room-temperature, single bolt. It says nothing about the joint being clamped — gasket creep, soft flanges and thermal cycling all lose preload after assembly.
How it works
Torque is a proxy, and a poor one. What holds a joint together is preload — the tension stretched into the bolt — and torque is only the crude instrument used to reach it.
T = K × D × F, where D is the nominal diameter, F the preload, and K the nut factor. Somewhere between 85% and 90% of that torque is spent on friction under the head and in the threads. Barely a tenth of it becomes tension.
That is why the lubrication setting changes the answer so violently. Going from dry to waxed halves K, so the same torque produces nearly twice the preload — enough to yield the bolt. Torque figures published without a stated condition are close to meaningless, and a chart pulled off the internet and applied to anti-seized threads is a standard way to snap a stud.
The 75% default is where reusable joints live. It puts the bolt at three quarters of proof load, leaving elastic range for the working load without permanent stretch. Critical joints are sometimes taken to 90% or deliberately past yield — that is what torque-to-yield head bolts are, and it is also why they are single-use.
If the joint genuinely matters, stop measuring torque. Measure the stretch, use an angle-controlled sequence, or use load-indicating washers. Torque wrenches scatter preload by roughly ±25% even when everything is done right.
Torque for a specific bolt
Common questions
Why does lubrication change it so much?
Because almost all the torque goes into friction, not tension. Halving the friction coefficient roughly doubles the preload for the same torque. Applying dry-torque figures to a lubricated bolt is one of the most reliable ways to break it.
Can I reuse a bolt?
At 75% of proof, yes — the bolt stayed elastic. Torque-to-yield fasteners, and anything taken past proof deliberately, are single-use: they have already stretched permanently and will not reach the same preload again.
What about the thread it goes into?
This sizes the bolt, not the hole. A steel bolt in aluminium strips the aluminium first unless engagement is one and a half diameters or more; in plastic, use an insert. The weaker thread always governs.
My torque wrench only reads lb·ft.
All three units are given. Note that lb·ft and lb·in differ by twelve, and small fasteners are usually specified in lb·in — an M4 at 3 N·m is 2.2 lb·ft but 26 lb·in.