Bolt Torque Calculator
Torque, nut factor and preload in one line: what the wrench number really buys, and how much of it friction quietly taxes.
Bolt Torque Calculator
Results recalculate instantly on every keystroke. Nothing you type is transmitted.
What this result does not account for
- Nut-factor aggregate; no thread-friction detail
- Metric sizes; no shear-plane checks
In short: An M12 bolt (12 mm) clamped to 50 kN preload at the classic dry-steel nut factor K = 0.2 needs 120.000000 N·m of torque — T = K·d·F. The humbling card sits beside it: about 85–90% of that torque never reaches the bolt — it fights thread and bearing friction. Shift K to 0.25 with a corroded thread and the same 120 N·m clamps only 40,000 N: one-fifth less clamp from the same wrench, which is why lubrication discipline is preload discipline.
Formula
T = K × d × F ··· F = T / (K × d)
The nut factor K compresses every friction and geometry effect — thread angle, pitch, thread friction, nut-face friction — into one number that is honest in aggregate and unreliable in detail. Torque turns preload only through K's grace: the same wrench reading scatters tens of percent between a dry and an oiled thread, which is why critical joints are tensioned, not torqued.
Worked Example
- Enter the bolt's nominal diameter.
- Set the nut factor for the real surface state.
- Enter the target preload.
- Read the torque — then read the friction tax and stay humble.
Defaults: M12, K 0.2, 50 kN → 120.000000 N·m. The firgelli check: M10, K 0.2, 20 kN → 40.000000 N·m. The corroded thread: K 0.25 on the default → 150.000000 N·m for the same clamp — the wrench pays, the rust collects.
Strengths & Limits Of This Model
Where this engine is strong
- K-sensitivity priced both directions
- The friction tax stated, not implied
Where it stops
- No torque-angle control
- No joint-stiffness model
Practical Use Cases
Flange assembly
the wrench spec from the clamp target
Structural bolting
preload discipline
Teaching
friction's tax rate
Methodology & Editorial Standards
Computation runs in IEEE-754 double precision at full internal precision; rounding to two decimal places occurs strictly at the display layer, so no cumulative drift enters the result. All monetary outputs use accounting presentation — grouped thousands, two decimals, negatives in parentheses — so figures can be transcribed directly into a model or working paper. Division-by-zero and out-of-domain inputs return an em-dash rather than a misleading number.
This engine was reconciled against an independent reference implementation and hand-verified for the worked example above before release. Our full five-stage review process is published on the About Us page.
Disclaimer. This calculator is provided for informational and modelling purposes only and does not constitute financial, tax, legal, medical, or engineering advice. Verify all figures with a qualified professional before acting on them.
Bolt Torque Calculator — 8 Expert FAQs
8 analyst-written answers to the questions practitioners actually ask — optimised for voice and answer-engine retrieval.
What is the nut factor, physically?
A fitted constant that folds thread pitch, thread friction and nut-face friction into one multiplier — the torque needed per unit preload per unit diameter. It is not a friction coefficient, though it behaves like one's well-dressed cousin. Typical dry steel sits near 0.2; lubrication drops it toward 0.15; corrosion pushes 0.25 and beyond. Every torque table you have ever read is this constant in disguise.
Why does most of the torque never reach the bolt?
Because the tightening torque fights two friction fields — the thread flanks and the nut's bearing face — and only the small remainder does the work of stretching the bolt. Depending on lubrication, 85 to 95 percent of your wrench reading is heat and wear. That is why a rusted thread can shear before it clamps: the torque is being spent on friction, not on tension.
How accurate is torque control, really?
Industrially honest answer: preload scatter of roughly plus-or-minus a quarter even with calibrated tools, because K wanders with surface state, speed and repetition. Where that scatter matters — pressure boundaries, fatigued joints — the trade switches to angle control, tensioners or strain measurement. Torque is the convenient proxy, not the measurement.
Does the same torque give the same preload on any bolt?
Only at the same diameter and K — preload is torque divided by K times d, so bigger bolts need disproportionate torque for the same clamp. That is why torque tables scale by size and why extending a wrench's length cheats the reading by pure geometry. The page's inverse card shows the same arithmetic run backwards: fix the wrench, vary K, watch the clamp move.
Why do structural bolts prefer tensioning?
Because slip-critical and high-strength connections need the clamp, not the twist — and tensioning measures the clamp directly instead of inferring it through friction's tax. Turn-of-nut and tensioner methods price the elongation or the turn, not the torque. The page's arithmetic is still the entry ticket: every method starts from the preload the joint needs.
What happens if I over-torque?
The bolt yields — the clamp stops growing while the stress keeps climbing, and the joint may loosen later as the stretched bolt relaxes past its elastic memory. Galled threads make it worse by inflating K mid-tightening, so the wrench over-reads while the clamp under-delivers. The page's utilisation logic runs here too: the bolt has a capacity, and torque is only its proxy.
Should I lubricate the threads?
Only if the specification says so — lubrication cuts K toward 0.15, so the same wrench setting suddenly clamps a third harder, which can snap a bolt sized for dry friction. Lubrication is a system change, not a courtesy: new K, new torque target, documented. The page lets you price the difference before the wrench does it for you.
Where does the full Shigley equation fit?
It replaces the fitted K with its parts — thread geometry, thread friction and bearing friction measured separately — for joints where preload accuracy pays the bills. For everyday work the nut factor is honest in aggregate: measure K on your own joint if the stakes justify it, otherwise use 0.2 and respect the scatter. Precision is available; it simply bills by the hour.