Tightening torque ≈ 0.2 × preload × bolt diameter (dry steel).
T = K·F·d with a nut factor K ≈ 0.2 for unlubricated steel. Lubrication drops K toward 0.12–0.15 — same torque, much higher preload.
Put anti-seize on the same M12 bolt and the nut factor drops from about 0.20 to 0.12. The same 100 N·m now delivers roughly 65% more preload — enough to take a grade 8.8 fastener past yield while your torque wrench reads exactly what the table told you.
At 0.14 μ the rule says 0.2 and the exact answer is 0.1948 — 2.7% high. It holds to within 10% from 0.13 μ to 0.16 μ, and drifts outside that.
K = 0.2 is not a property of the bolt, it is a friction guess wearing a bolt’s clothes. It is right at μ ≈ 0.145 and nowhere else: on a lubricated thread the real factor is 0.12, so torquing to the rule puts about 60% more tension in the bolt than you asked for. That is the direction that snaps things.
The rule against the exact answer, computed across the range. Inside the shaded band the shortcut is close enough to use; outside it, reach for the calculator.
Estimate with the rule, then check it against the calculator that models it properly.
Open Bolt Torque & Preload Calculator →Tightening torque ≈ 0.2 × preload × bolt diameter (dry steel). T = K·F·d with a nut factor K ≈ 0.2 for unlubricated steel.