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Bolt torque calculator

Estimated tightening torque for a target preload of 75% of proof load, from T = K · F · d. The nut-factor (K) assumption is shown on every result - it is the single biggest source of torque uncertainty.

M8 × 1.25 · preload 75% of proof

Preload and estimated tightening torque for M8 × 1.25 by fastener grade or property class, at K = 0.20.
Grade / classPreloadTorque
Class 8.815.92 kN26 N·m
Class 10.922.79 kN37 N·m
Class 12.926.63 kN43 N·m

Strengths are ISO 898-1:2013 Table 3 for d ≤ 16 mm. Class 8.8 is size-banded - 580 MPa proof at or below 16 mm, 600 MPa above - while 10.9 and 12.9 hold one value across the range.

Computed with K = 0.20 (dry / as-received steel) via T = K · F · d. The usual default when no lubricant is specified.
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Torque for every lubrication condition

The free estimate uses the dry / as-received default (K = 0.20). Premium unlocks the full nut-factor selector - zinc-plated, lightly oiled, and waxed / anti-seize - so your torque figure matches how the joint is actually assembled.

Unlock the torque calculatorBrowse the free size reference

Every size page, all dimensions, tap & clearance drills, a default-K torque estimate, the thread identifier and the tap-drill calculator stay free forever.

Why the K factor matters

The short-form torque equation is T = K · F · d, where F is the target preload, d the nominal diameter and K the nut factor - a lumped friction coefficient for the threads and the bearing face. A dry, as-received steel fastener is usually taken as K = 0.20; oil drops it toward 0.18, and wax or anti-seize can push it to 0.12 or lower.

Because torque scales directly with K, a lubricated joint torqued to a dry-K figure will be badly over-preloaded. That is why ThreadRef always prints the K it used, and why the lubrication selector is part of the Premium calculator.

The other reason two torque charts disagree is the diameter banding in the standards themselves - SAE J429 steps Grade 2 down above 3/4 in and Grade 5 above 1 in, and ISO 898-1 steps class 8.8 up above 16 mm. The guide on bolt grade markings and why torque specs differ works through all of it with real figures, and the guides index covers tap drills, pitch selection and thread identification.

These are estimates for reference. Always follow the fastener manufacturer's torque specification and your joint's engineering requirements for anything load-bearing.