TOOLS / FASTENERS

Bolt torque & preload calculator

Estimate bolt torque and clamp force from bolt size, property class, lubrication and target preload. A starting estimate, not a certified joint.

Starting torque88.0 Nm
Target preload36.66 kN

Tensile stress area: 84.3 mm2. Friction dominates torque, so torque-only tightening scatters about +/-25 to 35 percent; this is a starting estimate, not a certified joint.

Safety-critical: have a qualified person verify the joint. Confirm grade, lubrication, thread condition, joint stiffness and the assembly specification before tightening a load-bearing connection. Under-preload loosens; over-torque yields or strips the bolt.

Formula

Torque T = K x D x F, where K is the nut factor, D is the nominal diameter and F is the preload. Tensile stress area As = pi / 4 x (d - 0.9382 x pitch) squared. Preload here = target percentage x proof strength x As.

Standard / source

ISO 898-1 (property classes and proof strengths), ISO 16047 and VDI 2230 (torque-preload practice), as cited in the MetalCalz tool-expansion research (2026-10-27). Typical K values used: 0.20 dry, 0.15 to 0.17 oiled, 0.09 to 0.12 anti-seize. No code table is reproduced here.

Worked example

M12 x 1.75, class 8.8 (proof 580 MPa), K = 0.20 dry, target 75% of proof load: As = pi / 4 x (12 - 0.9382 x 1.75) squared = 84.27 mm2. Preload = 0.75 x 580 x 84.27 = 36,656 N = 36.66 kN. Torque = 0.20 x 12 x 36,656 / 1000 = 87.97 Nm, shown as 88.0 Nm. These are the calculator defaults.

Safety note

Safety-critical: qualified-person gate. Torque is a starting estimate only; torque-only tightening scatters about +/-25 to 35 percent because friction dominates. Under-preload loosens and over-torque yields or strips the bolt. A qualified person must verify grade, lubrication, thread condition, joint stiffness and the assembly specification before tightening a load-bearing connection. This result is not a certified joint.