Calculate the tightening torque needed to reach a target bolt preload using T = K·D·F, for metric class 8.8/10.9/12.9 and SAE Grade 2/5/8 fasteners.
Pick thread size, grade, and lubrication. Torque T = K·D·F develops the target preload (default 75% of proof load).
About this tool
The Bolt Torque Calculator is a free, in-browser fastener tool that gives the tightening torque required to develop a target preload (clamp force) in a bolt, using the short-form torque-tension relation T = K·D·F. Here K is the nut factor for the fastener's condition, D is the nominal diameter, and F is the preload. Pick a thread size, a grade or property class, and a lubrication condition. All arithmetic runs locally in your browser.
The preload defaults to a percentage of proof load: F = (preload %) × At × σ_proof, where At is the tensile stress area of the thread and σ_proof is the proof strength of the grade — metric class 8.8 uses 580 MPa, 10.9 uses 830 MPa, and 12.9 uses 970 MPa (SAE Grade 2, 5, and 8 use 379, 586, and 827 MPa). Note that the standard ISO 898-1 proof stress for class 10.9 is 830 MPa, not the 720 MPa sometimes quoted. The nut factor K reflects friction: about 0.20 dry, 0.22 zinc-plated, 0.16 lightly lubricated, and 0.10 for moly or wax. Torque follows as T = K·D·F and is reported in both N·m and lbf·ft, alongside the preload in newtons and pounds-force. You can also override the preload directly in newtons.
As a worked example, an M10 class 8.8 bolt (At = 58 mm², proof 580 MPa) tightened to 75% of proof has a preload F = 0.75 × 58 × 580 ≈ 25,230 N; dry (K = 0.20) the torque is T = 0.20 × 0.010 × 25,230 ≈ 50.5 N·m (about 37 lbf·ft). These are estimates — the nut factor scatters by ±25% in practice, so use a calibrated method (angle or tension control) for critical joints.
Frequently asked questions
What is the torque-tension formula?
The short-form relation is T = K·D·F, where T is torque, K the dimensionless nut factor (friction condition), D the nominal bolt diameter, and F the target preload or clamp force. It is an approximation — the nut factor typically scatters by about ±25%, so it estimates rather than guarantees the preload.
How is the preload determined?
By default the tool sets preload to a percentage (75% by default) of the bolt's proof load: F = %·At·σ_proof, where At is the tensile stress area and σ_proof the grade's proof strength (8.8 = 580 MPa, 10.9 = 830 MPa, 12.9 = 970 MPa). You can also type an exact preload in newtons to override this.
What nut factor K should I use?
K captures the combined thread and under-head friction: roughly 0.20 for plain dry steel, 0.22 zinc-plated, 0.16 lightly oiled, and 0.10 for molybdenum-disulfide or wax coatings. A lower K means less of the torque is lost to friction, so the same torque produces more preload.
Why is 10.9 proof strength 830 MPa here?
The nominal proof stress in ISO 898-1 for property class 10.9 is 830 MPa; a value of 720 MPa is sometimes quoted incorrectly. This tool uses the standard 580 / 830 / 970 MPa for classes 8.8 / 10.9 / 12.9, so its preload and torque figures match published fastener tables.