Bolt Torque Calculator

Use the torque-tension equation T = K × D × F, where T is torque, K is the nut factor for the surface condition, D is the nominal bolt diameter, and F is the target clamp force. Multiply the three values with consistent units.

The nut factor K is a torque coefficient that bundles bolt material, thread finish, plating, and lubrication into one number. Dry steel runs about 0.20, oiled steel 0.15, and anti-seize 0.12 to 0.13. Higher K means more torque for the same clamp load.

It depends on bolt size, grade, and lubrication. For a dry 1/2"-13 Grade 5 bolt, about 75 ft-lb at 75% of proof load. For an M12 Class 8.8, about 88 to 100 N·m depending on the clamp convention. Use the reference charts on this page or the calculator for your exact size.

Only if the specification assumes lubrication. Lubricating a dry-speced bolt cuts the required torque by 15 to 25%, and applying the dry torque to a lubricated bolt can overload it past yield. Always match the lubrication condition to the torque value.

Grade 5 has a proof stress of 85,000 psi and Grade 8 of 120,000 psi. Grade 8 carries about 40% more clamp load at the same size, so it takes about 40% more torque. Never substitute a lower grade and apply a higher-grade torque.

These are metric ISO 898 property classes. Class 10.9 has a proof stress of 830 MPa versus 580 MPa for Class 8.8, so it reaches about 43% more clamp force at the same size. Class 12.9 sits above both at 970 MPa.

A bolt produces roughly 10 to 15% of its applied torque as clamp force after friction losses. To estimate it, rearrange the formula to F = T / (K × D), or use this calculator's reverse mode.

Because friction consumes the rest. About 50% of the applied torque fights friction under the bolt head or nut, about 40% fights friction in the threads, and only the remaining 10 to 15% stretches the bolt into clamp load.

No. Head bolts, gasketed flanges, and structural steel joints per AISC or RCSC need manufacturer or code values, not general torque tables. Use this tool for general assembly and design reference only.

When exact preload matters, because torque control scatters preload by ±25 to 30%. Angle control, or turn-of-nut, tightens to a rotation after snug and bypasses most friction scatter. It is standard for torque-to-yield fasteners and critical structural joints.

A calibrated torque wrench is typically accurate to about ±25%, and actual preload can vary from −30% to +50% of target because the nut factor varies. For critical joints, verify with angle control or direct tension measurement.

For a dry M10 Class 8.8, about 57 N·m; for dry M12 Class 8.8, about 88 to 100 N·m depending on the clamp convention. Class 10.9 runs higher: 82 N·m for M10 and 143 N·m for M12. See the metric torque chart above.