Select bolt type, size, grade, and surface condition to calculate the recommended tightening torque.
The calculator applies the short-form torque equation T = K × D × F. D is the nominal bolt diameter. F is the target clamp load, set at 75% of the bolt's proof load — proof stress from ISO 898-1 (metric) or SAE J429 (imperial) multiplied by the tensile stress area of the selected thread. K is the nut factor for the surface condition you choose, from 0.10 for waxed threads to 0.25 for hot-dip galvanized.
Everything is computed in your browser from those inputs, so the result matches the table on the matching bolt page and the working is shown under the result. Diameter-dependent proof stress is handled automatically: class 8.8 uses 600 MPa above M16, and Grades 2 and 5 drop to their lower values above 3/4" and 1" respectively. Class 9.8 is not offered above M16 because ISO 898-1 does not define it there.
The biggest uncertainty is K, not arithmetic. Real nut factors scatter by ±25% or more with thread finish, washer condition and tightening speed, so use the result as a starting point and follow the equipment manufacturer's figure wherever one exists.