How to Calculate Bolt Tightening Torque: T = k x D x P and Nut Factors

Key things to remember

The essentials from this document in plain words. Check the full document before you rely on a figure.

• ✓Formula: T = k x D x P, where T is torque in Nm, k is the nut factor, D is the bolt's nominal diameter in mm and P is the target clamp load in kN. The recommended target clamp load is 65% of the proof load.
• ✓Proof load in kN is proof stress (MPa) times tensile stress area (mm²) divided by 1,000, per AS 4291.1 and ISO 898.1.
• ✓Common k factors, as a guide only: plain or dry 0.20, zinc 0.14, highly lubricated 0.10, hot dip galvanised 0.42, lightly oiled galvanised 0.22.
• ✓Torque is affected by the nut factor, finish, property class, surface hardness, roughness and flatness, the type of joint and the number of fasteners in it.
• ✓Where clamp load is critical, calibrate on site: assemble 10 to 12 bolts as built, torque them to failure with a calibrated wrench, average the maximum torques and use 65 to 70% of that average as the seating torque.
• ✓The most accurate method is to simulate the joint with a tension indicating device and note the torque that reaches the target load. For critical structural bolting, the document points to EN 14399-3 K2 assemblies, with a clamp force of 0.7 times the bolt's ultimate load.

Who it helps: Engineers, fitters and staff who need to explain or work out a tightening torque.