T = F·(0.16P + 0.58μd₂ + μDₖₘ/2)This free bolt torque calculator helps engineers, mechanics, and DIYers estimate the required tightening torque for metric and inch fasteners. It calculates preload from the bolt diameter, material strength class (ISO 8.8, 10.9, 12.9 / SAE Grade 5, 8) and the tightening torque from thread and head friction for various lubrication conditions.
Instead of the simplified bolt torque equation T = K × D × F, the calculator uses the ISO 16047 / VDI 2230 formula. It splits torque into thread pitch, thread friction and friction under the head, so results stay realistic for small screws and large bolts, where a single nut factor K drifts.
Whether you need a screw tightening torque calculator for automotive work, a bolt stress calculator for engineering analysis, or a general fastener torque calculator for industrial applications, this tool provides reliable estimates. It also works as a nut torque calculator for hex nuts and threaded fasteners. Looking for a table instead? See the bolt torque chart for every size and grade.
The short form is T = K × D × F, where T is torque (N·m), K is the nut factor, D is bolt diameter (m), and F is clamping force (N). This calculator uses the more precise T = F × (0.16·P + 0.58·μ·d₂ + μ·Dₖₘ/2), with thread pitch P, pitch diameter d₂, friction coefficient μ and effective head diameter Dₖₘ, so it stays accurate for small screws and large bolts alike.
Preload is F = As × σy × (percentage of yield), where As is the tensile stress area and σy the yield strength of the strength class. The calculator shows the stress area and resulting clamping force for the selected bolt.
Dry, clean steel is about μ = 0.15 (K ≈ 0.20). Oiled threads are about μ = 0.10–0.11 (K ≈ 0.15). Anti-seize and PTFE compounds go down to μ = 0.05–0.06 (K ≈ 0.09–0.10). Lower friction means less torque for the same preload. Always verify with manufacturer specifications for critical applications.
Yes. The torque values apply to both bolt heads and nuts. Use the same diameter, strength class and friction for accurate nut torque calculations.
ISO metric classes 3.6 to 12.9 (ISO 898-1), SAE Grade 1, 2, 5, 5.2, 8 and 8.2 (SAE J429), ASTM A307, A325, A354 BD, A449, A490 and A574, and stainless steel A2 and A4 in classes 50, 70 and 80 (ISO 3506-1).