Bolt Torque Calculator

Bolt tightening torque is commonly used as an indirect way to create clamp load in a bolted joint. Turning the nut or bolt stretches the fastener slightly, and that elastic stretch produces preload. This calculator applies the widely used nut-factor relationship: torque equals nut factor multiplied by preload multiplied by nominal bolt diameter. With preload in kilonewtons and diameter in millimeters, the numerical product gives newton-meters after the unit conversions cancel. The nut factor represents friction in the threads and beneath the rotating bearing surface. Friction consumes most of the applied torque, so a small change in lubrication, coating, surface finish, washer, or reuse condition can cause a large change in achieved preload. A value near 0.20 is often used for an unlubricated steel estimate, while lubricated assemblies may use a lower value such as 0.15. These are planning assumptions, not universal material constants. Target preload must come from a suitable joint design. Designers often select a fraction of bolt proof load, then check separation, slip, fatigue, bearing, thread stripping, and member strength. The nominal diameter is the fastener designation rather than its tensile-stress diameter. This simplified torque equation does not separately model thread pitch, prevailing torque, locking compounds, gasket relaxation, embedment, or tool scatter. The calculator reports both newton-meters and pound-force inches. The conversion is included only for convenience; all input dimensions remain metric. Keep units exactly as labeled because entering newtons instead of kilonewtons or inches instead of millimeters would change the result by a large factor. Use a calibrated torque tool over its recommended operating range and follow the specified tightening sequence for multi-bolt joints. For safety-critical, structural, pressure-containing, rotating, or fatigue-loaded connections, use the fastener manufacturer’s data and the applicable engineering standard. Confirm the actual friction condition through controlled testing or a validated torque-tension procedure. Direct tension indicators, ultrasonic elongation, turn-of-nut methods, or hydraulic tensioning may provide better preload control where torque scatter is unacceptable. This result is therefore a transparent preliminary estimate, not a substitute for an approved tightening specification.