Bolt Torque Calculator

A bolt holds by stretching like a stiff spring and clamping the joint, and torque is how you put that stretch in. Too little and the joint works loose, too much and the bolt yields or snaps. This gives a sensible tightening torque for a metric bolt from its size, grade and thread condition, along with the clamp load that torque sets up.

Torque is a proxy for clamp load

What actually holds a bolted joint is clamp load, the tension in the bolt pulling the parts together. You cannot measure that directly with a wrench, so you torque to a value that produces it. The catch is that most of the torque, often eighty to ninety percent, goes to overcoming friction under the head and in the threads, and only the rest stretches the bolt. That is why friction, set by the thread condition, swings the number so much.

Why lubrication changes the figure

A lubricated or anti-seize thread is slippery, so less torque is lost to friction and more goes into stretching the bolt. Use the dry torque on a lubed bolt and you overshoot the clamp load, sometimes enough to yield or break it. That is why this has a thread-condition setting. If you oil or anti-seize a thread, drop to the lubricated figure, and if you are unsure, dry is the safer assumption for not over-tightening.

Grade sets how hard you can pull

The grade stamped on the head, 8.8, 10.9 or 12.9, is the bolt's strength, and a stronger bolt can take and hold more clamp load, so it gets more torque. These numbers target seventy-five percent of the bolt's proof load, a common, safe working tension that leaves margin before yield. A higher grade in the same size lets the joint clamp harder, which is often why a design calls for 10.9 or 12.9 instead of plain 8.8.

Where to trust a real spec instead

These are general-purpose figures with typical friction, fine for a bracket, a mount or a machine frame. They are not a substitute for an engineered torque spec on anything structural, pressure-bearing or safety-critical, where the number accounts for the exact joint, gasket and lubricant. For those, use the published value and a calibrated torque wrench, and treat this as a sanity check rather than the authority.

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