Workshop and Metalwork Calculators

Bolt torque.

Pick the bolt size and its strength grade. The tool takes the published proof load, applies the clamp percent you choose and your friction coefficient, and returns the tightening torque in N m, ft lb and in lb. Runs in your browser; nothing is sent or stored.

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Inputs

%

0.15 gives K near 0.20. Lower for lubricated threads.

Result

Fill the form and press Calculate.

Answer first. Torque = K x clamp load x bolt diameter. With friction 0.15 on both threads and under the head, K is about 0.20, and a typical target clamp load is 75 percent of the bolt's proof load. Always use the torque the manufacturer specifies when there is one.

Worked example

An M10 class 8.8 bolt: proof load 33,700 N. At 75% of proof the clamp load is 25,275 N. With friction 0.15 the nut factor K is 0.197, so torque = K x clamp load x diameter = 0.197 x 25,275 N x 0.010 m = 49.7 N m, about 37 ft lb.

How it works

How the number is built. The proof load of each metric size and property class is taken from the Engineering ToolBox table . For inch bolts the stress area is pi/4 x (d - 0.9743 / threads per inch)^2 and the proof strength is the SAE value in the Wikipedia screw article: grade 2 is 55 ksi up to 3/4 in (33 ksi above), grade 5 is 85 ksi and grade 8 is 120 ksi. The clamp load is a percent of proof load; the source says an acceptable clamp load is typically 75 percent.

Nut factor. Wikipedia gives T = K x preload x d, with K = (dm / 2d) x (tan psi + mu sec alpha) / (1 - mu tan psi sec alpha) + 0.625 mu_c, using the pitch diameter dm and a 30 degree half thread angle. This tool sets thread friction and under-head friction to the same coefficient you enter. At 0.15 the result is close to the source's K of about 0.20.

Friction is the weak point. Lubricant, plating, dirt and thread condition change the friction coefficient and so the clamp load for the same torque. The source describes a test of 1/2-20 bolts torqued to the same value where the standard deviation of preload was about 1,100 lbf for the unlubricated sample and about 680 lbf for the lubricated one, around the same mean preload of 7,700 lbf. Expect real preload to vary even when the torque wrench is perfect.

Not for critical joints. Wheels, engine and brake parts, structural, pressure and lifting connections have their own specified torque and often a tightening sequence or angle. Use that figure, not this estimate. Stainless, aluminum and coated fasteners are not covered.

Safety. Use a calibrated torque wrench, never use a cheater bar on a click-type wrench, and check the manufacturer specification for any safety-critical joint. A bolt tightened beyond its proof load can stretch or fail. This is an estimate, not professional engineering advice.

Sources

Proof loads for classes 4.6, 5.8, 8.8, 10.9 and 12.9 are the coarse-thread values from the first page for M3 to M24. 1 ft lb = 1.3558 N m.

FAQ

What is the torque for an M10 bolt?

For class 8.8 at 75 percent of proof load and friction 0.15, about 50 N m (37 ft lb). Class 10.9 and 12.9 give higher figures because their proof loads are higher.

What is the torque for a 1/2-13 grade 5 bolt?

At 75 percent of proof and friction 0.15, about 74 ft lb (100 N m). Grade 8 is higher, about 105 ft lb, because its proof strength is 120 ksi.

Should I lubricate the threads?

Lubricant lowers friction, so the same torque produces a higher clamp load. If a spec was written for dry threads, adding oil without reducing the torque can overload the bolt. Follow the spec; if you must estimate, lower the friction value here.

Why 75 percent of proof load?

The Engineering ToolBox page states that an acceptable clamp load is typically 75 percent of proof load. You can change the percent in the form.

Are these the same as the torque charts in my manual?

They can differ. Published charts assume particular friction values and sometimes different clamp percents. When a manufacturer gives a torque for your joint, use that.

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