Metric bolt torque chart: tightening torque and preload for 8.8, 10.9 and 12.9

How the bolt torque values are calculated

The preload FM is 90 % of the minimum yield strength of the property class (ISO 898-1), taking into account the twist that tightening adds to the tension in the thread. The torque MA is the preload times the thread pitch term and the friction in the thread and under the head:

MA = FM × (0.16 P + 0.58 d2 μG + μK DKm / 2)

The friction in the thread μG and under the head μK are taken as equal. The head is a hex head with the bearing diameter of ISO 4017 on a medium clearance hole of ISO 273, which is also valid for socket head cap screws to ISO 4762. These are the same assumptions as the published tables, and every value here is within 1.4 % of at least one of them. The only differences above 2 % are M3 in 10.9 and 12.9 and M4 in 10.9, where one of the two tables uses a lower preload.

Bolt torque values and friction: choosing μ

Friction depends on the surface finish and the lubrication of the bolt, the nut and the bearing face, and most published tables use μ = 0.12 as their default. The lower the friction, the more of the torque turns into preload, so applying the torque for a higher μ to a well-lubricated bolt overloads it. When the coating or the lubricant has a stated friction value, use the column for that value.

Bolt torque: common questions

What is the torque for an M10 8.8 bolt?

48.5 N·m at μ = 0.12, which gives a preload of 29.6 kN. At μ = 0.10 it is 42.6 N·m and at 0.14 it is 53.9 N·m.

What is the torque for an M8 8.8 bolt?

24.6 N·m at μ = 0.12, for a preload of 18.6 kN.

Can I use these values for stainless steel bolts?

No. A2 and A4 stainless bolts have other strength classes (50, 70, 80) and usually higher friction. Use the values given by the bolt supplier.