How to Calculate the Tightening Torque of Bolts

torq formula

It is essential to know how to calculate the tightening torque of bolts to ensure your application works safely, without breaking. A mistake in this calculation could cost tens of thousands of dollars, never mind the time and hassle of replacing broken equipment.

The Bolt Torque Formula

The formula that expresses the relationship between the load in a bolt and applied torque is:

T = K F d

K is the coefficient of friction. Your coefficient will depend on the material of your bolt/nut, the type of lubrication (or, rarely, the lack of lubrication) and the size of the bolt.

The nominal bolt diameter is d in this equation. It may be expressed in millimeters or inches.

F is the axial bolt force, expressed in pounds or Newtons.

You need to provide all the information in this formula (except, of course, for T, which is the torque that you are looking for).

In your calculations you should also remember that torque wrenches are not perfectly accurate. Almost all the torque wrenches you can buy or rent from us at Ultra Torq are +/- 3 percent.

An Example of Calculating Bolt Torque

 Let’s work through a typical example. Our K will be 0.2, which is a typical coefficient of friction (without lubrication). Our bolt’s diameter will be 0.5 inches, and the axial bolt force will be 11,175 lbs.

Put those values into the formula:

T = 0.2 * 11,175 * 0.5

T = 1,117.5 Inch lbs

Then convert to foot pounds by dividing by 12

T = 1,117.5 / 12

T = 93.125 ft lbs

Our T works out to 93 ft lbs.

torq bolt

Confirm for Peace of Mind

You have your torque, but you need to confirm it for peace of mind, and to ensure the safety and proper functioning of your equipment. A small error could result in a large discrepancy in your final figure.

There are three proven methods you can use to verify your calculations. Read more about these methods in our article that examines how to verify that your torque specification is correct.

If you have questions about how to calculate the tightening torque of bolts for your application, reach out to us at Ultra Torq.