T = K·F·d(1 - l/100)
Bolt type
Mild-steel, dry
Constant K
Diameter (d)
in
Lubrication
Lubricant
No lubricant
Lubrication factor (l)
%
Force and torque
Clamping force (F)
lbf
Torque (T)
ft-lb

This bolt torque calculator will ensure that you tighten up your bolts with the correct torque. Just as well, as a loose bolt can really ruin your day.

Read on to discover what is bolt torque, the bolt tightening torque calculation formula, how to calculate bolt torque, and typical torque values for bolts. Our bolt torque calculator supports different bolt materials and levels of lubrication. So gather your nuts and bolts together 🔩, and let's start building something!

What is bolt torque?

Torque is a measure of the conversion of linear force to rotational force at some distance from the axis of rotation. When applied to a bolt, this force becomes tension in the bolt threads, which in turn applies a clamping force (or load) between the two materials you are bolting together.

The amount of tension in the bolt is significant. For each application of a bolt, there will be an ideal bolt torque value. For example, in the picture below, the bike seat post states the required torque is 5 N⋅m. If we apply too little torque to a bolt, vibrations will tend to loosen the bolt over time, which may result in a failure.

Bolt torque requirement on a bike seat post bolt
Bolt torque requirement on a bike seat post bolt.

On the other hand, if you tighten up a bolt with too much torque, the bolt may stretch, causing the clamping force to actually reduce. If you increase the torque still further, the bolt may break during assembly or, even worse, during operation.

How to calculate tightening torque? - Bolt tightening torque calculation formula

The conversion from torque to clamping force depends on the type of bolt material, it's diameter, and how much lubrication we apply to the bolt. Here is the formula that takes these factors into account and relates bolt torque and clamping force:

T = K * F * d * (1 - l/100)

where:

  • T is the torque applied by the wrench to tighten the bolt (N⋅m or lbf⋅ft);
  • K is a constant factor that takes into account bolt material and size;
  • F is the clamping force the bolt applies to the surfaces being bolted (N or lbf);
  • d is the diameter of the bolt (m or ft); and
  • l is the amount of lubrication, given as a percentage.

You should note that this formula is only an approximation, as it does not take into account the bolt thread pitch (the angle and density of threads).

Bolt torque values range from around 4 to 300 lbf⋅ft (5 to 400 N⋅m), depending on the factors in the above bolt tightening torque calculation formula.

How to use this bolt torque calculator?

In this section, we will show you the steps required to use our bolt torque calculator. We will also demonstrate how you can use this same gadget as a bolt clamping force calculator.

Note that by changing the calculator units, it is both a metric bolt torque calculator and an imperial bolt torque calculator. You can even use mixed units if you like.

  1. The first thing is to set the constant K that depends on the bolt material. Either choose from the bolt types listed, or if you know the value of K for the bolts you are using, enter in the amount directly into the calculator. Note that the K values for the different bolt types are valid for bolts between 1/4 and 1 inch.
  2. Next, input the diameter of the bolt. To be clear, that's the body of the bolt, not its head.
  3. Moving on to the lubrication section, either select one of the available lubricants or directly input the lubrication factor for your lubricant.
  4. Finally, to calculate the torque required when tightening the bolt, enter the clamping force needed for your application. Or...
  5. If you know the bolt torque required, enter its value, and the calculator will compute the clamping force. You can use this same calculator both ways!
💡 To convert between different torque units, why not check-out Omni's N⋅m to lbf⋅ft converter.

Example calculations of bolt torque

Let's show you how to do the bolt torque calculation on paper so you understand how our calculator works. Let's say you have a zinc-plated mild-steel bolt, with a diameter of 3/4 inch, lubricated with SAE 30 oil (40% lubrication factor). The specification for this bolt fixing says the clamping force needs to be 25,000 lbs.

If we put all this information into the tightening torque calculation formula from the section above we get:

T = K * F * d(1 - l/100) =

(0.2) * (25000 lb) * (0.75 in) * (1/12 ft/in) * (1 - 40% / 100%) =

187.5 lbf⋅ft

Steven Wooding