Enter the tensile shear area of the bolt and the proof load of the bolt into the calculator to determine the bolt preload. The calculator will provide two outputs, one for connections requiring re-use and one for permanent connections.

Bolt Preload Formula

The following formula is used to calculate a bolt preload force. 

F = c * A * S
  • Where F is the preload force
  • c is a constant (.75 for reuse connections, .89 for permanent connections
  • A is the tensile shear area of the the bolt
  • S is the poof load of the bolt

To calculate the bolt preload, multiply the constant by the shear area and the proof load.

Bolt Preload Definition

A bolt preload is defined as the total tension force created in a fastener, in most cases a bolt, when it is tightened.

The optimal preload is reached when the working load placed on the bolt is distributed to the installation material, or in other words, the materials the bolt is holding together.

Bolt Preload Example

How to calculate bolt preload?

The following example problem will go over the step-by-step process of calculating an optimal bolt preload.

First, it must be determined whether the connection will be permanent or intended for re-use. For this example, the bolt will be permanent.

Next, the tensile shear area of the bolt must be determined. In most cases, this can be found on a manufacturer’s datasheet, but it can be calculated using a formula. For this problem, the shear area is calculated as 15 in^2.

Next, the proof load of the bolt must be determined. This example will use 85% of the yield strength of the bolt, which is 33,000 psi.

Finally, the bolt preload can be calculated using the formula above.

F = c* A*S = .85*33,000*15 = 495,000 lb-f.

bolt preload calculator