Enter the tensile stress area (thread stress area) of the bolt and the proof strength (proof stress) of the bolt into the calculator to determine the bolt preload. The calculator will provide two outputs, one for connections intended to be re-used and one for permanent connections.

Bolt Preload Calculator

Provide at least one of Tensile Stress Area or Proof Strength, plus one preload (Permanent or Re-use) to compute the rest. If Area and Proof Strength are both provided, both preloads will be computed.








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 (0.75 for re-use connections, 0.90 for permanent connections)
  • A is the tensile stress area (thread stress area) of the bolt
  • S is the proof strength (proof stress) of the bolt

To calculate the bolt preload, multiply the constant by the tensile stress area and the proof strength.

Bolt Preload Definition

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

A common design goal is to apply enough preload to keep the joint clamped under service loads without exceeding the bolt’s proof load.

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 stress area (thread stress 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 tensile stress area is 15 in².

Next, the proof strength (proof stress) of the bolt must be determined (typically from the bolt grade/specification). For this example, the proof strength is 33,000 psi.

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

F = c * A * S = 0.90 * 15 * 33,000 = 445,500 lbf.

bolt preload calculator