Calculate bolt preload (clamp force) from tightening torque, or the torque needed for a target preload, using bolt diameter and a typical nut factor.

Typical textbook values, not measured friction. Use the fastener maker’s K when you have it.

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Bolt Preload Formula

For the proof-load mode, the following formula calculates a selected preload force. The torque modes instead use T = K × F × d, with d in metres, F in newtons and T in N·m. Nut factor K depends on the fastener and friction conditions; the starting value 0.2 is an assumption.

F = c × A × S
  • Where F is the preload force
  • c is the selected design fraction of proof load (percentage divided by 100); it is not determined by permanent or reusable status alone
  • 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 Proof Load and Tightening Torque

Proof load is the tensile force a bolt must withstand in a proof test without permanent elongation. Multiply the tensile stress area for the exact diameter and thread pitch by the proof stress specified for that bolt size and grade. With area in mm² and proof stress in MPa, the product is in newtons. The calculator’s proof-load mode applies your selected fraction to that force.

For an actual tightening torque, use the service manual or installation specification for the assembly. Bolt size and grade help determine proof load, but torque also depends on the target clamp force, coating, lubrication and mating hardware. The torque mode estimates the torque for a target preload and nut factor. It cannot establish an installation specification for the joint.

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 a selected bolt preload.

First, choose a preload fraction justified for the joint and installation method. This arithmetic example assumes 90%; it does not establish that 90% is appropriate for a particular connection.

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 screenshot