Enter the bolt diameter, material thickness, and up to two additional parent material thicknesses to determine the proper bolt length.

Bolt Length Formula

The following formula is used to calculate a bolt length. 

BL = (D*2) + ( a + b + c...)
  • Where BL is the proper bolt length
  • D is the diameter of the bolt
  • a, b, and c are the material thicknesses of the main material and any additional packing/plies respectively

To calculate a bolt length, multiply the bolt diameter by 2, then add in any additional material thicknesses.

What is the correct bolt length?

The correct bolt length that should be used in an assembly is roughly 2 times the diameter of the bolt, plus the thickness of the material.

As can be seen in the formula above, most often there are additional plates or packers in the bolt assembly. The thicknesses of these should also be added to the calculation.

How to calculate a bolt length?

The following example shows the steps and information needed to calculate the bold length for an assembly.

First, determine the diameter of the bolt to be used. In this example, the diameter is measured to be 2 inches.

Next, determine the thickness of the main material being bolted. In this case, that is 5 inches.

Next, add in any additional thickness of packing/plies. For this example, there are two packing nuts that have a combined thickness of .250″.

Finally, calculate the bolt length using the formula above:

BL = (D*2) + ( a + b + c…)

BL = (2*2) + ( 5+.250)

BL = 9.25