Calculate bolt length from the bolt diameter and the thickness of the parts being clamped, with optional washers and packing, or estimate thread engagement.

Rough length estimate using a 2 × diameter allowance. This is not the RCSC structural bolt table.

Combined thickness of the parts being clamped.

Packing, washers and allowance
Gaskets, spacers or extra plates; leave blank if none.

An extra length allowance does not establish joint strength.

Typical planning value (0.15 in). Replace it with your washer’s actual thickness.

Typical planning value (0.31 in). Replace it with your washer’s actual thickness.

Bolt Length Formula

The calculator uses the following rough planning formula. Enter all lengths in compatible units; washer counts are whole numbers.

BL ≈ 2D + G + W + A
  • BL is estimated bolt length; D is diameter; G is main material plus packing; W is the total actual thickness of all washers; A is the additional allowance.
  • The 2D allowance used here differs from RCSC Table C-2.2, which gives separate allowances for heavy hex structural bolts. Use the applicable fastener and connection requirements for final selection.

For this estimate, multiply the bolt diameter by 2, then add the clamped material, packing, actual washer thickness totals and any additional allowance. Default washer thicknesses of 0.15 and 0.31 inches are planning assumptions; adjust them to your hardware.

What is the correct bolt length?

A rough starting estimate is twice the diameter plus the grip and washer thicknesses. Final selection also depends on the actual nut, thread projection and engagement, fastener geometry and joint requirements. The engagement mode multiplies diameter by a selected 1.0, 1.5 or 2.0 allowance; it does not establish a universally safe minimum.

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 estimate the bolt 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, packing pieces have a combined thickness of 0.250 inches, with no washers or additional allowance.

Finally, calculate the bolt length using the formula above:

BL = (D*2) + ( a + b + c…)

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

BL ≈ 9.25 inches as a rough planning estimate.