Rule Of Thumb For determining the Fillet Weld Size

How to Find Minimum and Maximum Fillet Weld Size?

This post gives simple calculations for determining the minimum and maximum fillet weld sizes to withstand the maximum stresses at the weld.

The fillet weld size will be the optimum weld size using this calculation and fits most of the weld applications. Any weld size above this value will not add any strength to the joint but instead goes wasted as additional added weld metal, manpower, and other costs.

So, I advise you not to weld more than the size you obtain with this calculation.

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Formula to Calculate the Fillet Weld Size

When selecting the size of fillet weld for plate or pipe to achieve full strength weld apply the below rules for fillet weld leg length:

  1. The minimum size of the Leg length of the fillet shall be 0.75 (3⁄4) of the material thickness. For example, if the material thickness is 10 millimeters, you should have a fillet size of 7 millimeters.
  2. In a situation of dissimilar plate thickness, consider the thinner member.

Any Weld above leg (z) size will resist the maximum stress for a T-joint under any scale & loading side. The equation to find out the minimum fillet weld size with the above assumption shall be given as:

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The above equation to find fillet weld size as a rule of thumb is based on the below considerations: 

  1. Both sides welded fillet weld,
  2. Full-length fillet weld,
  3. In a situation of dissimilar plate thickness, consider the thinner member.

Below is an example of a fillet weld having a leg length (z) of 0.375 inches on both sides welded with half-inch plate size.

Considering the above weld was designed only for joining these two plates, then the weld stresses in the joint will be less than 1⁄3 of the material yield strength.

In this case, a fillet weld of leg length size 0.25 to 0.375 of the material thickness is sufficient. Stich weld can be carried out in place of full-length fillet weld for joining the parts.

In another case, if welded from the other side is not feasible and only one side fillet weld can be made, then the leg length size shall be doubled.

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You can refer to below table to find out the fillet weld size for different material thicknesses ranging from 1⁄4 inches to 3 inches considering the above formula. The tables advise fillet weld size based on a strength weld or joining weld (rigidity weld). 

AWS has also suggested the minimum fillet weld size in AWS D1.1. in the below table you can find the recommended minimum fillet weld size by the AWS.

As you know, in the USA fillet size measurements are carried out using leg length (z) hence, AWS gives the sizes for leg length and not the throat size. While the EN standards state that the minimum throat size should be 3 millimeters.

The approach given here is a simple and reliable calculation of fillet weld size based on standards. Although, It is advised to follow the designer’s recommendation when joining critical components.

These are fast and reliable methods to find fillet weld dimensions. Weld joints of critical applications shall be considered carefully.

Stress calculation in fillet and butt weld joints

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How to calculate throat size or leg length size in a fillet weld?

The required fillet size for this design application was determined to be 5/16″. This technique can be used to size any fillet weld application.

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