What is Leg length in a Fillet Weld?

Leg length is the distance from the root of the weld to the toe. There shall be two legs in a fillet joint. Refer below photograph where there is a leg at the horizontal direction and another one is in the vertical plane. Leg length is often denoted by the letter ‘Z’.

What is Throat Thickness in a Fillet Weld?

The throat thickness (or throat size) is another important measurement in fillet welds. It is defined as the shortest distance from the root of the weld to the face of the weld. Throat thickness is significant because it directly affects how strong the weld will be. A thicker throat can carry more load before failing, making it an essential factor in welding design. Throat thickness is often denoted by the letter ‘a’.

The Relationship Between Leg Size and Throat Thickness

Now, let’s talk about how leg length and throat thickness are related. There are two key formulas that show this relationship, but it’s important to note that these formulas apply only when both legs of the fillet weld are equal in length:

1. Calculating Throat Thickness from Leg Size:
   • If you know the leg length, you can find out the throat thickness by using this formula:
   • a=0.707 x Z

Here, ‘a’ represents the throat thickness. This formula tells us that the throat thickness is about 70.7% of the leg length, which means that the throat thickness will always be less than the leg length.

1. Calculating Leg Size from Throat Thickness:
   • If you know the throat thickness,  you can find out the leg size by using this formula:
   • Z = 1.414 X a

We will illustrate this with some example questions.

Q1. In a fillet weld where both legs are equal, if the leg length is 8 mm, what is the throat thickness?

1. 4.0 mm
2. 5.0 mm
3. 5.66 mm
4. 6.66 mm

Correct Answer: 3) 5.66 mm

Explanation:

To find the throat thickness (a) from the leg length (Z), we use the formula:

a=0.707 X Z

Substituting leg length (Z) = 8 mm in the above formula;

a =0.707×8 = 5.656 mm

Thus, the throat thickness is 5.656 mm which will approximately equal to 5.66 mm

Q2. In a fillet weld where both legs are equal, if the throat thickness is 6 mm, what is the corresponding leg length?

1. 7.0 mm
2. 8.484 mm
3. 9.34 mm
4. 10.0 mm

Correct Answer: 2) 8.484 mm

Explanation:

To find the leg length (Z) from the throat thickness (a), use the formula:

Z = 1.414 X a

Substituting throat (a) = 6 mm in the above formula;

Z = 1.414 X 6

Thus, the throat thickness is 8.484 mm.