Fillet weld size is used to determine whether a weld is strong enough for a particular project. It is also important not to put too large of a weld as it can warp the piece and add cost with no value. The fillet weld size, S, is the distance of the throat, T, times the width of the weld, W.

Determine the leg distance, L, using the multipurpose weld gauge by placing it on one surface of the weld and adjusting it vertically so the fin touches the crosspiece that was joined by the weld just above the weld.

Read the gauge at the mark. If the weld is flat or convex, then L is equal to the fillet weld size, S, and your calculations are complete.

Measure T actual at the point of concavity using the multipurpose weld gauge if the weld is concave.

Determine the actual throat distance, T. In the special case of a right angle weld, T equals the cosine of half the angle times the distance of S. In this case, half the angle is 45 degrees and the cosine of 45 degrees equals 0.7. So in the special case of a right angle weld, the actual throat distance equals 0.7 times the distance of S, the fillet weld size.

Divide the actual throat distance by 0.7 to determine the fillet weld size, S.