What is the minimum diameter for a hole or slot fillet weld?

• A. No less than the thickness of the material
• B. Twice the thickness
• C. Thickness plus 12 mm
• D. Thickness plus 8 mm (5/16")

Answer: (D)

When a hole or slot is welded, you need enough weld around the edge to provide reinforcement and to distribute stress evenly around the cutout. The standard rule for hole/slot fillet welds is that the minimum overall diameter of the weld around the hole is the plate thickness plus 8 mm (which is 5/16 inch). This extra amount ensures there’s enough throat depth and coverage around the circumference to maintain joint integrity without unnecessary excess that could cause distortion or waste filler metal.

So, if the plate is 6 mm thick, the minimum hole weld diameter would be 14 mm (6 + 8). The requirement keeps the weld practical and effective for typical loads, rather than being just the material thickness or being excessively large.