What is the recommended size of hole for a fillet weld to be completed in the middle of a plate?

• A. Three times the fillet size
• B. Four times the fillet size
• C. Six times the fillet size
• D. Six times the fillet size in millimeters

Answer: (C)

When a fillet weld is to be completed in the middle of a plate, a clearance hole is used to allow the molten metal to flow and consolidate without being blocked by the opposite plate edge. The size of that hole should scale with the fillet weld itself, not be a fixed number of millimeters. The recommended diameter is six times the fillet size, so the hole accommodates the bead as it grows and helps prevent burn-through, porosity, or lack of fusion at the center. For example, if the fillet size is 3 mm, the hole would be about 18 mm in diameter. Smaller multiples or a fixed millimeter value don’t adapt to different weld sizes, which is why six times the fillet size is the best rule.