What is the minimum diameter of the hole for a plug weld?

• A. Thickness of the material plus 8 mm (5/16")
• B. Thickness of the material
• C. Thickness plus 2 mm
• D. Thickness plus 12 mm

Answer: (A)

For a plug weld, you need enough weld metal around the edge to develop the joint’s shear strength. If the hole were only the thickness of the plate, the weld around the circumference would be too small to carry the load. Increasing the hole diameter by a fixed amount provides that extra weld area without being excessive. The standard minimum diameter is the material thickness plus 8 mm (5/16 in), which gives enough circumference for a proper weld throat to fuse with both plates and transfer the load effectively. Smaller increases (like thickness alone or thickness plus a tiny amount) can leave the weld undersized, while a much larger diameter adds unnecessary fabrication and material removal. So, thickness plus 8 mm (5/16 in) is the appropriate minimum.