The product image is not proportional to the bit so I have included this photo showing what a 5/16 inch looks in relation to the shaft. As you can see in the top left corner the hole diameter is confirmed to be 5/16" and shank diameter 1/4" In terms of performance itself these drill bits are fantastic. I never thought I'd have fun using a drill bit, but here it is. The entrance is INCREDIBLY clean and the bits themselves are very strong and well made.