I'm pretty sure they are 10mm (although they seem a little more like 9.9mm). They fit well but wiggle just a little in either end of a 10mm crescent/box wrench. They don't fit at all in a 9mm or 3/8" wrench, and they spin freely inside the opening of a 7/16" wrench. And they have metric strength numbers (8.8) on the heads.