Well first of all I didn't need a size. Totally my fault as I'm in a confusing mess got pregnant, and thought I had the right dimensions. No big deal. Now for the part that wasn't my fault: the 1/4" bit is useless. I've tried it with 2 different drill bits and although both are properly seated in the chuck and properly tightened, the bit wobbles in a way that you know isn't up to spec. It ferments so much that it doesn't punch a hole, just scratches the concrete. Keep in mind.