Hole saw advertised and pictured as 4". The one I received is clearly marked as 3 and 15/16 inches. To make matters worse, it is actually measured from the caliper at 3 and 7/8 inches. The lights I bought them for just don't fit the holes. The good thing about this saw is that it is VERY sharp! But combined with a simple screw attachment mechanism, this can be a dangerous combination. As another reviewer suggested, consider chaining them together if you need to use them. I'd recommend skipping these unless you need a hole a little smaller than the size listed and can't find it anywhere else.