"Both the previous reviewers have misunderstood the way these cam locks are sized. Which means Screwfix could have explained it better, but as they didn't, then I'll try here. 18mm refers NOT to the drilling diameter, but to the thickness of the board for which it is intended. Thus on this "18mm" cam the centre of the slot for the dowel is nearly 9mm below the surface with the screwdriver slots. In this way you will be drilling your 18mm board end-on (to receive the dowel) right in the _centre_ of the board thickness, so after assembly the surface of the cam lock is just below flush with the board surface. Drilling is always 15mm but of course you need to use a special drill to get the flat bottom to the hole - Screwfix used to sell a nice Erbauer set of these but I don't see it now. "MAD" bit 35782 will do, but in an over-the-top way."
"Its size is 18 mm. It is indicated on the pack the diameter is 18 mm. However, I checked two branches, the size is 15 mm. It is exactly same as 15 mm cam lock. Please check the size before you buy the drills and make the holes."