Hi Russ, it is the diameter of the bolt that determines the drill size required, do you have M6, M8 or M10 expansion bolts? M6 = 12mm, M8 = 14mm and M10=16mm for normal substrates such as brick, block or concrete. For aerated lightweight blocks such as Thermalite™ or similar you might want to go down a size. A good rule of thumb to be 100% sure is to drill a hole in a piece of similar material to check and always start with a smaller pilot drill first to correctly position the drill to avoid run-away of the drill tip if the surface is riven/ textured.