Hi Trevor,  M8 = 14mm for normal substrates such as brick, block or concrete. For aerated lightweight blocks such as Thermalite™ or in mortar 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 (4 – 6mm) pilot drill first to correctly position the drill to avoid run-away of the drill tip, especially if the surface is riven/ textured.