Hi Ray, you need a 16mm masonry drill,  but I would advise that you go up in stages with smaller sizes starting with a 4mm or 5mm to accurately locate the centre of the hole, then increase to a 10mm and finally the 16mm. If you try drilling in one go using the 16mm, the tip will wander and the centre of the hole will change and the overall diameter will be oversize. These expansion bolts are not load rated, so not suitable for work requiring Local Authority or Building Control sign-off, but ideal for many DIY tasks around the home, garage and garden.