1/2"-20 which is like most in the USA I believe. The "20" means twenty teeth per inch, but you'll probably also want quantity 20. I've tried 3 sets, all work. Make sure they're cone-shaped (bulge seat, most common type), not flat-end (old style). Almost any will fit. They come in various sizes of lug wrench (13/16" etc) but that doesn't matter which size you get. Even with locking hubs and open center caps (like mine), you have enough room for any lug wrench, just be careful not to scratch the hubs (if applicable).