What size drill bit to use with #8 sheet metal screw?

If you are drilling into solid metal you need to drill the hole and tap threads into it. Sheet metal screws are not the best screw to use. I would use machine screws or bolts. Stainless if it is outdoors. Match the tap to the screw. The tap should have marking that will tell you what size drill to use. It would also be best to use cutting oil when drilling. Drilling into iron will be tough going and you may break a couple bits before you are done. You may even break a tap.