How do find the size of drill bit for self tapping screws. I'm going to hang wooden door on 4x4 box steel secured in concrete.i will be drilling the iron to put wood stile on to it.?

The drill size for self tapping screws is the size of the barrel. ie,the bit that forms the body of the screws inside the thread. If you are using 1/8 screws then the body would be about 3/32. If I'm doing an unfamiliar size, I always experiment with 2-3 for best size. -An easier way is to simply use 'drill tipped screws' -These are available at an drywall wholesaler.