Hi Mike,

Look up "controlled impedance trace calculations." Also look up "trace impedance calculator."

Check out this link
http://www.polarinstruments.com/

Good luck.

Joe


P.S. Just so you know, I'm not trying to be coy by giving you sparse answers. It's just that that this is a very complex subject and there is no simple answer to your question. There are simple approximations that you can use to get close, but even that will require that you study the subject in significantly greater depth than you're currently exhibiting. Don't just look for a couple of "plug & play" equations. You will not be well served for having done so. Look up some literature on the subject and read. Then try to apply what you read to your problem. In the end you'll still need to do some approximating on your calculations, but you'll at least be doing so knowingly.

And if you're sending your gerber data out to a board shop, there's a good chance that all you'll need to do is say, "Make these traces X ohms."