I do not know of a formula to calculate spans, there are many span tables for your quick reference.
Span tables I have seen do deviate greatly according to species of wood. IE, Oak or hickory will span alittle further than southern pine.
I would imagine that you could do simple search on Google or Yahoo and use the key words, "Lumber Spans" or something like that, but you want wood not metal.
I deal with spans all the time, but they are mainly concerning decks. This is very simple and easy, but when you get into homes, you have all sorts of variations.
For example, I have a engineered span that is made from that stuff they advertise an elephant can walk on it upstairs. Its actually OSB board with 2x4 sides, some engineering marvel.
Then there are guidelines for drilling holes in these joists for electical and plumbing needs. I can show you those also.
I believe as Dodgeman stated below, and I thoroughly agree with, "sistering" is not a cure all for illegal spans. Sistering, in my opinion only works when you actually go the same distance as the inferior span and then on both sides greatly helps too. Then the fastening and nailing or screwing order has much to do for strengh.
I get touchy when I hear of so many people thinking that if I sister, the problem is over. You have added more weight and the percent of span help is not that great.
To properly fix an illegal span, you need a set of posts and beams or kings and beams.