I am interested in the same thing. I have been AG for a few years now and am starting to try to "predict" what will occure when I brew. I have looked and looked for an answer to this question and still have not found the promised land. Someone please help!
You savior has a name, and it's name is Promash. There are free programs that can do what you want out there, but none of them will be as complete and easy to use as Promash. No, I don't own the company, just a delighted customer.
To answer your original question, each type of grain has a potential gravity associated with it. For standard American 2 row, it's 1.036. That means, if you grind it to flour, mash and convert completely, 1 pound of grain will yeild 1 gallon of wort at a gravity of 1.036. That's assuming you are 100% efficient, which is why it's called "potential". Every system will have a reduction in the actual gravity you get due to the efficiency factor.
To do the calculations yourself, use the following formula:
WG = (Lbs x PPG x Eff)/WV
WG = Predicted Wort Gravity in points like 45 or 50 or 60
Lbs = Pounds of grain used
PPG = Gravity potential of the grain (points per pound per gallon, i.e. 36 or 38 for typical base malt)
WV = Wort Volume in gallons
Eff = Your system efficiency
If you want a wort gravity of 1.060 in 1 gallon of water, and we'll assume 70% efficiency and a PPG of 36 for the grain, then your equation looks like this:
WG = 60
PPG = 36
WV = 1
Eff = 0.7
Solve for Lbs
Lbs = (60 x 1)/(36 x 0.7)
Lbs = 2.38 lbs
It's much easier for Promash to figure this for you, but an understanding of the math is good.