It's going to depend on what specific malts you used. Each has a different amount of potential sugar to give to the wort.
This chart will give you an idea:
http://www.onebeer.net/grainchart.html
There's an easy to use online calculator here:
http://www.brewersfriend.com/brewhouse-efficiency/
Most introductory brewing books will tell you how to do it. I learned on Daniels book, "Designing Great Beers". I recommend you buy it if you haven't already. It costs less than a grain bill and it really improved my brewing techniques.
That being said, most any brewing software will calculate this out for you. Nevertheless, it's good to know the calculations yourself. It will help you understand malts and grain bills a little better.
If you want to calculate it all out yourself here is an example:
Let's say you've got 10 pounds of 2-row, which has a potential of 1.037 and you mash it in 5 gallons of water.
Take 10 (the weight in pounds of the malt) x 37 (drop the 1. and just keep the 037) / 5 (mashing water volume). That all equals 34.
So 34 would be 100% efficiency in this case, so take your post-mashing hydrometer reading and divide it by this number. So lets say your OG reading was 25. 25/34 = .7352, or about 73.5%.
The potential extract for all grains is different, so you need to calculate the potential of each malt seperately just as above and add the numbers together to see what the total potential extract is. Numbers are often given as ranges, because there is always some variation from malt to malt, year to year, etc. I just use a number in the middle of the range.
Post you malt bill if you're unsure of the math and I or someone else can calculate it for you. Once you see the equations and do it a few times it will become second nature.
Happy brewing!
