Here is a handy calculator
I created a small spreadsheet so that I can work offline. Here is the formula for mash volume based on grain bill and mash thickness:
mash volume [gal] = weight of grain [lbs] * (0.08 + mash thickness [qts/lb] / 4)
I use a 50 quart cooler for 10 gallon batches and can easily handle 25 pounds of grain.