Calculating calories for any beer is very simple, especially for the beer you brew because you already have the information you need.
Calories per Pint: S.G. in Plato x 18
Calories per 12 oz: S.G. in Plato x 13.5
oz. pint of Imperial Stout at 17.6 degrees Plato S.G.:
12 oz. can of National-Brand Lager at 10.2 degrees Plato S.G.: 10.2 x 13.5 = 137.7 Calories.
I tested the above computation for 12 ounces of beer at Dogfish Head with QC Supervisor Chad Collier and his amazing $32,000 analysis machine. Calorie computations are logarythmic and not a straight line, however Chad was impressed with how close the calculations will get you to the actual Calorie count.
And wouldn't you rather have a 300+ calorie pint of Imperial Stout instead of a piece of chocolate cake anyway?