alcohol has 7 calories per gram.
Ther are 28 grams in an ounce. A shot is 1 and 1/8 oz. Captain is 70 proof.
Therefore about 105 cals per shot. In a Captain and Diet Coke, there coul dbe a few oz. of Captain so add it up.
However the real problem with alcohol induced weight gain is because alcohol slows digestion, and you usually eat like shit when you drink.