Calorie Comparisons. A 2.5-ounce classic vodka martini has about 156 calories, which is about the same as what you would get from the same size classic dirty martini. Choose a 4.5-ounce classic dirty martini, which is the size often served in bars, and each drink will set you back about 240 calories. Whether you choose a martini made with gin or vodka won't make a big difference in the calorie count, as both contain the same number of calories per ounce.