When you read the label, the calories listed should match whatever macros are listed. For example if the label says:

Calories: 250 calories
Protein: 21 grams
Carbs: 19 grams
Fats: 10 grams

You don't really need to do the math, because it has already been done for you e.g.

Protein: 21 grams = 21g x 4cal = 84cal
Carbs: 19 grams = 19g x 4cal = 76cal
Fats: 10 grams = 10g x 9cal = 90cal

84+76+90=250cal

The manufacturer has already done the math to tell you how many calories are in the product. Just be careful though because these macros and calories are usually listed by serving size and many products contain more than 1 serving per package.