calories ?

    A calorie is a unit of energy.
  1. The small calorie or gram calorie (symbol: cal) is the approximate amount of energy needed to raise the temperature of one gram of water by one degree Celsius at a pressure of one atmosphere.
  2. The kilocalorie, also known as large calorie, kilogram calorie, food calorie, and similar names (symbols: kcal, Cal or C) is 1000 small calories.

CALORIES are needed to provide energy so the body functions properly. The number of calories in a food depends on the amount of energy the food provides. The number of calories a person needs depends on age, height, weight, gender, and activity level.
When you hear something contains 100 calories, it's a way of describing how much energy your body could get from eating or drinking it.

You can find out how many calories are in a food by looking at the nutrition facts label. The label also will describe the components of the food — how many grams of carbohydrate, protein, and fat it contains.

Here's how many calories are in 1 gram of each:
Fat: 9 calories
Protein: 4 calories
Carbohydrates: 4 calories
Alcohol: 7 calories

That means if you know how many grams of each one are in a food, you can calculate the total calories. You would multiply the number of grams by the number of calories in a gram of that food component.
For example, if a serving of potato chips (about 20 chips) has 10 grams of fat, 90 calories are from fat. That's 10 grams x 9 calories per gram.