They’re so small, but they add up so quickly. I would go over the technical definition of calories, the chemistry of a calorie, and calories versus kilocalories (which is what we actually use to quantify our food) but for the sake of brevity, I’ll just say that a calorie is measured as the amount of food energy per unit of mass. Calories equal energy.
Grams of fat, protein, and carbohydrates have caloric values. Fat has nine calories per gram, while protein and carbohydrates have four each. This means that something with five grams of fat, 20 grams of carbs, and 10 grams of protein would have 165 total calories (45+80+40).
If you want to lose weight, your calories consumed must be less than the calories burned. Three thousand five hundred calories equal a pound. So if you want to lose a pound a week then you need to cut an average of five hundred calories a day from what you consume, or burn an extra 500 in the gym (or some combination of the two like consuming 250 less and burn 250 more at the gym).
To figure out how many calories that you need to consume, you must first determine how many calories a day you burn. To get a rough estimate of how many calories you burn a day, you need to figure out how many you burn in the gym (several heart rate monitors can do that) and what your BMR is.
Basal metabolic rate, also known as BMR, is the minimum number of calories that your body burns to function. Basically it is the number of calories burned in a 24 hour period if you stayed in bed the entire time. Many websites can help you get a rough estimate. Obviously you don’t always know how many calories you burn outside of the gym doing regular every day activities so that’ll be a little extra.
You can see now that you can use calories to your advantage. They no longer have to be a hindrance, but a tool that you can use and manipulate in order to reach your goals.