Calories Explained: BMR, TDEE, and the 500-Calorie Rule
Calories are the currency of every diet, but most people have only a vague idea of what they actually are. They know that eating fewer of them leads to weight loss and eating more leads to gain — but the numbers underneath are useful to understand, especially if you're trying to lose or gain weight deliberately.
What a calorie is
A calorie (technically a kilocalorie, which is what food labels mean) is a unit of energy. It's the amount of energy needed to raise the temperature of one kilogram of water by one degree Celsius. In food terms, it's the energy your body can extract from what you eat.
The three macronutrients
- Protein — 4 calories per gram
- Carbohydrates — 4 calories per gram
- Fat — 9 calories per gram
Alcohol contributes about 7 calories per gram, which is why drinks add up quickly. Fat is the most calorie-dense, which is why a small amount of oil can have more calories than a large serving of vegetables.
Your total daily energy expenditure (TDEE)
Your body burns calories through four mechanisms:
- Basal Metabolic Rate (BMR) — calories burned at rest, just keeping you alive. Typically 60–70% of total.
- Thermic effect of food — energy used digesting what you eat. About 10%.
- Exercise — intentional physical activity. Usually 5–15%.
- Non-exercise activity thermogenesis (NEAT) — fidgeting, walking, standing. Varies a lot between people.
The 500-calorie rule
A commonly used heuristic: eating 500 fewer calories per day than your TDEE produces roughly one pound of weight loss per week, because a pound of body fat stores about 3,500 calories. Similarly, a 500-calorie surplus produces about one pound of gain per week.
The heuristic has limits — your metabolism adapts, the composition of weight lost includes muscle as well as fat, and hormones influence appetite — but it's a reasonable starting point. Real-world weight loss slows over time.
What "a calorie is a calorie" misses
For weight loss specifically, total calories dominate. But the source of those calories affects satiety, muscle retention, and how you feel. 500 calories of chicken and vegetables keeps you full longer than 500 calories of candy, and provides protein that helps preserve muscle during a deficit. Both are "500 calories" for the ledger, but not for your body.
Finding your number
A calorie calculator gives you an estimate based on age, sex, weight, height, and activity level. Treat it as a starting point, not a precise prescription — track your intake and weight for a few weeks, and adjust based on real results. Your body's actual energy needs can differ from any formula by 10–20%.