Learn how to estimate your Total Daily Energy Expenditure (TDEE) and set calorie goals for weight loss, maintenance, or muscle gain.
The Science Behind Calories
The word "calorie" comes from the Latin calor (heat). French physicist Nicolas Clément first used the term in 1819 to describe a unit of heat energy. In nutrition, one kilocalorie (kcal) — what people commonly call a "calorie" — is the energy needed to raise one kilogram of water by one degree Celsius.
In the 1890s, American agricultural chemist Wilbur Atwater became the first person to systematically measure the caloric content of foods using a respiration calorimeter. He established that protein and carbohydrates each yield about 4 kcal/g and fat yields 9 kcal/g — the Atwater factors still printed on nutrition labels today.
The first practical formula to estimate human energy needs came in 1919 with the Harris-Benedict equation. It was revised and a more accurate version — the Mifflin-St Jeor equation — was published in 1990 and remains the gold standard for estimating Basal Metabolic Rate (BMR) in healthy adults.
Key Terms
Basal Metabolic Rate (BMR): The calories your body burns at complete rest just to keep vital functions running — breathing, circulation, cell repair. It accounts for roughly 60–70% of total daily calorie burn.
Total Daily Energy Expenditure (TDEE): Your BMR multiplied by an activity factor. This is the number of calories you burn in a typical day including movement, exercise, and daily activities.
Calorie deficit: Consuming fewer calories than your TDEE. A deficit of 500 kcal/day theoretically produces ~0.5 kg of fat loss per week.
Calorie surplus: Consuming more calories than your TDEE to support muscle growth. A modest surplus of 200–300 kcal/day minimises fat gain while building muscle.
The Mifflin-St Jeor BMR Formula
Example: a 30-year-old woman, 65 kg, 165 cm → BMR = (10×65) + (6.25×165) − (5×30) − 161 = 650 + 1031.25 − 150 − 161 = 1370 kcal/day at rest.
Activity Multipliers (TDEE = BMR × Factor)
| Activity Level | Multiplier | Who it fits |
|---|---|---|
| Sedentary | × 1.2 | Desk job, little or no exercise |
| Lightly active | × 1.375 | Light exercise 1–3 days/week |
| Moderately active | × 1.55 | Moderate exercise 3–5 days/week |
| Very active | × 1.725 | Hard exercise 6–7 days/week |
| Extra active | × 1.9 | Physical job + daily training |
Setting Your Calorie Goal
Lose weight
Eat 300–500 kcal below TDEE. This produces ~0.3–0.5 kg of loss per week — fast enough to see results, slow enough to preserve muscle.
Maintain weight
Eat at TDEE. Your weight stays stable. Focus shifts to food quality and hitting macronutrient targets.
Gain muscle
Eat 200–300 kcal above TDEE. A small surplus maximises muscle growth while minimising fat gain. Pair with resistance training.
Macronutrients at a Glance
4 kcal/g — 1.6–2.2 g per kg body weight for muscle retention/growth
9 kcal/g — Minimum 0.5 g/kg; 20–35% of total calories for hormonal health
4 kcal/g — Fill remaining calories; prioritise whole grains, fruits, and vegetables
Common Mistakes to Avoid
- 1
Underestimating portion sizes — studies show people underreport food intake by 20–50%. Weigh food on a kitchen scale rather than estimating visually.
- 2
Overestimating exercise calories — fitness trackers overestimate burn by 30–90%. Do not "eat back" all exercise calories.
- 3
Using a fixed calorie target forever — recalculate every 4–6 weeks as your weight and activity level change.
- 4
Cutting calories too aggressively — very low calorie diets (below 1200 kcal for women, 1500 kcal for men) trigger adaptive thermogenesis, reducing TDEE and making fat loss harder.
- 5
Ignoring protein — insufficient protein causes muscle loss during a calorie deficit, making the body composition outcome worse even if the scale drops.
How to Calculate Your Daily Calories
Find your BMR
Use the Mifflin-St Jeor formula above. Enter your weight in kg, height in cm, and age. Men add +5 at the end; women subtract −161.
Multiply by your activity factor
Pick the activity level that best matches a typical week. If you sit at a desk but go to the gym three times, choose "Moderately active" (×1.55).
Set your calorie goal
Decide your objective: subtract 300–500 kcal for weight loss, eat at TDEE to maintain, or add 200–300 kcal to build muscle.
Track your macros
Prioritise protein (1.6–2.2 g/kg). Set fat at a minimum of 0.5 g/kg. Fill remaining calories with carbohydrates.
Recalculate every 4–6 weeks
As your weight changes, your BMR and TDEE change too. Update your numbers monthly to keep your goal on track. Use our Calorie Calculator to automate this.
Try our free tool
Calorie Calculator
Frequently Asked Questions
Q: How accurate is TDEE?
A: TDEE formulas are estimates with ±10–15% variance. Use it as a starting point, then adjust based on real weight changes over 2–3 weeks.
Q: Can I eat below 1200 kcal/day?
A: Very low calorie diets risk nutrient deficiency and muscle loss. Consult a doctor before going below 1200 kcal (women) or 1500 kcal (men).
Q: Does metabolism slow on a diet?
A: Yes — adaptive thermogenesis can reduce TDEE by 100–300 kcal during prolonged calorie restriction. Diet breaks and higher protein intake help minimise this.