About Calorie Needs
A calorie (technically a kilocalorie, or kcal) is a unit of energy. Everything you eat provides energy, and everything your body does — from running a marathon to sleeping — burns it. Your daily calorie needs are the total energy your body requires to maintain its current weight, and this depends on three main factors: your basal metabolic rate, your physical activity, and the thermic effect of food.
Eating fewer calories than you burn causes weight loss. Eating more causes weight gain. The math is not perfectly linear — the body adapts, hormones shift, and metabolism slows with prolonged deficits — but for most people, tracking the deficit or surplus over weeks is the single most reliable way to control weight.
BMR — Basal Metabolic Rate
Your BMR is the number of calories your body burns at complete rest, just to stay alive — breathing, circulating blood, regulating temperature, growing cells. It accounts for 60–75% of most people's total daily calorie burn. The Mifflin-St Jeor equation is the most accurate BMR formula for the general population and is what this calculator uses:
| Gender | Formula |
|---|---|
| Male | BMR = 10 × weight (kg) + 6.25 × height (cm) − 5 × age + 5 |
| Female | BMR = 10 × weight (kg) + 6.25 × height (cm) − 5 × age − 161 |
For a 30-year-old man at 175 cm and 70 kg: BMR = 700 + 1,093.75 − 150 + 5 = 1,649 kcal/day. The same calculation for a woman: 700 + 1,093.75 − 150 − 161 = 1,483 kcal/day. The difference is about 166 kcal — the average gap between male and female metabolic rates of similar size.
TDEE — Total Daily Energy Expenditure
Your TDEE is your BMR multiplied by an activity factor that accounts for everything else you do — walking, working, exercising, fidgeting. It is the number of calories you actually burn on a typical day.
| Activity level | Multiplier | Description |
|---|---|---|
| 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 |
| Extremely active | ×1.9 | Athlete training twice a day |
A moderately active 30-year-old man at 70 kg has TDEE = 1,649 × 1.55 ≈ 2,556 kcal/day. Eating around that amount maintains his current weight.
The three goal calorie targets
| Goal | Calorie target | Expected change |
|---|---|---|
| Weight loss | TDEE − 500 | ~0.5 kg / 1 lb per week |
| Maintain | TDEE | Stable weight |
| Weight gain | TDEE + 500 | ~0.5 kg / 1 lb per week |
The ±500 kcal/day rule comes from a simple approximation: 1 kg of body fat stores roughly 7,700 kcal, and 1 lb stores roughly 3,500 kcal. Spread over 7 days, that is about 500 kcal/day for either goal. Larger deficits (up to 1,000 kcal/day) accelerate loss but are harder to sustain and increase the risk of muscle loss; smaller deficits (200–300 kcal/day) are slower but easier to maintain.
Macronutrient breakdown
Once your calorie target is set, the next question is how to divide those calories between protein, carbohydrates and fat. The three macronutrients provide energy at different rates:
| Macronutrient | Energy | Role |
|---|---|---|
| Protein | 4 kcal/g | Muscle repair, satiety, metabolic rate |
| Carbohydrate | 4 kcal/g | Primary fuel for exercise and the brain |
| Fat | 9 kcal/g | Hormones, cell membranes, vitamin absorption |
This calculator uses a 30% protein / 40% carbs / 30% fat split — a well-balanced default that works well for both fat loss and general health. Different goals warrant different splits: for fat loss, higher protein (35–40%) preserves muscle; for endurance athletes, higher carbs (50–60%) fuels training; for keto, fat dominates (70%+).
Why the formula is an estimate
Even the best formula is only accurate to about ±10% for any individual. Real-world factors that shift the number include:
- Muscle mass. Muscle burns more calories at rest than fat, so two people with the same height and weight can have different BMRs.
- NEAT (Non-Exercise Activity Thermogenesis). Fidgeting, standing, walking around — varies hugely between people and can differ by 400+ kcal/day.
- Hormones. Thyroid function, insulin sensitivity and cortisol all affect metabolic rate.
- Adaptive thermogenesis. During prolonged dieting, the body reduces energy expenditure by 5–15% to defend against weight loss.
- Sleep, stress and diet quality. All influence measured energy expenditure.
The best way to use any calorie calculator is to start with its estimate, eat at that level for 2–3 weeks, and adjust based on what actually happens on the scale.
Common use cases
Weight Loss
Set a sustainable deficit that loses 0.5 kg / 1 lb per week.
Muscle Building
Eat at a slight surplus to gain lean mass without excess fat.
Sports Nutrition
Calculate fuelling needs for training and recovery.
Health Maintenance
Understand what keeps your body running at rest.
Meal Planning
Plan macros around your daily calorie target.
Nutrition Education
Learn the relationship between BMR, TDEE and macronutrients.
Privacy and accuracy
This calorie calculator runs entirely in your browser. All calculations happen locally in JavaScript with no server calls. Your age, gender, height and weight are never transmitted, stored or logged.