What are Calories & TDEE?
A calorie (technically a kilocalorie, kcal) is a unit of energy. Your body requires a certain amount of energy each day to maintain basic physiological functions (breathing, circulation, cell repair) and fuel physical activity. Understanding your caloric needs is the foundation of any weight management strategy.
Basal Metabolic Rate (BMR) is the number of calories your body burns at complete rest — the energy required just to keep your organs functioning. It typically accounts for 60–75% of total daily energy expenditure.
Total Daily Energy Expenditure (TDEE) is your BMR multiplied by an activity factor that accounts for exercise, non-exercise activity thermogenesis (NEAT), and the thermic effect of food (TEF). TDEE represents the total calories you burn in a day and is the key number for determining your caloric target.
The principle of energy balance is straightforward: consume fewer calories than your TDEE to lose weight, more to gain weight, or equal to maintain. One pound (~0.45 kg) of body fat contains approximately 3,500 kcal, so a daily deficit of 500 kcal results in roughly 0.5 kg of weight loss per week.
Mifflin-St Jeor Formula
The Mifflin-St Jeor equation (1990) is considered the most accurate predictive equation for estimating BMR in healthy adults, and is recommended by the Academy of Nutrition and Dietetics:
Activity Level Multipliers
| 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 |
| Active | 1.725 | Hard exercise 6–7 days/week |
| Very Active | 1.9 | Very hard exercise, physical labor, or athletic training |
Energy Balance Diagram
Macronutrient Breakdown
While total calories determine weight change, the macronutrient composition of your diet affects body composition (muscle vs. fat), satiety, performance, and health. The three macronutrients are:
| Macronutrient | Calories per Gram | Recommended Range | Key Functions |
|---|---|---|---|
| Protein | 4 kcal/g | 25–35% of calories | Muscle repair, enzyme synthesis, satiety, highest TEF (~20–30%) |
| Carbohydrates | 4 kcal/g | 40–50% of calories | Primary energy source, brain fuel, glycogen stores, fiber |
| Fat | 9 kcal/g | 20–30% of calories | Hormone production, cell membranes, fat-soluble vitamins (A, D, E, K) |
This calculator uses a balanced macro split of approximately 30% protein / 45% carbohydrates / 25% fat, which is suitable for most general health and moderate weight management goals. Athletes, ketogenic dieters, or individuals with specific medical conditions may require different ratios.
Sustainable Weight Loss
Evidence-based guidelines for healthy and sustainable weight management:
- Aim for 0.5–1 kg per week: Faster weight loss often leads to muscle loss, metabolic adaptation, and is harder to maintain long-term
- Never go below BMR: Consuming fewer calories than your BMR for extended periods can impair organ function, hormone levels, and metabolism
- Prioritize protein: Higher protein intake (1.6–2.2 g/kg) preserves muscle mass during a caloric deficit and increases satiety
- Combine diet with exercise: Resistance training during weight loss preserves lean mass; cardio increases the deficit through energy expenditure
- Track progress, not perfection: Weight fluctuates daily due to water, glycogen, and food volume. Use weekly averages instead of daily weigh-ins
- Diet breaks and refeeds: Periodic maintenance calorie periods (1–2 weeks every 8–12 weeks of dieting) can help manage metabolic adaptation and psychological fatigue
Minimum safe calorie intake: Generally, women should not consume fewer than 1,200 kcal/day and men not fewer than 1,500 kcal/day without medical supervision. Very-low-calorie diets (<800 kcal) should only be followed under physician guidance.
Worked Example
A 30-year-old male, 70 kg, 170 cm, moderately active, wants to lose 0.5 kg/week:
Macros at 30/45/25 split:
- Protein: 2007 × 0.30 ÷ 4 = 150 g (602 kcal)
- Carbs: 2007 × 0.45 ÷ 4 = 226 g (903 kcal)
- Fat: 2007 × 0.25 ÷ 9 = 56 g (502 kcal)
Frequently Asked Questions
Why use Mifflin-St Jeor instead of Harris-Benedict?
The Mifflin-St Jeor equation (1990) has been shown to be more accurate than the older Harris-Benedict equation (1919) in modern populations. A 2005 systematic review by the ADA found that Mifflin-St Jeor predicted measured RMR within 10% in more non-obese and obese individuals than any other equation studied.
How accurate are these calorie calculators?
Predictive equations like Mifflin-St Jeor have a standard error of approximately 10–15%. Individual variation depends on body composition, genetics, hormones, and metabolic adaptation. Use the calculated value as a starting point and adjust based on real-world results over 2–4 weeks.
Should I eat back exercise calories?
It depends. If your activity factor already accounts for your exercise routine, eating back exercise calories may lead to overeating. If you do additional exercise beyond what your activity factor covers, consider eating back 50–75% of estimated exercise calories (as most trackers overestimate caloric burn by 30–50%).
What happens if I eat below my BMR?
Chronically eating below your BMR can lead to metabolic adaptation (reduced metabolic rate), muscle loss, hormonal disruption (reduced thyroid function, sex hormones), nutrient deficiencies, fatigue, and impaired immune function. It is generally unsustainable and counterproductive for long-term weight management.
How important is meal timing?
For most people, total daily calorie and macronutrient intake matters far more than meal timing. That said, distributing protein evenly across 3–5 meals (20–40 g per meal) can optimize muscle protein synthesis. Pre- and post-workout nutrition becomes more relevant for competitive athletes.
Do I need to count calories forever?
No. Calorie counting is a useful tool for building awareness of portion sizes and food energy density. Once you develop a sense of appropriate portions and eating patterns, many people transition to intuitive eating or simpler tracking methods (e.g., hand-portion method) while maintaining their results.