BMI and calorie needs: the formulas
Body mass index, basal metabolic rate, calorie calculations: the reference formulas, no judgment or personalised advice.
Body mass index (BMI)
BMI = weight (kg) / height (m)²
| BMI | Category (WHO, adult) |
|---|---|
| < 18.5 | Underweight |
| 18.5 - 24.9 | Normal weight |
| 25 - 29.9 | Overweight |
| ≥ 30 | Obese |
BMI is a population-level indicator, not an individual diagnosis: it doesn't distinguish muscle mass from fat mass and is no substitute for medical advice.
Basal metabolic rate (Mifflin-St Jeor equation)
Men: (10 × weight kg) + (6.25 × height cm) − (5 × age) + 5 Women: (10 × weight kg) + (6.25 × height cm) − (5 × age) − 161
The result is your basal metabolic rate (BMR): the energy spent at complete rest over 24 hours.
Activity factor to estimate total needs
| Activity level | BMR multiplier |
|---|---|
| Sedentary (little to no exercise) | × 1.2 |
| Lightly active (1-3 days/week) | × 1.375 |
| Moderately active (3-5 days/week) | × 1.55 |
| Very active (6-7 days/week) | × 1.725 |
These formulas give estimates, not absolute truths — every metabolism varies roughly ±10-15% from the theoretical calculation.
Calorie balance: the basic logic
Calories eaten > total needs → potential weight gain Calories eaten < total needs → potential weight loss Calories eaten ≈ total needs → stable weight
A surplus or deficit of roughly 7,700 kcal corresponds approximately to 1 kg of fat mass — a useful theoretical benchmark, but one that ignores individual variation (water retention, body composition, metabolic adaptation).
Limits of metabolism formulas
| Factor not accounted for | Impact |
|---|---|
| Muscle mass | Two people of the same weight/height can have different BMRs depending on body composition |
| Genetics and hormones | Can shift actual metabolism by several hundred kcal |
| Metabolic adaptation | The body adjusts its energy expenditure after a prolonged period of restriction |
These formulas are useful theoretical benchmarks for a general estimate, not individual prescriptions. For a specific health or weight goal, a healthcare professional or dietitian remains the reference.
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