Basal metabolic rate is the energy your body uses at complete rest, keeping your heart beating, your brain running and your temperature stable. It accounts for the majority of daily energy use in most people — typically 60 to 70 percent.
Weight in kilograms, height in centimetres, age in years. This is the Mifflin-St Jeor equation, published in 1990, which is more accurate for modern populations than the older Harris-Benedict formula it replaced.
From BMR to total expenditure
Multiplying by an activity factor gives total daily energy expenditure. The factors are coarse and self-reported activity is notoriously overestimated, so treat the result as a starting figure to be adjusted against what actually happens over a few weeks rather than a target to be trusted.
What the equation cannot account for
Body composition is the largest omission. Muscle is more metabolically active than fat, so two people of identical height, weight, age and sex can differ by several hundred kilocalories a day. Thyroid function, genetics, medications, sleep and recent dieting history all shift the figure too.
Metabolic rate also adapts. Sustained large energy deficits reduce BMR beyond what weight loss alone would predict, which is one reason very aggressive approaches tend to stall.
Using the number sensibly
This is a population estimate, not a prescription. Anyone making significant changes to how they eat or train — and particularly anyone with a medical condition or a history of disordered eating — should work with a doctor or a registered dietitian rather than a formula.