BMR Calculator (Basal Metabolic Rate)

Basal Metabolic Rate (BMR) is the amount of energy your body uses at complete rest to keep the basic biological machinery running — heartbeat, breathing, cell maintenance, temperature regulation, kidney and liver function. It is the largest component of daily calorie expenditure, typically 60-75% of total daily energy needs for most people.

BMR is measured by indirect calorimetry (measuring oxygen consumption and carbon dioxide production during complete rest, ideally after a 10-12 hour fast). Because this measurement requires specialised equipment, most people estimate BMR using validated predictive equations. This calculator uses the Mifflin-St Jeor (1990) formula, the current clinical standard.

BMR is often confused with Resting Metabolic Rate (RMR) or Resting Energy Expenditure (REE). Technically BMR is measured immediately upon waking after 8 hours of sleep and 12 hours of fasting; RMR is a less-strict version measured after rest but without the sleep/fasting requirements. RMR is typically 5-10% higher than true BMR because of the small metabolic effects of prior food and activity. In everyday use the terms are used interchangeably.

Three widely-cited formulas estimate BMR from height, weight, age, and sex:

BMR is only the resting portion of daily energy expenditure. Total Daily Energy Expenditure (TDEE) — computed by the Calorie Calculator — includes energy burned through movement (physical activity) and the thermic effect of food (energy used to digest and store nutrients). TDEE is what you actually need to maintain body weight.

The standard method for estimating TDEE from BMR is to multiply by an activity factor:

BMR typically declines about 1-2% per decade in adulthood, driven primarily by loss of lean body mass. Skeletal muscle is metabolically active tissue — it burns roughly 6-10 kcal per pound of muscle per day at rest, versus 2 kcal per pound of fat. Losing 5-10 pounds of muscle per decade (typical without resistance training) drops daily BMR by 30-100 kcal.

Resistance training preserves and even builds muscle into old age, largely preventing this BMR decline. Studies of adults 60-90 following progressive strength programs show BMR maintained or increased. Adequate protein intake (0.7-1.0 g per pound of lean body mass daily) supports muscle preservation during this training.

Diet-induced weight loss also lowers BMR proportionally as body mass drops, and by an additional "adaptive thermogenesis" effect where the body downregulates metabolic rate during caloric restriction. This is why crash diets produce unsustainable results — the metabolic rate at the new lower weight is significantly lower than the linear prediction, making maintenance harder.

BMR estimation formulas capture only ~70% of the variance in actual measured BMR between individuals. The remaining 30% comes from genetic differences, thyroid function, medication effects, and body composition details not captured by height/weight/age/sex.

Thyroid function has a large effect. Hypothyroidism can lower BMR by 15-40%; hyperthyroidism can raise it by 25-50%. If your measured or perceived metabolism seems dramatically inconsistent with formula predictions, thyroid testing is a reasonable clinical step.

Genetic variation in mitochondrial efficiency, uncoupling proteins, and neuroendocrine set-point regulation produce BMR variation of 100-300 kcal/day between individuals of otherwise identical body composition. This is the "some people just seem to burn more" phenomenon, and it is real but modest — not enough to explain large weight differences between people with similar diets and activity.

For weight loss: subtract 500 kcal from TDEE for approximately 1 lb/week fat loss (a 3,500 kcal deficit per week). More aggressive deficits (750-1,000 kcal/day) speed loss but risk muscle loss, metabolic adaptation, and unsustainability. For most people, 500 kcal/day is the sustainable maximum for long-term work.

For weight gain (muscle building): add 250-500 kcal to TDEE, combined with a resistance-training program and adequate protein intake. Faster gains typically produce more fat gain per pound of muscle. Genetic ceiling on muscle-building rate is roughly 0.5-2 lb of muscle per month for men and 0.25-1 lb per month for women, depending on training experience.

For maintenance: eat approximately TDEE. Track weight over 2-4 week windows rather than day-to-day — daily weight fluctuates 2-5 lb from water and gut content changes even at perfect energy balance.

These are starting points. Actual TDEE deviates from the formula prediction by ±15% for most people. Adjust intake based on 4-week weight trend, not initial calculation.