What BMR Actually Measures
Basal Metabolic Rate is the calorie cost of keeping you alive while you do nothing. Lying still in a thermally neutral room, awake but completely at rest, with your last meal long enough ago that digestion is finished — that is the state BMR is defined for. Your heart still beats, your lungs still breathe, your brain still consumes its share of glucose, and every cell in your body continues its slow metabolic turnover. The total energy that scaffolding requires is what we call BMR, and for most adults it falls somewhere between 1,200 and 2,000 kcal per day. This is the single largest component of total daily energy expenditure for nearly everyone except elite endurance athletes — typically around 60–70% of all calories burned in a day.
BMR is sometimes used interchangeably with Resting Metabolic Rate (RMR), but the two are slightly different. BMR is measured under stricter laboratory conditions (overnight fast, fully relaxed, controlled temperature). RMR is measured more loosely and tends to run a few percent higher because it includes a small amount of digestive activity and incidental movement. For most non-clinical purposes the gap is negligible, and the calculators you see online — including this one — give you a number that lands between the two.
The Mifflin-St Jeor Formula
The Mifflin-St Jeor equation, published in 1990 in the American Journal of Clinical Nutrition, is the modern standard for estimating BMR in healthy adults. It replaced the older Harris-Benedict equation (1919) because it predicted measured resting energy expenditure within roughly 5% across a much wider population, including people with obesity. The equation uses four inputs: weight in kilograms, height in centimetres, age in years, and a sex-based constant.
Men: BMR = 10 × weight(kg) + 6.25 × height(cm) − 5 × age + 5
Women: BMR = 10 × weight(kg) + 6.25 × height(cm) − 5 × age − 161
The 166-point gap between the male and female constants reflects average differences in body composition, not a claim about any specific individual. Other equations exist — Harris-Benedict, Cunningham, Katch-McArdle (which uses lean body mass instead of total weight) — but Mifflin-St Jeor remains the default unless you have a body-fat measurement, in which case Katch-McArdle is usually more accurate.
How to Use This Calculator
- Pick a unit system. The math runs in metric internally; imperial inputs are converted (1 lb = 0.45359237 kg, 1 in = 2.54 cm).
- Select sex. The constant changes by 166 kcal between the two equations — small but not negligible.
- Enter age in whole years. The age coefficient subtracts 5 kcal per year, so even a one-year change shifts the number.
- Enter your current weight and height. The result updates as you type.
- Read the BMR in kcal/day. Round to the nearest whole number — Mifflin-St Jeor is precise to about ±10%, so finer precision is false confidence.
Worked Examples
Example 1 — Adult man, average
30-year-old, 70 kg, 175 cm. BMR = 10(70) + 6.25(175) − 5(30) + 5 = 700 + 1,093.75 − 150 + 5 = 1,649 kcal/day. That is the floor: any movement, work, or exercise adds on top.
Example 2 — Adult woman
28-year-old, 60 kg, 165 cm. BMR = 10(60) + 6.25(165) − 5(28) − 161 = 600 + 1,031.25 − 140 − 161 = 1,330 kcal/day. Eating below this number for any extended period reliably depresses metabolic rate as the body down-regulates.
Example 3 — Older adult, lower BMR
65-year-old man, 80 kg, 178 cm. BMR = 10(80) + 6.25(178) − 5(65) + 5 = 800 + 1,112.5 − 325 + 5 = 1,593 kcal/day. The age term costs him 175 kcal compared to a 30-year-old with the same body — a real number you can see in the equation.
What Drives BMR Up or Down
- Lean body mass. Muscle tissue burns roughly 6 kcal/lb/day at rest; fat burns about 2 kcal/lb/day. The single largest individual lever for raising BMR is gaining muscle.
- Age. BMR declines roughly 1–2% per decade after age 20, primarily because lean mass drops with age unless you actively train.
- Sex. Men typically have higher BMRs than women of the same height and weight because of higher average lean-mass percentage. The 166 kcal constant in the equations approximates this.
- Thyroid function. Hypothyroidism can lower BMR by 10–20%; hyperthyroidism can raise it by 30% or more. Persistent fatigue with weight changes warrants a TSH test.
- Genetics. Identical-twin studies suggest 30–50% of BMR variance is heritable.
- Crash dieting. Severe, prolonged calorie restriction can suppress BMR by 10–15% beyond what weight loss alone would predict — a phenomenon called adaptive thermogenesis.
- Climate and ambient temperature. Cold environments slightly raise BMR through non-shivering thermogenesis; the effect is modest but measurable.
- Pregnancy and lactation. Both raise energy needs, but BMR changes vary widely; specific maternal nutrition guidance is more reliable than generic equations.
BMR, RMR, TDEE — How They Connect
BMR is one floor of a layered model of daily energy expenditure. Stack on the thermic effect of food (about 10% of intake spent on digestion), then non-exercise activity thermogenesis (NEAT — walking, fidgeting, posture), then exercise activity thermogenesis (EAT — structured workouts), and you get TDEE. For most desk workers, TDEE = BMR × 1.2; for hard-training athletes it can exceed BMR × 1.9. If BMR is the engine at idle, TDEE is the engine under the load of a normal day. To plan calorie intake for weight loss, gain, or maintenance, TDEE is the number you actually want; BMR sets the floor you should not eat below.
Common Misconceptions
- "BMR is what I should eat to lose weight." No. BMR is the floor; eating at BMR over time pushes adaptive thermogenesis. A sustainable cut sits between BMR and TDEE — typically 15–25% below TDEE.
- "Eating breakfast boosts metabolism." Meal timing has very small effects on BMR; meal size and composition dominate the thermic effect of food.
- "Drinking ice water burns hundreds of extra calories." The thermogenic effect is real but tiny — measured at roughly 5 kcal per 500 ml of cold water.
- "My BMR is broken because of years of dieting." Adaptive thermogenesis is real but largely reversible. Several months of eating at maintenance with progressive resistance training restores most of the suppression.
Frequently Asked Questions
How accurate is the Mifflin-St Jeor equation?
Studies comparing it against indirect calorimetry — the gold standard — find it lands within ~10% of measured BMR for about 80% of healthy adults. It tends to overestimate slightly in older adults and underestimate slightly in athletes with high lean mass.
Why do men and women have different formulas?
Average body composition differs — men typically carry more lean mass for the same height and weight, and lean mass burns more at rest. The 166-kcal constant difference approximates that gap on average, but it is a population-level adjustment, not a personal one.
Should I eat above or below my BMR?
Above, almost always. Aim for TDEE for maintenance, TDEE × 0.8 for a moderate cut, TDEE × 1.1 for a moderate surplus. Eating below BMR for extended periods triggers metabolic adaptation and tends to be unsustainable.
Does muscle really burn that many extra calories?
Less than gym-folklore suggests. Skeletal muscle burns about 6 kcal per pound per day at rest, versus 2 kcal/lb/day for fat. Adding 5 lb of muscle nets you roughly 20 extra kcal/day — meaningful over a year, not in a week.
Does my BMR change with the seasons?
Slightly. Cold exposure increases non-shivering thermogenesis through brown fat activation, which can nudge BMR a few percent in winter, especially if you spend time outdoors.
Is my data stored?
No. Every calculation runs in your browser. Age, weight, height, and sex are never sent to a server, never logged, and never persisted after the page closes.
References
- Mifflin MD, St Jeor ST, et al. A new predictive equation for resting energy expenditure in healthy individuals. American Journal of Clinical Nutrition, 1990;51(2):241–247.
- Harris JA, Benedict FG. A Biometric Study of Basal Metabolism in Man. Carnegie Institution of Washington, Publication No. 279, 1919.
- Frankenfield D, Roth-Yousey L, Compher C. Comparison of predictive equations for resting metabolic rate in healthy nonobese and obese adults. Journal of the American Dietetic Association, 2005;105(5):775–789.
- Müller MJ, Bosy-Westphal A. Adaptive thermogenesis with weight loss in humans. Obesity (Silver Spring), 2013;21(2):218–228.
- U.S. National Institutes of Health, Office of Dietary Supplements. Public reference on basal metabolic rate and energy balance.