BMR Calculator (Basal Metabolic Rate)

What is Basal Metabolic Rate?

Basal Metabolic Rate (BMR) is the minimum energy required to keep your body alive at complete rest: breathing, blood circulation, nutrient processing, cell production, and brain function. It is measured under strict conditions: overnight fast, supine position, thermoneutral environment, no recent exercise.

BMR estimates resting energy use, while daily needs also include digestion and movement. Even a sedentary routine usually places total expenditure above BMR. The difference varies with food intake, mobility, illness, and activity.

The NIH Bookshelf summary on energy notes that resting expenditure equations use weight, age, and sex because lean mass differences explain most between-person variation at a given body size.

This calculator estimates BMR using the Mifflin-St Jeor equation. Mifflin and colleagues published the equation in the American Journal of Clinical Nutrition after testing 498 healthy adults with indirect calorimetry. Frankenfield's systematic review and the Academy of Nutrition and Dietetics Evidence Analysis Library both favor this formula when measured resting rate is unavailable.

Consumer calculators call this value BMR, but Mifflin-St Jeor predicts resting energy expenditure measured under less restrictive conditions than a classical basal test. The distinction matters in research and clinical measurement. For everyday planning, both are starting estimates below total daily expenditure.

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The Mifflin-St Jeor formula

Men: BMR = 10 × weight (kg) + 6.25 × height (cm) - 5 × age (years) + 5. Women: BMR = 10 × weight (kg) + 6.25 × height (cm) - 5 × age (years) - 161.

The equation was derived from regression analysis of measured resting energy expenditure in 498 healthy subjects aged 19 to 78. It outperformed the original Harris-Benedict, revised Harris-Benedict, and Owen equations in predicting measured values within 10% of actual.

In Frankenfield's 2013 validation cohort of 337 community-dwelling adults, Mifflin-St Jeor was unbiased relative to indirect calorimetry and reached roughly 82% accuracy within 10%. Accuracy was lower in obese than nonobese participants.

Enter current weight, height, age, and sex. The result is kilocalories per day: what the body would burn over 24 hours in bed without eating.

BMR typically falls between 1,200 and 2,000 kcal for most adults, with men and heavier individuals at the upper end. Values outside this range warrant review of inputs or consultation with a healthcare provider.

Worked example: a 30-year-old woman who weighs 65 kg and is 165 cm tall has an estimate of 10 × 65 + 6.25 × 165 − 5 × 30 − 161 = 1,370 kcal/day after rounding. This is not her maintenance intake. Light activity at 1.375 would produce an initial TDEE estimate near 1,884 kcal/day.

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BMR vs RMR

Basal metabolic rate and resting metabolic rate describe similar concepts but differ in measurement protocol. BMR requires more restrictive conditions and typically reads 10 to 20% lower than RMR measured under everyday resting conditions.

Predictive equations like Mifflin-St Jeor estimate resting expenditure rather than a value measured under a strict basal protocol, but the label "BMR" persists in consumer calculators and fitness literature. For general nutrition planning, both values require an activity estimate.

If you need clinical precision, for example in hospital nutrition support, indirect calorimetry measures actual resting energy expenditure rather than relying on formulas.

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Factors that influence BMR

Total body mass and lean mass are the strongest predictors. Muscle tissue consumes more energy at rest than adipose tissue, so two people of equal weight but different body composition can have different BMRs not captured by weight-only equations.

Age reduces BMR mainly through sarcopenia, progressive loss of muscle after about age 30.

Sex differences in equations reflect average lean mass differences between men and women at the same age and weight. Genetics, thyroid status, ambient temperature, caffeine, nicotine, and menstrual cycle phase cause smaller but measurable variations. Fever and injury substantially raise metabolic rate temporarily.

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How to use your BMR

Multiply BMR by an activity factor to estimate total daily energy expenditure (TDEE). Sedentary: ×1.2. Lightly active (1 to 3 days exercise): ×1.375. Moderately active (3 to 5 days): ×1.55. Very active (6 to 7 days): ×1.725. Extra active (physical job plus training): ×1.9.

Eat below TDEE to lose weight, at TDEE to maintain, and above TDEE to gain. BMR is only the starting component of total expenditure.

Validate estimates empirically: track food and weight for 2 to 3 weeks. Stable weight means your intake equals your true TDEE, regardless of what any equation calculates.

Do not use BMR itself as an intake target. Safety depends on total energy availability, nutrient intake, body size, health, and training, not whether intake sits a few calories above or below one predicted resting value. Set goals from TDEE and clinical context.

The National Academies notes that lean body mass differences explain most sex and age gaps in resting energy expenditure. Weight-only equations miss that variation, which is why composition-aware formulas exist for people who know their body fat percentage.

Illness, fever, and injury can raise BMR 10 to 30% above baseline as the immune system and tissue repair demand extra energy. Predictive equations assume healthy resting conditions and may underestimate needs during acute illness.

Use one equation consistently, select an activity level that reflects the whole week, and round the result to a practical target. Adding exercise calories to a TDEE that already represents normal training counts activity twice. Revisit the estimate when weight, health, or routine changes.

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Measurement and equation accuracy

Indirect calorimetry estimates resting expenditure from oxygen consumption and carbon dioxide production. Proper preparation includes rest and restrictions on recent food, exercise, caffeine, and nicotine according to the laboratory protocol. Calibration and a stable measurement period also affect quality.

Validation studies often define an accurate prediction as within 10% of measured expenditure. Mifflin-St Jeor performs well on average but still misses that band for a meaningful minority. A 10% error on 1,500 kcal is 150 kcal/day before activity is added.

Request measurement when a wrong estimate could affect clinical nutrition, when repeated weight trends contradict careful records, or when body composition and disease make general equations unreliable. One measured value still requires interpretation if health or treatment changes.

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Validate BMR-based targets with weight trends

BMR cannot be checked directly with scale data because the scale reflects total energy balance. Multiply BMR by an activity factor, log intake for 14 to 28 days, and compare seven-day average weights under consistent morning conditions.

If estimated TDEE is 1,884 kcal but weight remains stable at a logged average of 1,750, use 1,750 as the better field estimate of maintenance after checking logging accuracy. If weight trends down, add the estimated energy equivalent of loss to intake to approximate expenditure.

Water shifts make the 7,700 kcal/kg rule approximate, so use several weeks. Adjust intake by 100 to 200 kcal, hold the change, and repeat. Recalculate after substantial weight loss because both resting cost and movement cost decline with a smaller body.

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Population and clinical limitations

Mifflin-St Jeor was developed in healthy adults aged 19 to 78, with normal-weight and obese participants. Children, pregnant or lactating people, elite athletes, frail older adults, and some racial or ethnic groups were not adequately represented for universal individual accuracy.

Critical illness, burns, fever, severe malnutrition, thyroid disease, cancer treatment, and major fluid shifts can change needs beyond the equation. Hospital teams may use indirect calorimetry and diagnosis-specific methods rather than an online result.

Seek care for unexplained weight loss, fainting, repeated hypoglycemia, chest pain, menstrual loss, or persistent severe fatigue. People with diabetes, kidney disease, an eating-disorder history, pregnancy, or prescribed nutrition therapy should follow their clinical plan.

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How it works

Mifflin-St Jeor (1990): men = 10×weight + 6.25×height − 5×age + 5; women = same − 161.

Frequently asked questions

• BMR vs RMR?BMR is measured under strict fasting and rest conditions. RMR is slightly higher and more commonly measured.
• How do I use BMR?Multiply BMR by an activity factor to estimate TDEE (maintenance calories).

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References

1. Mifflin et al.. A new predictive equation for resting energy expenditure in healthy individuals
2. Frankenfield et al.. Comparison of predictive equations for resting metabolic rate in healthy nonobese and obese adults
3. National Academies. Dietary Reference Intakes for Energy
4. Willis et al.. Cross-validation of resting metabolic rate prediction equations
5. Levine et al.. Non-exercise activity thermogenesis
6. Trexler et al.. Metabolic adaptation to weight loss
7. CDC. Losing Weight
8. Academy of Nutrition and Dietetics. Academy of Nutrition and Dietetics: Assess Energy Needs
9. Frankenfield. Bias and accuracy of resting metabolic rate equations in non-obese and obese adults
10. NIH/National Academies. Energy - Recommended Dietary Allowances