Understanding your BMR result
BMR is the foundation of daily energy expenditure. Total needs are estimated by multiplying BMR by an activity factor — the standard convention ranges from about 1.2 for sedentary lifestyles to about 1.9 for very active ones.
| Component | What it covers | Share of daily energy (typical) |
|---|---|---|
| Basal metabolic rate | Vital functions at complete rest | ≈ 60 – 75% in sedentary adults |
| Thermic effect of food | Digesting and processing meals | ≈ 10% |
| Physical activity | Exercise and all other movement | ≈ 15 – 30%, highly variable |
- Predictive equations estimate the average person of a given sex, age, weight and height; individual measured values commonly deviate by roughly ±10%, and the Frankenfield 2005 review found larger errors in obese individuals for most equations.
- Neither equation accounts for body composition — a muscular and a less muscular person with identical inputs receive the same estimate, though muscle tissue raises real resting expenditure. The Katch-McArdle equation uses lean mass for that purpose.
- BMR declines with age and falls during sustained calorie restriction (adaptive thermogenesis), so estimates should be revisited as circumstances change.
- BMR is not an eating target: consuming only your BMR ignores activity needs, and personalized calorie planning is best done with a registered dietitian or healthcare professional.
What is basal metabolic rate?
Basal metabolic rate (BMR) is the rate of energy expenditure at complete physical and mental rest, measured under standardized conditions — after sleep, fasted, in a thermoneutral environment. It represents the largest component of most people's daily energy expenditure, typically around 60–75% in sedentary adults, and covers essential processes such as breathing, circulation, temperature regulation and tissue maintenance.
Because direct measurement requires laboratory calorimetry, predictive equations estimate BMR from sex, age, weight and height. The Mifflin-St Jeor equation, published in the American Journal of Clinical Nutrition in 1990, was derived from indirect calorimetry in 498 healthy adults. A systematic review by Frankenfield and colleagues (2005) found it the most reliable of the common equations, predicting measured resting metabolic rate within 10% in more nonobese and obese adults than the alternatives.
The original Harris-Benedict equations date from 1918. Roza and Shizgal re-derived them in 1984 using a larger dataset; that revised form is the second estimate shown by this calculator. The two equations usually differ by a few percent for the same person, which illustrates the inherent approximation in all predictive formulas.
BMR is not a calorie target. Total daily energy expenditure adds activity on top of BMR, and health agencies advise that individual energy needs be interpreted with a healthcare professional or registered dietitian, since muscle mass, medical conditions, medications and genetics all shift metabolic rate away from any formula's prediction.
How to use this BMR calculator
- Select your sex — both equations have separate male and female forms.
- Enter your age in years.
- Enter your weight and height, using the Metric/Imperial toggle if needed.
- Compare the Mifflin-St Jeor and revised Harris-Benedict estimates; the difference between them reflects normal formula uncertainty.
- To estimate total daily calories, multiply BMR by an activity factor or use the TDEE calculator.
The formulas behind BMR
Both equations estimate resting energy needs in kilocalories per day from weight in kilograms, height in centimetres and age in years. Mifflin-St Jeor uses the same coefficients for both sexes with a different constant; the revised Harris-Benedict uses fully separate coefficients.
Common mistakes
- Treating BMR as a daily calorie target — total daily energy expenditure includes activity and is always higher than BMR.
- Mixing units, such as entering weight in pounds into a metric formula; this calculator handles conversion, but manual calculations often fail here.
- Expecting the formula to reflect body composition — standard equations ignore muscle mass, so they underestimate needs for very muscular people and can overestimate for others.
- Comparing BMR values from different equations or calculators and interpreting the gap as a real change in metabolism.
- Assuming a low formula result means a 'slow metabolism' — measured individual variation of around ±10% is normal, and only laboratory calorimetry measures actual resting expenditure.
Frequently asked questions
What is a normal BMR?
BMR depends on sex, age, weight and height, so there is no single normal value. As an illustration, the Mifflin-St Jeor equation estimates about 1,700 kcal/day for a 30-year-old man of 175 cm and 70 kg, and about 1,400 kcal/day for a 30-year-old woman of 165 cm and 60 kg. Individual measured values commonly differ from estimates by around 10%.
Which BMR formula is most accurate?
A systematic review by Frankenfield and colleagues (Journal of the American Dietetic Association, 2005) concluded that the Mifflin-St Jeor equation predicted measured resting metabolic rate within 10% of the true value more often than other common equations, in both nonobese and obese adults. That is why this calculator shows it as the primary result.
What is the difference between BMR and TDEE?
BMR is energy used at complete rest. Total daily energy expenditure (TDEE) adds the thermic effect of food and all physical activity on top of BMR. TDEE is conventionally estimated by multiplying BMR by an activity factor between about 1.2 (sedentary) and 1.9 (very active).
What is the difference between BMR and RMR?
Basal metabolic rate is measured under strict conditions — fasted, rested, thermoneutral, immediately after sleep. Resting metabolic rate (RMR) is measured under looser conditions and runs slightly higher. Predictive equations like Mifflin-St Jeor were actually validated against resting measurements, so in practice the terms are often used interchangeably for estimates.
Does muscle mass increase BMR?
Yes. Lean tissue, including muscle, is more metabolically active at rest than fat tissue, so people with more muscle burn more energy at rest. Standard BMR equations do not capture this because they use only sex, age, weight and height; equations based on lean body mass, such as Katch-McArdle, address it when body composition is known.
Should I eat at my BMR to lose weight?
BMR is not designed as an intake target. Energy needs include activity, and health agencies caution against very low calorie intakes without medical supervision. Guidance from a registered dietitian or healthcare professional is the appropriate way to set a personal calorie plan.
References
- Mifflin MD, St Jeor ST, Hill LA, Scott BJ, Daugherty SA, Koh YO. A new predictive equation for resting energy expenditure in healthy individuals. American Journal of Clinical Nutrition 1990; 51(2): 241–247.
- Roza AM, Shizgal HM. The Harris Benedict equation reevaluated: resting energy requirements and the body cell mass. American Journal of Clinical Nutrition 1984; 40(1): 168–182.
- Harris JA, Benedict FG. A biometric study of human basal metabolism. Proceedings of the National Academy of Sciences 1918; 4(12): 370–373.
- Frankenfield D, Roth-Yousey L, Compher C. Comparison of predictive equations for resting metabolic rate in healthy nonobese and obese adults: a systematic review. Journal of the American Dietetic Association 2005; 105(5): 775–789.
- National Institutes of Health (NIH). Body Weight Planner and energy-balance research resources. niddk.nih.gov.