How this instrument works
Resting metabolic rate (RMR) is roughly how many calories the body burns over 24 hours doing nothing but staying alive — keeping organs running, maintaining body temperature, breathing — before anything is added for movement, exercise, or digesting food. The Mifflin-St Jeor equation estimates it from four inputs: weight in kilograms, height in centimeters, age in years, and sex, using a single constant at the end that differs between men and women. It was published in 1990 by Mifflin, St Jeor, and colleagues, derived from indirect calorimetry measurements on 498 healthy adults.
This site also has separate calculators built on the older Harris-Benedict equation and on Katch-McArdle, which uses lean body mass directly. Mifflin-St Jeor is the newer of the two weight-based options and is generally considered the more accurate default for modern populations — a 2005 systematic review by Frankenfield, Roth-Yousey, and Compher, comparing several predictive equations against measured resting metabolism in healthy nonobese and obese adults, found it the most consistently accurate of the equations compared, including Harris-Benedict. Katch-McArdle can be more precise still, but only if you already know your body-fat percentage to calculate lean mass from — Mifflin-St Jeor needs no such extra measurement.
RMR by itself isn't total daily calorie burn — it's the resting baseline. Getting to total daily energy expenditure means adding calories for daily movement and exercise on top of this number. It's also a population-based estimate rather than a direct measurement of any one person: the equation was fit to averages across hundreds of people, so an individual's true resting rate can differ from this estimate by a meaningful margin, more so at the extremes of body size. Indirect calorimetry — actually measuring oxygen consumption and carbon dioxide production — remains the accurate way to know an individual's real number.
- Set "Sex is female" to Yes or No — the formula adds 5 for men and subtracts 161 for women.
- Enter weight in kilograms — divide pounds by 2.2 first if needed.
- Enter height in centimeters — multiply inches by 2.54 first if needed.
- Enter age in whole years.
- Read the estimated resting metabolic rate in kcal/day.
Worked example — 70 kg, 170 cm, 30-year-old man
A 70 kg man, 170 cm tall, 30 years old: 10 × 70 + 6.25 × 170 − 5 × 30 + 5 = 700 + 1062.5 − 150 + 5 = 1617.5 kcal/day. That's his estimated energy burn at rest over a full 24 hours, before adding anything for daily activity.
For comparison, a 60 kg, 160 cm, 25-year-old woman: 10 × 60 + 6.25 × 160 − 5 × 25 − 161 = 600 + 1000 − 125 − 161 = 1314 kcal/day — the identical equation, just with the women's constant substituted at the end in place of the men's.
Questions
Is RMR the same thing as BMR (basal metabolic rate)?
They're closely related and often used interchangeably in everyday conversation. Strictly, BMR is measured under very controlled conditions — after waking, after a 12-hour fast, at complete rest — while RMR is typically measured under somewhat less strict conditions and so tends to come out a little higher in practice. Equations like Mifflin-St Jeor were built from data collected under RMR-style conditions, and in everyday use, the terms describe essentially the same idea: energy burned at rest.
Why does this site have Harris-Benedict, Katch-McArdle, and this Mifflin-St Jeor calculator?
Each reflects a different era and a different set of inputs. Harris-Benedict dates to 1919 (later revised in 1984) and was the standard for decades. Mifflin-St Jeor, published in 1990 from a larger and more contemporary dataset, has since been found more accurate for most healthy adults in comparative studies — notably a 2005 systematic review by Frankenfield and colleagues. Katch-McArdle instead uses lean body mass directly, which can be more precise, but only for someone who already knows their body-fat percentage from another measurement.
Does this number include exercise or daily activity?
No. RMR is only the energy cost of staying alive at rest — it doesn't include movement, exercise, or the extra calories spent digesting food. Estimating total daily energy needs means scaling this resting figure up by an activity factor that reflects how physically demanding a typical day is, from largely sedentary to very physically active. This calculator returns only the resting number.
Why does the equation add 5 for men but subtract 161 for women?
In the original 1990 regression, Mifflin, St Jeor, and colleagues found that men and women differ systematically in resting metabolic rate even after weight, height, and age are already accounted for — largely reflecting that men, on average, carry more metabolically active lean mass at a given weight. Rather than requiring a separate body-composition measurement, the equation captures that difference with one constant at the end, +5 for men versus −161 for women.
How accurate is the Mifflin-St Jeor equation for an individual?
In the 2005 Frankenfield, Roth-Yousey, and Compher systematic review comparing predictive equations against measured resting metabolism, Mifflin-St Jeor came out as the most consistently accurate of the equations studied for healthy nonobese and obese adults. Even so, any group-derived equation carries real individual error — accuracy tends to be lower at higher BMI and for people whose metabolism deviates from the population average — so treat any single result as a reasonable estimate rather than a measured value.
Can this calculator be used for children?
No — the Mifflin-St Jeor equation was derived from adult data and isn't validated for children or adolescents, whose metabolic rates follow different growth-related patterns. Pediatric energy needs should come from a formula or reference specific to children, ideally with a pediatrician or dietitian involved.
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, Am J Clin Nutr, 1990
- Frankenfield D, Roth-Yousey L, Compher C — Comparison of predictive equations for resting metabolic rate in healthy nonobese and obese adults: a systematic review, J Am Diet Assoc, 2005
Read this first: This instrument computes a screening figure from population formulas — it is not a diagnosis, and it cannot see the whole picture a clinician can. Use it to inform a conversation, not to replace one.