BMR Calculator

How to use the BMR calculator

Pick the sex constant, then enter age, weight and height, and the tool returns what the Mifflin-St Jeor equation estimates for basal metabolic rate in kilocalories per day. The fields arrive pre-filled with a representative profile, a 30-year-old at 70 kg and 175 cm, so you can watch the arithmetic before you swap in your own figures.

The sex toggle matters more than it looks: the two forms of the equation share their first three terms exactly and differ only in a closing constant of plus 5 or minus 161, so that single choice moves the published estimate by 166 kilocalories with nothing else changed. Weight and height each carry a unit toggle, and the tool converts to kilograms and centimetres internally because Mifflin-St Jeor is defined in metric.

Basal metabolic rate is the energy a body uses at complete rest across a full twenty-four hours simply to stay alive: the heart pumping, the lungs breathing, the kidneys filtering, cells repairing and replacing themselves. The measurement it approximates assumes an awake but motionless subject, in a neutral temperature, who has not eaten for twelve hours and is not digesting anything.

That is a laboratory condition, produced in the original studies with a ventilated hood and indirect calorimetry, and it is not a description of anybody at home. What the equation gives you is a statistical prediction of what such a measurement would have returned for a body of your sex, age, weight and height, drawn from the sample that Mifflin and colleagues measured in 1990.

Read the number as a floor rather than a budget. Almost nobody spends a day at complete rest, so the energy actually spent over a real day is higher, and turning a resting estimate into a whole-day one means multiplying by an activity factor, which is what the tdee-calculator does with this same figure.

This tool does not tell you what to eat and cannot: it reports what one published equation predicts for a body of certain dimensions, and that is a different thing from a target.

Why two respected formulas disagree

The table above sets Mifflin-St Jeor next to the Harris-Benedict equation, first published in 1919 and revised by Roza and Shizgal in 1984, so you can see how far two respected formulas disagree on identical inputs. The gap runs from around 13 kilocalories to over 100 across the profiles listed, and it is not constant.

Because Mifflin-St Jeor subtracts 5 kilocalories per year of age while the revised Harris-Benedict subtracts 5.677 for men and 4.330 for women, the two formulas converge with age for male bodies and diverge for female ones. That behaviour is a property of the two regressions rather than of any real metabolism, and it is the plainest available illustration that a predicted resting rate is a model output with real uncertainty attached.

The formula, worked line by line

Mifflin-St Jeor takes weight in kilograms, height in centimetres and age in years, applies a fixed coefficient to each, and closes with a constant set by sex. It was published by M. D. Mifflin, S. T. St Jeor and colleagues in the American Journal of Clinical Nutrition in 1990, derived from 498 healthy adults, 247 women and 251 men, aged 19 to 78, whose resting energy expenditure was measured directly by indirect calorimetry under a ventilated hood.

The two forms of the equation are identical except for that closing constant, which is what makes the sex toggle on this page so blunt an instrument. The equation has no term for muscle mass, body composition, thyroid function, genetics, medication or anything else that genuinely moves a resting metabolic rate; it has four inputs because those were the four the regression could use.

male:   BMR = 10 x kg + 6.25 x cm - 5 x age + 5
female: BMR = 10 x kg + 6.25 x cm - 5 x age - 161
Harris-Benedict (Roza-Shizgal 1984), male:
        BMR = 88.362 + 13.397 x kg + 4.799 x cm - 5.677 x age

Run the same body through the 1984 Roza-Shizgal revision of Harris-Benedict for comparison: 88.362 + 13.397 x 70 + 4.799 x 175 - 5.677 x 30 comes to 1,695.67, about 1,696 kcal, which is 47 kilocalories above the Mifflin-St Jeor figure. Roza and Shizgal rebuilt the 1919 equation from 337 subjects, 168 men and 169 women, and reported correlations of 0.77 for men and 0.68 for women.

A systematic review published in the Journal of the American Dietetic Association in 2005 found Mifflin-St Jeor the most reliable of the common predictive equations for non-obese and obese adults alike, which is why it became the default in clinical practice, but reliable here means most often within 10 percent of a measured value, not exact.

That 10 percent band is the honest way to hold any of these numbers. On a predicted 1,649 kcal it spans roughly 1,484 to 1,814, and a meaningful minority of people fall outside it entirely.

Muscle mass is the largest single factor the equation cannot see: lean tissue is metabolically more active than fat, so two people identical on all four inputs can differ substantially in measured resting expenditure. Thyroid function, some medications, illness, pregnancy and a long history of restricted eating all move the real figure too.

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