BMR Calculator: Basal Metabolic Rate (Mifflin-St Jeor)

What is BMR?

Basal metabolic rate (BMR) is the energy a body uses at complete rest, just to keep working: breathing, pumping blood, staying warm, repairing cells, running the brain. A BMR calculator estimates that daily energy from four inputs: sex, age, height and weight. This one applies the Mifflin-St Jeor equation and reports the answer in kilocalories per day (kcal/day) and in kilojoules per day (kJ/day).

The output is a statistical estimate for a population, not a measurement of one person. A true measurement needs indirect calorimetry, which records the oxygen a person uses and the carbon dioxide they breathe out under controlled resting conditions.

BMR formula

The calculator uses the equation published by Mifflin, St Jeor and colleagues in 1990. Weight goes in as kilograms, height as centimetres, age as years, and the result comes out as kcal/day.

Men:

$BMR = 10 \times \text{weight (kg)} + 6.25 \times \text{height (cm)} - 5 \times \text{age (y)} + 5$

Women:

$BMR = 10 \times \text{weight (kg)} + 6.25 \times \text{height (cm)} - 5 \times \text{age (y)} - 161$

The two versions are identical except for the final constant: plus 5 for men, minus 161 for women. Both constants come from the same 1990 study, and the calculator does not extrapolate past them.

Two conversions run before the equation does. Imperial height becomes centimetres as (feet × 12 + inches) × 2.54, and pounds become kilograms as pounds × 0.45359237. Both factors are exact by definition under the 1959 International Yard and Pound Agreement. The kilojoule figure is the same energy in SI units, since 1 kilocalorie is exactly 4.184 kilojoules (NIST Special Publication 811).

One naming point: the "calorie" on a food label is a kilocalorie. A 2,000 calorie diet means 2,000 kcal, so the kcal figure here is on the same scale as food labels.

How to calculate BMR

1. Put weight in kilograms and height in centimetres, converting from pounds and feet or inches first if needed.
2. Multiply weight in kilograms by 10.
3. Multiply height in centimetres by 6.25.
4. Multiply age in years by 5.
5. Add the first two products, subtract the third, then add 5 for a man or subtract 161 for a woman.

To read the result in kilojoules, multiply the kcal figure by 4.184.

Worked example

Metric example. A man aged 30, 175 cm tall, weighing 70 kg.

$10 \times 70 = 700$ $6.25 \times 175 = 1093.75$ $5 \times 30 = 150$ $BMR = 700 + 1093.75 - 150 + 5 = 1648.75$

The calculator displays 1,649 kcal/day. In kilojoules that is 1648.75 × 4.184 = 6898.37, displayed as 6,898 kJ/day.

Imperial example. A woman aged 45, 5 feet 4 inches tall, weighing 150 lb.

First the conversions: 5 × 12 + 4 = 64 inches, and 64 × 2.54 = 162.56 cm. Then 150 × 0.45359237 = 68.0388555 kg, which the calculator prints as 68.04 kg.

$10 \times 68.0388555 = 680.388555$ $6.25 \times 162.56 = 1016$ $5 \times 45 = 225$ $BMR = 680.388555 + 1016 - 225 - 161 = 1310.39$

The calculator displays 1,310 kcal/day, and 1310.388555 × 4.184 = 5482.67, displayed as 5,483 kJ/day.

The arithmetic runs on the unrounded numbers and rounds once, at the end. Redoing a line by hand from the rounded height and weight can land one digit away from the figure on screen.

What the number covers, and what it does not

BMR is only the resting share of daily energy use. It leaves out walking, working, exercise, and the energy spent digesting food. Total daily energy expenditure (TDEE) is the whole amount, and it is usually estimated as BMR multiplied by a physical activity level factor. That multiplication is a separate calculation and this page does not perform it.

The Mifflin-St Jeor study measured resting metabolic rate (RMR), not strictly basal rate. A basal measurement requires an overnight fast and a full rest period, usually on waking; a resting measurement is taken under looser conditions and comes out a few percent higher. The label BMR is the common one and this page uses it, but the equation predicts RMR.

Limits of the estimate

• The equation was fitted on 498 healthy adults aged 19 to 78. It was not validated for children, for adolescents, during pregnancy, or for adults older than that range. The calculator marks an age outside 19 to 78 with a note.
• A systematic review by Frankenfield and colleagues in 2005 found the equation predicted measured resting metabolic rate to within 10 percent for about 82 percent of adults who were not obese, and about 70 percent of adults who were obese. Individual error can pass 20 percent.
• The inputs include total body weight, not lean mass. Muscle burns more energy at rest than fat does, so at the same weight the estimate tends to read high for someone with a lot of body fat and low for someone very muscular.
• Accuracy falls off at the extremes of height and weight, where the study population held few people.
• Because the equation is a straight line, very small bodies combined with a high age can push it to zero or below. The calculator reports that as an error rather than printing a number the equation cannot support.

Frequently asked questions

What counts as a normal BMR? There is no cutoff value. BMR tracks body size closely, so a bigger body has a higher BMR, and comparing two people only makes sense alongside their height, weight, age and sex. The two examples above, 1,649 and 1,310 kcal/day, show how far ordinary adult figures can sit apart.

Why do men and women get different results? The published equation has one coefficient set for each, differing only in the final constant. On average, men in the study population carried more lean mass at the same height and weight, and the two constants absorb that difference.

Is BMR the same as the calories a person burns in a day? No. BMR is the resting portion only. The daily total also includes movement, exercise and digestion, so it is higher.

How does Mifflin-St Jeor compare with Harris-Benedict? Harris-Benedict, first published in 1919 and revised in 1984, is the older equation. The 2005 Frankenfield review compared several predictive equations and concluded that Mifflin-St Jeor was the one most likely to land within 10 percent of measured resting metabolic rate in both nonobese and obese adults.

Why show kilojoules as well as kilocalories? The kilojoule is the SI unit of energy, and food labels in many countries print kJ. The conversion is exact: 1 kcal is 4.184 kJ.

What happens if the age entered is outside 19 to 78? The equation still returns a number, and the calculator shows it with a note that the age falls outside the population the equation was fitted on. The estimate carries more uncertainty there.

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. doi:10.1093/ajcn/51.2.241
• 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. doi:10.1016/j.jada.2005.02.005
• National Institute of Standards and Technology. Guide for the Use of the International System of Units (SI), NIST Special Publication 811, 2008 edition, Appendix B.