BMR Calculator

What basal metabolic rate measures

Basal metabolic rate (BMR) is the energy your body spends in a day just to stay alive while completely at rest: breathing, pumping blood, keeping organs working and holding body temperature steady. It does not include digesting food, walking around or exercise.

In a laboratory, BMR is measured by indirect calorimetry — recording how much oxygen you use and carbon dioxide you breathe out — after an overnight fast, lying still in a quiet, comfortably warm room. Most people never have that test, so calculators estimate it from body size, age and sex using equations fitted to groups of people who were measured.

Strictly, the equations on this page predict resting energy expenditure, measured under slightly less strict conditions than a true basal test. The two terms are used interchangeably in practice, and this calculator does the same.

Why the calculator asks for sex. The published equations were fitted separately to data from men and from women, so they need to know which set of coefficients to apply. They were not developed or tested for people whose body composition doesn’t match either group (for example during gender-affirming hormone therapy); in that case, the results are less certain and a clinician can advise which estimate to use.

The three BMR equations

Mifflin-St Jeor (1990) is the main result. It was derived from 498 healthy adults aged 19 to 78, both normal-weight and obese, whose resting energy expenditure was measured:

W

weight in kilograms

H

height in centimeters

A

age in years

Harris-Benedict dates from 1919. Roza and Shizgal refitted it in 1984 on the original data plus 98 further subjects, giving the revised version used here:

Katch-McArdle ignores sex and age and uses lean body mass (everything that isn’t fat), the single strongest predictor of resting energy use. The same equation was proposed by Cunningham in 1991 from a review of studies:

To estimate total daily calories, BMR is multiplied by an activity factor. The factors below are the conventional ones used with these equations; they are rules of thumb rather than measured values:

For comparison, the FAO/WHO/UNU expert report on human energy requirements describes typical physical activity levels of 1.40–1.69 × BMR for sedentary or lightly active lifestyles, 1.70–1.99 for active ones and 2.00–2.40 for vigorous ones. The 1.2 factor is therefore a low, cautious figure for someone who moves very little.

Worked example

A 35-year-old woman who is 5 ft 5 in tall and weighs 150 lb:

1. Height: 65 in × 2.54 = 165.1 cm. Weight: 150 lb × 0.45359237 = 68.04 kg.
2. Mifflin-St Jeor: 10 × 68.04 + 6.25 × 165.1 − 5 × 35 − 161 = 680.4 + 1,031.9 − 175 − 161 = 1,376 kcal/day
3. Revised Harris-Benedict: 447.593 + 9.247 × 68.04 + 3.098 × 165.1 − 4.330 × 35 = 1,437 kcal/day
4. If her body fat is 30%: lean mass = 68.04 × 0.70 = 47.63 kg, and Katch-McArdle gives 370 + 21.6 × 47.63 = 1,399 kcal/day
5. Daily calories from the Mifflin-St Jeor figure: 1,651 (sedentary), 1,892 (lightly active), 2,133 (moderately active), 2,374 (active) and 2,614 kcal (very active).

The three equations land within about 60 kcal of each other here. The gap is often wider for heavier people: for a 45-year-old man who is 180 cm tall and weighs 110 kg, Mifflin-St Jeor gives 2,005 kcal and revised Harris-Benedict 2,170 kcal. Neither is exact, which is a useful reminder of how approximate any single figure is.

Who these equations are not for

• Children and teenagers. The equations were built on adults (Mifflin-St Jeor on ages 19–78). Growth adds energy needs that they don’t capture.
• Pregnancy and breastfeeding. Both raise energy needs in ways the equations don’t include.
• Athletes and very muscular people. Weight-based equations tend to underestimate BMR when a lot of the weight is muscle. Katch-McArdle, with an accurate body fat measurement, usually fits better.
• Medical conditions. Thyroid disorders, fever, injury, recent surgery, some medications and malnutrition all change metabolism. Roza and Shizgal found Harris-Benedict unreliable in malnourished patients.
• Groups missing from the data. A 2005 systematic review noted that older adults and US ethnic minorities were under-represented both when these equations were developed and when they were tested.