BMR calculator
For a typical desk worker, basal metabolic rate accounts for around 60 to 70 per cent of total daily energy use. Digestion takes another ten. Deliberate exercise, even a solid hour of it, is usually the smallest slice. Diet changes therefore move weight faster than exercise does, and burning off a large meal takes considerably longer than eating it did.
An estimate from a population equation. Individual metabolic rate varies by 10–15% from the predicted figure even between people of identical size, and more if body composition is unusual.
Basal metabolic rate is the energy the body uses at complete rest. Mifflin–St Jeor estimates it from weight, height, age and sex: on the panel defaults, a 30-year-old woman of 175 cm and 75 kg comes out at 1,533 kcal a day, and the same figures for a man give 1,699. Multiply by an activity factor of 1.2 to 1.9 for the daily total.
How to calculate BMR
The equation is a regression fitted to 498 adults. Mifflin and colleagues measured resting energy expenditure by indirect calorimetry in 247 women and 251 men aged 19 to 78, roughly half of them normal weight and half obese, and found that weight, height, age and sex together explained about 71% of the variance. That is what the panel is running. The missing 29% has a name: the same paper reports that fat-free mass alone, a single variable, explained 64%. Body composition is doing most of the work, and this equation cannot see it.
Every regression also carries the population it was fitted on. A 2005 systematic review for the American Dietetic Association compared four commonly used equations and put Mifflin–St Jeor first, landing within 10% of measured resting rate more often than Harris–Benedict, Owen or WHO/FAO/UNU, with the narrowest error range. The same review flagged what the validation work had not covered: older adults and US ethnic minorities were underrepresented both in deriving the equations and in testing them. An equation fitted mainly on middle-aged white adults is a weaker claim about a 78-year-old or about someone whose ancestry was barely present in the sample, and the review recommends treating it as such rather than reading the output as a measurement.
For an individual, then, expect a spread of 10 to 15 per cent either side, driven by lean mass, thyroid function and simple measurement error. If you know your body fat percentage from a reasonable method, the Katch–McArdle equation works from lean mass instead and tends to fit better at the extremes of build, where weight-based equations struggle most.
What people use it for
- Setting a calorie target before dieting
- Seeing how much of the day is resting rate and how much is activity
- Feeding a macro split
- Checking a figure an app produced against the published equation
Questions
The energy the body uses at complete rest to run its basic functions: breathing, circulation, brain activity, tissue repair. Strictly, basal rate is measured after an overnight fast in a thermoneutral room; what most calculators including this one estimate is resting rate, a few per cent higher.
BMR is at rest. TDEE multiplies it by an activity factor to cover digestion and everything you do. TDEE is the figure to eat against.
Mifflin–St Jeor for most people, on the strength of the 2005 review that compared it with three alternatives. Katch–McArdle is better if you have a credible body fat percentage, because it works from lean mass, which predicts resting rate more directly than bodyweight does.
Mifflin's own paper found the 1919 Harris–Benedict equations overestimated measured resting expenditure by about 5% in its sample. How much that is depends entirely on who you are. At the panel defaults — 30 years old, 175 cm, 75 kg — the original 1919 equations give 1,556 for a woman against Mifflin's 1,533, a gap of 23 kcal a day. Switch the sex selector to male and the same comparison is 1,771 against 1,699, a gap of 72. Either way an activity multiplier of up to 1.9 magnifies whichever gap applies to you.
Barely. Skeletal muscle costs roughly 13 kcal per kilogram per day at rest, so several kilograms of hard-won muscle buys well under a hundred calories. Organs do most of the resting burning, and you cannot train those.
Population equations carry a 10 to 15 per cent spread for an individual, and self-reported intake is systematically lower than actual intake. Adjust from measured weight change rather than from the equation.
Less reliably. The 2005 review specifically noted that older adults were underrepresented in both the derivation and the validation of these equations, and resting rate falls with age for reasons beyond the linear age term the formula uses.
Check the sex selector. It defaults to female, and the equation carries a fixed offset of minus 161 for women against plus 5 for men: 166 kcal a day at identical height, weight and age.