The equation, written out
Mifflin-St Jeor, published in 1990, is the standard first choice. In metric it reads: 10 times weight in kilograms, plus 6.25 times height in centimetres, minus 5 times age in years, then plus 5 for the male form or minus 161 for the female form. The result is kilocalories per day at complete rest, in a thermally neutral room, awake but not moving, in the fasted state.
Working the example in the lead: 5 foot 10 is 177.8 cm, 170 lb is 77.11 kg. Ten times 77.11 is 771. 6.25 times 177.8 is 1,111. Minus 5 times 30 is minus 150. Plus 5. That totals 1,737 kcal per day. The American Dietetic Association reviewed the available predictive equations in 2005 and found this one closest to measured resting expenditure in non-obese and obese adults, which is why it displaced Harris-Benedict as the default.
Closest available is not close
"Best of the predictive equations" sets a low bar. Validation studies typically find Mifflin-St Jeor within 10 percent of measured resting expenditure for around 80 percent of people, which means one person in five is outside that band and the equation gives no signal about which group you are in. On a 1,700 kcal basal rate, 10 percent is 170 kcal a day.
The reason is that the equation has four inputs and metabolic rate has more determinants than that. Lean mass is the dominant one, and height and weight are only a proxy for it. Two people matched on all four inputs can differ meaningfully in resting expenditure through muscle mass, thyroid function, organ size relative to body size, and recent dieting history. If you know your body fat percentage, the Katch-McArdle equation works from lean mass directly and is generally a better fit for people whose composition is far from average in either direction.
The activity multiplier is the loose part
Look at the table in the output. The spread between the sedentary and very heavy multipliers is larger than the whole error band on the basal rate itself. Choosing between 1.375 and 1.55 changes the daily total by several hundred kilocalories, and that choice is a five-item dropdown standing in for everything you do in a day.
| Multiplier | What it actually describes |
|---|---|
| 1.2 | Desk work, little deliberate exercise, mostly seated outside of it |
| 1.375 | Desk work plus one to three training sessions a week |
| 1.55 | Three to five real sessions a week, or a job with movement in it |
| 1.725 | Six or seven sessions a week, or heavy daily physical work |
| 1.9 | Manual labour plus structured training on top |
Self-reported activity skews high with striking consistency in the research literature. Non-exercise movement — walking, standing, fidgeting, the general restlessness of a day — varies between people by hundreds of kilocalories and is not captured by counting gym sessions at all. When you are genuinely unsure between two levels, take the lower one and let two weeks of weight data correct it.
Why the basal rate drifts with age
The age term subtracts 5 kcal per year, which over two decades is 100 kcal a day. That coefficient is capturing an association in the fitted data rather than a mechanism, and the mechanism is mostly loss of lean mass rather than age itself. Resistance training slows that loss, which is the practical version of the observation. The often-repeated claim that metabolism collapses in middle age has not held up well: a large 2021 analysis of doubly-labelled-water data found total expenditure adjusted for body composition stays fairly stable from about 20 to 60 before declining. What changes over those decades is mostly what the body is made of and how much it moves.
None of that is a reason to treat the output as a budget. It is a reason to treat it as one estimate among several, worth about as much as the assumptions you fed it.
Questions people ask
What is the difference between BMR and RMR?
Basal metabolic rate is measured under strict conditions: after an overnight fast, at complete rest, in a thermally neutral room, having not moved much before the measurement. Resting metabolic rate is the same idea measured under more relaxed conditions and comes out roughly 5 to 10 percent higher. In practice the terms are used interchangeably by most calculators, including this one, and the difference is far smaller than the error band on the prediction itself, so it rarely matters for anything you would do with the number.
Why does my smartwatch or gym scale give a different figure?
They are running different estimates from different inputs. Bioimpedance scales estimate lean mass from an electrical resistance measurement and feed that into a lean-mass-based equation, which will disagree with a height-and-weight equation whenever your composition is away from average. Wearables layer heart rate and movement data on top of a baseline they estimated the same way this page does. All of them are estimates, none is a measurement of your metabolism, and a gap of 100 to 300 kcal between two of them is ordinary rather than a sign that one is broken.
Can I raise my basal metabolic rate?
Adding lean mass raises it, but less dramatically than is usually claimed. Skeletal muscle at rest burns on the order of 13 kcal per kilogram per day, so a few kilograms of hard-won muscle is a few tens of kilocalories a day at rest. The larger effects of resistance training are the energy spent training and the recovery cost afterwards, plus the preservation of lean mass over years. Beyond that, most of what is marketed as metabolism-boosting has effects too small to measure reliably against the noise in this estimate.
Should I eat below my BMR?
That is a question for a clinician or a registered dietitian, not for a calculator, and this page deliberately does not produce a target intake. What can be said neutrally is that very low intakes make it physically difficult to meet protein and micronutrient requirements from food, and that medically supervised low-calorie programmes exist precisely because they need supervision. If you have a medical condition, take medication, are pregnant or breastfeeding, or are under 18, the numbers on this page are background reading and not a plan.