The short answer
BMR is what your body burns at complete rest. Mifflin-St Jeor is the formula to use: for a 30 year old man at 180 cm and 80 kg it returns about 1,780 calories per day. Do not eat at this number, it is your resting cost, not your daily need.
What BMR actually measures
Your basal metabolic rate is the energy cost of staying alive with no activity at all: keeping your heart beating, your lungs working, your body temperature stable, your cells repairing themselves. For most people it accounts for 60 to 70 percent of everything they burn in a day.
That share surprises people who assume exercise dominates. A hard 45 minute session might account for 400 calories. Simply existing accounts for four times that before you get out of bed.
BMR is driven mostly by how much tissue you have, which is why body weight is the strongest input in every formula, and why larger people have higher BMRs. Age lowers it gradually, and men typically sit higher than women at the same weight and height because they carry more lean mass on average.
The three formulas
| Formula | Year | Inputs | Best for |
|---|---|---|---|
| Mifflin-St Jeor | 1990 | Sex, weight, height, age | The general population |
| Harris-Benedict (revised) | 1984 | Sex, weight, height, age | Historical comparison |
| Katch-McArdle | 1996 | Lean mass | Lean, muscular people |
Mifflin-St Jeor is the default, and it is the one this calculator highlights. A 2005 systematic review by the Academy of Nutrition and Dietetics compared the available equations against measured resting energy expenditure and found Mifflin-St Jeor was within 10 percent of the true value more often than any alternative in non-obese and obese adults alike.
Harris-Benedict is the oldest, first published in 1919 and revised by Roza and Shizgal in 1984. It tends to run slightly higher than Mifflin-St Jeor. It is included here for comparison because it still appears widely, not because it is the better choice.
Katch-McArdle takes a different route: it ignores sex entirely and works from lean body mass, on the reasoning that muscle is what burns the energy and sex is only a proxy for how much of it you have. That makes it the most accurate of the three for lean, muscular people, and it needs a body fat percentage, which is why it only appears above once you supply one. If you do not have that figure, the body fat calculator will estimate it from tape measurements.
Why we do not average them
It would be easy to add the three together and present a single confident number. That would be worse.
The formulas disagree by 100 to 200 calories for the same person, and that gap is real information: it is the honest width of the estimate. Averaging it away produces a figure that looks precise to the calorie and is not. Any of these numbers can be off by 10 percent for an individual even when the formula is working as designed, because they are regressions fitted to populations and you are not a population.
Use the spread as a sanity range. If two formulas say 1,700 and 1,850, your real BMR is somewhere in that neighbourhood, and no amount of extra decimal places will narrow it.
What to do with the number
BMR on its own is not actionable. It becomes useful in two ways.
Multiply it to get your TDEE. Your total daily energy expenditure is BMR times an activity factor, roughly 1.2 if you are sedentary and up to 1.9 for physical work plus training. That total is what you set a calorie target against. Our TDEE calculator does this step, and its BMR figure comes from the same Mifflin-St Jeor equation you see here.
Treat it as a floor. Deliberately eating below your BMR for extended periods is where crash dieting goes wrong: you lose lean tissue alongside fat, which lowers the BMR itself and makes the weight harder to keep off. If a calorie target lands below this number, the target is too aggressive.
The number you cannot calculate
Here is the asymmetry worth sitting with. Your BMR is fixed by your body and changes slowly, over months, and mostly through changes in lean mass. You cannot negotiate with it.
What you eat is the side that moves daily and the side you actually control. It is also the side almost nobody measures accurately: validation research using doubly labelled water finds people underreport intake by a meaningful margin without meaning to, simply because recall is unreliable and logging is tedious.
CalAlly attacks the tedious part. Describe the meal out loud, the AI turns it into ingredients, calories and macros, one tap logs it, about 3 seconds. The AI coach then works from what you actually ate rather than what you remembered eating.