BMR Calculator: What Your Body Burns at Rest
Your basal metabolic rate is the energy you would still spend lying perfectly still for a whole day. Enter four numbers and you get your BMR from the Mifflin-St Jeor equation, the working shown line by line, all four of the standard formulas side by side, and what the figure becomes once you start moving. Nothing is stored and nothing is sent anywhere.
Leave this empty if you do not have a measurement you trust. It only unlocks the two lean-mass formulas, Katch-McArdle and Cunningham.
At complete rest your body burns about
—
calories a day
That is the energy spent on breathing, pumping blood, running your kidneys and brain, and holding your temperature at 37 degrees. It is normally 60 to 70 per cent of everything you burn in a day, before you take a single step.
How we got your number
Mifflin-St Jeor equation, published in the American Journal of Clinical Nutrition in 1990 and recommended for healthy adults by the Academy of Nutrition and Dietetics. The same maths the BodyPal app uses.
From resting burn to a real day
| How you live | Multiplier | Calories a day |
|---|
Your BMR multiplied by an activity factor gives your total daily energy expenditure, the number you would actually eat to in order to stay the same weight. Pick the row that matches an honest average week, not your best one.
The four BMR formulas side by side
| Formula | BMR | What it uses |
|---|
The headline number uses Mifflin-St Jeor. Katch-McArdle and Cunningham work from lean body mass instead of total weight, so they need a body fat percentage before they can say anything.
Why your BMR is not what you should eat
Nobody lies still for twenty four hours. The moment you stand up, walk to the kitchen, type, fidget, digest a meal or carry shopping, you are burning above your BMR, and those additions are not small: for a desk worker they add about 20 per cent, and for someone on their feet all day closer to 80 per cent.
Digestion alone takes roughly 10 per cent of what you eat, which is why the lightest activity factor in the table above is 1.2 rather than 1.0. Eating at your BMR is therefore not maintenance and not a gentle deficit; it is a steep one, and it is the most common reason a plan falls apart in week three.
Use the resting figure as a floor and a reference point, then work from your daily total. The daily calorie calculator takes the same inputs and adds a goal, a pace, macros and the date you would reach a target weight.
BMR, RMR and what a lab actually measures
A true basal reading requires you to sleep at the testing site, wake without sitting up, and breathe into a hood after a twelve-hour fast in a temperature-controlled room. Almost nobody does this outside a research setting.
What clinics measure instead is resting metabolic rate: twenty minutes lying quietly, no overnight stay. The looser conditions let a little digestion and muscle tone creep in, so RMR reads about 10 per cent higher than BMR. Both are measured by indirect calorimetry, which tracks the oxygen you consume and the carbon dioxide you produce.
The equations on this page were fitted to measured resting data, so what they print sits between the two definitions. For deciding what to eat, the gap does not matter. For comparing one calculator against another, it explains a fair share of the difference.
Basal metabolic rate is the energy your body spends doing nothing at all. Breathing, circulating blood, filtering through your kidneys, rebuilding tissue, holding 37 degrees, and keeping your brain lit, which on its own takes around a fifth of the total. It is the largest single line in your daily energy budget, usually 60 to 70 per cent of everything you burn, and you have almost no minute-to-minute control over it.
BMR and RMR get treated as synonyms, and they are not quite. A basal reading demands an overnight stay, a twelve-hour fast and no movement at all before the mask goes on. A resting reading asks only that you lie still for twenty minutes, which is why clinics measure RMR in practice and why it lands about 10 per cent higher. The equations here were fitted against measured resting data, so the figure they print sits somewhere between the two labels.
Mifflin-St Jeor is the default on this page because it was built in 1990 on people who resemble people now. Harris-Benedict came from 1919 subjects who were leaner and more active than the modern average, and even the 1984 revision by Roza and Shizgal tends to read a few per cent high. A 2005 evidence review for the Academy of Nutrition and Dietetics compared the equations against indirect calorimetry and found Mifflin-St Jeor came within 10 per cent of the measured rate more often than the alternatives.
Treat any equation as a starting estimate. Even the best of them is within 10 per cent for roughly four people in five, which at 1,600 calories is a band of 160 either way, and the fifth person sits further out than that. Height, weight, age and sex cannot see how much muscle you carry, and muscle is metabolically busier than fat. That is where Katch-McArdle and Cunningham earn their place: both ignore total weight and work from lean body mass, so they describe a lean 85 kg athlete far better than a weight-based formula can, and they describe a very lean person better too. They also need a body fat figure, and a guessed one makes them worse than the equation they replaced. If the number matters to you, treat it as a hypothesis, eat to it for two or three weeks, watch the scale, and adjust from what actually happened.
More free calculators
Work out your full daily burn with the daily calorie calculator, check your BMI, see a healthy range with the ideal weight calculator, price a workout with the calories burned calculator, or total up a meal with the recipe nutrition calculator. The full list is on the calculators page.
Frequently asked questions
What is a normal BMR?
There is no single normal. Most adult women land between 1,200 and 1,600 calories a day, and most adult men between 1,500 and 2,000. Bigger bodies burn more because there is more tissue to keep alive, so a tall 90 kg man can sit above 2,000 while a small 50 kg woman sits near 1,150, and both are entirely ordinary. A number that looks low for your size usually means low weight or high age rather than a broken metabolism.
Does BMR slow down with age?
Yes, though less dramatically than people assume. Mifflin-St Jeor subtracts 5 calories for every year of age, so a decade costs about 50 calories a day on paper. Work published in Science in 2021 found that energy expenditure per kilo of tissue is remarkably stable from roughly 20 to 60, and that most of the real-world decline comes from losing muscle and moving less rather than from age itself.
Can you increase your BMR?
Only modestly. Muscle is the lever, and a kilo of muscle burns roughly 13 calories a day at rest, so a hard year of training might add 50 to 100 calories. That is real but small. Eating enough, sleeping enough and not living in a deep deficit for months matter more, because chronic under-eating pushes resting burn down. The large swings in what you burn come from activity, not from BMR.
Is BMR the same as RMR?
Close, but not identical. A true BMR reading is taken after a night of sleep in the lab, lying still, fasted for twelve hours, in a temperature-controlled room. RMR, resting metabolic rate, is measured after a shorter rest under looser conditions and comes out about 10 per cent higher. Every calculator, including this one, prints a figure from an equation fitted to measured resting data, which is why the two terms get used interchangeably in practice.
Why is my BMR different on other calculators?
Because they use a different equation, round differently, or quietly fold in an activity factor. Mifflin-St Jeor, Harris-Benedict, Katch-McArdle and Cunningham can disagree by 200 calories for the same person, which is why all four are shown side by side above. Compare the formula, not the number. If another site shows a figure far higher than yours, check whether it is showing resting burn or a full day's total.
Do I need my body fat percentage?
Not for the headline number. Mifflin-St Jeor needs only sex, age, height and weight. Body fat unlocks the two lean-mass formulas, Katch-McArdle and Cunningham, which read from muscle rather than total weight. They are worth having if you are visibly muscular or very lean, where weight-based equations tend to underestimate you. A rough body fat figure makes them worse than the formula they replace, so leave the field empty unless you have a measurement you trust.
Should I eat my BMR to lose weight?
No. BMR is the floor, not a target. Eating at your resting rate means eating nothing at all for everything else you did that day, which is a far steeper deficit than it looks and usually ends in a rebound. Work out your full daily burn first, then take 300 to 500 calories off that.
How this calculator works
Sources
- Mifflin MD, St Jeor ST et al. (1990). A new predictive equation for resting energy expenditure in healthy individuals. American Journal of Clinical Nutrition. pubmed.ncbi.nlm.nih.gov
- National Academies of Sciences, Engineering, and Medicine. Dietary Reference Intakes: acceptable macronutrient distribution ranges. www.ncbi.nlm.nih.gov
This calculator is for general information and is not medical advice. It is not suitable if you are under 18, pregnant or breastfeeding, or being treated for an eating disorder or a condition that affects weight. Speak to your GP or a registered dietitian before acting on the numbers.