BMR Calculator

A BMR number is the most misused figure in fitness. People find it, see something like 1,700 calories, and decide that is what they should eat. It is not. Basal metabolic rate is the energy cost of lying completely still in a warm room with an empty stomach, doing nothing at all. It is the floor underneath your calorie needs, not the target.

What a BMR estimate is genuinely good for is understanding the scale of your metabolism, seeing how much of it you actually control, and feeding the activity step that produces a real calorie target. This calculator gives you three published equations at once instead of one, because the honest answer is that they disagree with each other by a few percent, and hiding that behind a single confident number is how calculators mislead people.

How this calculator works

Every constant below comes from the paper that published it. Nothing here is invented or tuned.

• Mifflin-St Jeor, 1990 (default): Mifflin et al. 1990 in the American Journal of Clinical Nutrition, derived from 498 healthy adults. For men, BMR = 10w + 6.25h − 5a + 5. For women, BMR = 10w + 6.25h − 5a − 161, with weight in kilograms, height in centimetres and age in years. It is the default because the Frankenfield, Roth-Yousey and Compher 2005 systematic review in the Journal of the American Dietetic Association compared the commonly used equations against measured values and found this one landed within 10 percent of measured resting metabolic rate in 82 percent of nonobese adults and 70 percent of adults with obesity, with the narrowest error range of any equation tested.
• Harris-Benedict, revised 1984: the 1919 original re-fitted by Roza and Shizgal 1984 in the American Journal of Clinical Nutrition. For men, BMR = 88.362 + 13.397w + 4.799h − 5.677a. For women, BMR = 447.593 + 9.247w + 3.098h − 4.330a. It is shown for comparison, not as the recommendation. Roza and Shizgal reported that the equation predicts resting energy expenditure with a precision of about 14 percent, and in practice it returns a number 3 to 9 percent above Mifflin-St Jeor for the same person.
• Katch-McArdle, when body fat is supplied: BMR = 370 + 21.6 × lean body mass in kilograms. The equation was published by Cunningham 1991 in the American Journal of Clinical Nutrition as a general prediction equation from fat-free mass, and it circulates in fitness under the Katch-McArdle name. It has no term for age, sex or height, because fat-free mass already carries most of that information. ten Haaf and Weijs 2014 in PLoS One tested it against measured values in 90 recreational athletes and found it accurate within 10 percent in 85 percent of men and 78 percent of women, while the general-population equations managed under 50 percent.
• Likely measured range: the recommended estimate plus or minus 10 percent. That band is not decoration. It is the accuracy the best available equation achieves in the population it was built for, per Frankenfield et al. 2005, and on a typical adult BMR it spans 300 to 400 calories.
• Activity illustrations: the sedentary and moderate day figures multiply the recommended BMR by 1.2 and 1.55. Those are the standard activity factors used with Mifflin-St Jeor. They are shown to make the scale concrete, not to set a target. The activity step belongs in the TDEE calculator, where the multiplier choice gets the explanation it deserves.

BMR vs RMR vs TDEE, in plain terms

The three acronyms get used interchangeably and they are not the same thing.

BMR is the strictest of the three. A true basal measurement happens after an overnight fast, on waking, with the person lying still in a thermoneutral room and no exercise in the preceding day. Compher and colleagues (2006) laid out those conditions in detail for the Journal of the American Dietetic Association, and they are demanding enough that very few published datasets meet them fully.

RMR, resting metabolic rate, is the same measurement taken under looser conditions: awake, still, supine or seated, after a shorter rest. McMurray and colleagues (2014) in Medicine and Science in Sports and Exercise describe the two as differing only in how stringent the pre-test conditions are, and note that RMR is often the more useful indicator of real daily energy needs. Because the conditions are looser, RMR comes out slightly higher than true BMR.

Here is the part most BMR calculators never admit: the studies behind Mifflin-St Jeor, the revised Harris-Benedict and the Cunningham lean-mass equation all measured resting energy expenditure, not strict basal metabolic rate. So this page, like every other BMR calculator on the internet, is really predicting your RMR. The gap between the two is small enough that the distinction does not change what you should do, but it is worth knowing that the label is looser than it sounds.

TDEE, total daily energy expenditure, is the whole day. It is resting burn plus the thermic effect of food, which Westerterp (Endotext, 2022) puts at about 10 percent of the energy you eat, plus structured exercise, plus everything else you do while upright. That last component, nonexercise activity thermogenesis, is the wild card: Levine 2004 in the American Journal of Physiology showed it can vary by hundreds of calories per day between two adults of the same size. TDEE is the number you set calories from. BMR is only its largest ingredient.

What this BMR number means for eating

Almost nothing, on its own. That is the honest answer, and it is why this page does not hand you a diet.

To turn a BMR estimate into a calorie target you have to do the activity step, and the activity step is where most of the error lives. Multiply the recommended BMR by an activity factor to get TDEE, then set a deficit or surplus from there. The TDEE calculator walks through the five standard multipliers and flags the two that people over-select. The macro calculator then splits the result into grams of protein, fat and carbohydrate. The ISSN position stand on diets and body composition (Aragon et al. 2017) is the source for the goal-setting logic in both.

What matters more than the arithmetic is whether you train consistently enough for any of it to count. If fat loss is the goal, the training side is the half that decides the outcome, and the app you use is mostly a question of whether you still open it in week nine. We compared them in the guide to the best workout apps for weight loss.

How accurate are predictive BMR equations?

Plus or minus 10 percent, for most healthy adults, using the best equation available. On a BMR of 1,700 calories that is a band from roughly 1,530 to 1,870. It is a wide band, and every BMR calculator that reports a single number to the calorie is quietly pretending it is narrower.

The figure comes from Frankenfield et al. 2005, which is still the reference comparison. Mifflin-St Jeor hit within 10 percent of measured values in 82 percent of nonobese adults and 70 percent of adults with obesity. Read that the other way around: even the winner misses by more than 10 percent in roughly one adult in five, and in nearly one in three adults with obesity.

The reason is that sex, age, height and weight do not fully describe a metabolism. Two people with identical inputs can carry very different amounts of lean tissue, and lean tissue is what burns. That is exactly why the Cunningham lean-mass equation exists, and why a trustworthy body fat measurement upgrades the estimate rather than decorating it. If you do not have one, the body fat calculator gives a field estimate from tape measurements or skinfolds, with its own error band stated up front.

Who these equations misestimate

Very lean and athletic adults

This is the clearest failure case. ten Haaf and Weijs (2014) measured resting energy expenditure in 90 recreational athletes aged 18 to 35 who trained about 9 hours a week. The general-population equations, including both Harris-Benedict versions and Mifflin-St Jeor, were accurate within 10 percent in fewer than half of the subjects and tended to underestimate. The lean-mass equation was accurate in 85 percent of the men and 78 percent of the women. If you train seriously and carry above-average muscle, get a body fat measurement and use the Katch-McArdle option.

Adults with obesity

Prediction gets less reliable as body mass rises, because a larger share of body weight is adipose tissue, which is far less metabolically active per kilogram than lean tissue. Frankenfield and colleagues (2003) validated the established equations in obese and nonobese people and recommend measured resting metabolic rate by indirect calorimetry, rather than any prediction, when clinical decisions depend on the number. At a BMI above 40 in particular, treat a calculated value as a rough bracket.

Older adults

The equations were fitted mostly on younger and middle-aged adults, and they travel poorly into later life. Cioffi, Marra, Pasanisi and Scalfi (2021) reviewed 14 studies of resting energy expenditure prediction in healthy older adults for Clinical Nutrition and found a consistent tendency toward overestimation across equations, with poor accuracy at the individual level. Age-related loss of lean mass is part of it: two adults at the same weight and height, one 30 and one 70, can differ in fat-free mass far more than the age coefficient allows for.

Worked examples

Ten reference profiles with both age-based equations computed, so you can sanity-check the calculator against a body like yours without running it. The final two rows include a body fat percentage, so the Katch-McArdle column is filled in.

Two patterns are worth noticing. The revised Harris-Benedict equation sits above Mifflin-St Jeor in every single row, by 3 percent at the young and lean end and by 9 percent for the 70-year-old woman. And in the last row, where the man carries an average amount of body fat, the lean-mass equation lands below both, because it stops crediting adipose tissue with the same metabolic cost as muscle.

Three things a BMR number does not tell you

It does not tell you how much to eat

This is the mistake the number invites, and it is the reason the calculator above shows a sedentary and a moderate day beside it. Eating at your BMR means eating as though you spend 24 hours a day motionless. Nobody does. Calorie targets come from total daily energy expenditure, and the gap between BMR and TDEE is 20 to 90 percent depending on how much you move. If you want a target, do the activity step in the TDEE calculator instead of borrowing this one.

It does not tell you your metabolism is broken

A low-looking BMR is usually just a small body, an older body, or a body carrying less lean tissue. Metabolic adaptation after sustained dieting is real, but it is measured in the low hundreds of calories, not the thousands that the word "damaged" implies, and it is not what the calculator is showing you. If your weight has stalled while you believe you are in a deficit, the likely explanations are ranked in our piece on why the scale stops moving in a calorie deficit, and the levers that genuinely move the resting floor are ranked in the review of how to boost metabolism.

It does not tell you that age has wrecked your metabolism

Look at what the equation actually does with age. Mifflin-St Jeor subtracts 5 calories per year, so a 50-year-old and a 30-year-old of identical height, weight and sex differ by 100 calories per day. That is a cookie. The far larger driver of midlife change is the lean tissue people quietly lose while becoming less active, which shows up in the calculator as a lower weight-and-composition input rather than as an age penalty. Resistance training that defends lean mass defends most of the number.

When to ignore this calculator

The equations were built on healthy adults and they do not transfer everywhere. A few situations call for a clinician rather than a web tool.

• Pregnancy and lactation. Energy needs rise through the second and third trimesters and during lactation, in ways no BMR equation models. Defer to your obstetric care team.
• Thyroid disease. Hyperthyroidism and hypothyroidism shift resting metabolic rate in opposite directions and by amounts the equations cannot see. Work with an endocrinologist.
• Eating disorders, or recovery from one. A calculated floor can become a target in exactly the wrong way. A registered dietitian who specialises in eating disorders is the right support here, not a calculator.
• Chronic kidney disease, liver disease, cancer or other catabolic illness. Disease states change resting energy expenditure substantially. Nutrition targets should come from the clinical team.
• After bariatric surgery. Resting rate can sit away from predicted values for years afterward. Use measured values where available, or the surgical nutrition team's guidance.
• BMI above 40. Frankenfield et al. 2003 recommend measured resting metabolic rate by indirect calorimetry over any predictive equation in this group when a decision depends on the number.
• Children and adolescents under 18. Growth changes the arithmetic. Paediatric equations exist and this is not one of them.

For everyone else, the estimate above is a defensible starting point. Hold it for two to four weeks, weigh yourself under the same conditions each morning, average the readings weekly, and let the trend correct the equation.

Related reading

References

1. Mifflin MD, St Jeor ST, Hill LA, Scott BJ, Daugherty SA, Koh YO. "A new predictive equation for resting energy expenditure in healthy individuals." Am J Clin Nutr. 1990;51(2):241-247. doi:10.1093/ajcn/51.2.241
2. Frankenfield D, Roth-Yousey L, Compher C. "Comparison of predictive equations for resting metabolic rate in healthy nonobese and obese adults: a systematic review." J Am Diet Assoc. 2005;105(5):775-789. doi:10.1016/j.jada.2005.02.005
3. Roza AM, Shizgal HM. "The Harris Benedict equation reevaluated: resting energy requirements and the body cell mass." Am J Clin Nutr. 1984;40(1):168-182. doi:10.1093/ajcn/40.1.168
4. Cunningham JJ. "Body composition as a determinant of energy expenditure: a synthetic review and a proposed general prediction equation." Am J Clin Nutr. 1991;54(6):963-969. doi:10.1093/ajcn/54.6.963
5. ten Haaf T, Weijs PJM. "Resting energy expenditure prediction in recreational athletes of 18-35 years: confirmation of Cunningham equation and an improved weight-based alternative." PLoS One. 2014;9(10):e108460. doi:10.1371/journal.pone.0108460
6. Frankenfield DC, Rowe WA, Smith JS, Cooney RN. "Validation of several established equations for resting metabolic rate in obese and nonobese people." J Am Diet Assoc. 2003;103(9):1152-1159. doi:10.1016/s0002-8223(03)00982-9
7. Cioffi I, Marra M, Pasanisi F, Scalfi L. "Prediction of resting energy expenditure in healthy older adults: a systematic review." Clin Nutr. 2021;40(5):3094-3103. doi:10.1016/j.clnu.2020.11.027
8. McMurray RG, Soares J, Caspersen CJ, McCurdy T. "Examining variations of resting metabolic rate of adults: a public health perspective." Med Sci Sports Exerc. 2014;46(7):1352-1358. doi:10.1249/MSS.0000000000000232
9. Compher C, Frankenfield D, Keim N, Roth-Yousey L. "Best practice methods to apply to measurement of resting metabolic rate in adults: a systematic review." J Am Diet Assoc. 2006;106(6):881-903. doi:10.1016/j.jada.2006.02.009
10. Levine JA. "Nonexercise activity thermogenesis (NEAT): environment and biology." Am J Physiol Endocrinol Metab. 2004;286(5):E675-E685. doi:10.1152/ajpendo.00562.2003
11. Westerterp KR. "Control of Energy Expenditure in Humans." In: Feingold KR, et al., eds. Endotext. South Dartmouth, MA: MDText.com; 2022. NCBI Bookshelf NBK278963
12. Aragon AA, Schoenfeld BJ, Wildman R, et al. "International Society of Sports Nutrition Position Stand: diets and body composition." J Int Soc Sports Nutr. 2017;14:16. doi:10.1186/s12970-017-0174-y

How this calculator differs from typical BMR calculators

Most online BMR calculators do one of three things. They run a single equation, usually the 1919 Harris-Benedict or its 1984 revision, and present the output as a fact. They report a number to the calorie with no error band, which implies a precision no predictive equation has. Or they slide straight from BMR into a calorie target, which is the one thing a basal rate should never be used for on its own.

This one runs all three published equations at once and shows the disagreement instead of hiding it, because the spread between Mifflin-St Jeor and the revised Harris-Benedict is 3 to 9 percent for a typical adult and you deserve to see that. It defaults to Mifflin-St Jeor on the evidence of the Frankenfield et al. 2005 systematic review rather than on tradition, switches to the lean-mass equation when you supply a body fat percentage, and states the plus or minus 10 percent band around whichever estimate it recommends. It names the three groups the equations misestimate, with the papers attached, rather than pretending accuracy is uniform. And it deliberately stops short of handing you a diet, pointing at the activity step instead. The cost is a less tidy answer. The benefit is an answer you can defend.