BMR Calculator

Mifflin-St Jeor (1990) was developed from modern body-composition data and is generally considered the more accurate of the two for most people. Harris-Benedict (1919, revised 1984) runs a little higher — for a 70 kg, 175 cm, 30-year-old man the gap is about 47 kcal/day. Katch-McArdle (from lean body mass) joins the comparison whenever you enter a body fat % above.

Four factors set most of BMR. Body size sets the scale: larger bodies burn more at rest, which is why weight and height dominate every formula. Muscle sets the rate: lean tissue burns more than fat tissue at rest, so two people of the same weight can differ by hundreds of calories if one carries much more muscle, which is exactly what the Katch-McArdle formula prices when a body-fat percentage is supplied. Age lowers the level gradually, as muscle declines and the body becomes smaller and less metabolically active, and sex shifts the level through the average differences in size and composition that the formulas encode as different constants. Hormones and thyroid function matter too, in general terms: thyroid disorders can raise or lower resting expenditure substantially, which is why a BMR far from expectation, or a result that does not match measured experience, is worth discussing with a clinician rather than adjusting by guesswork.

Activity does not change BMR, which is defined at rest, but it dominates the step from BMR to daily calories, and it is where most estimation error enters. A day of 10,000 steps does not raise BMR; it raises the activity multiplier applied to it, and the step count itself is a rough proxy for the total movement of the day, including the uncounted fidgeting, standing, and carrying that make up NEAT, non-exercise activity thermogenesis. Accuracy of the BMR estimate itself is typically within about 10% for people near the average build the formulas were fitted on, and wider for very lean, very heavy, or very muscular people. Treat the figure as a starting point, test it against two to three weeks of averaged weights at the TDEE level, and adjust in steps of 100 to 200 calories.

So what is bmr, exactly? Your basal metabolic rate is the rate at which your body spends energy at complete rest — the calories burned in 24 hours by a body that does nothing but stay alive: heart beating, lungs breathing, liver filtering, brain signalling, cells repairing themselves and holding body temperature at 37 °C. The definition is deliberately strict. A true basal measurement is taken in the morning, after a 10–12 hour fast and a full night’s sleep, with the subject lying awake and motionless in a warm, quiet room. Almost nobody is ever measured that way outside a laboratory, which is why the equations on this page exist: they estimate that laboratory number from weight, height, age and sex.

It helps to see where the energy actually goes. Standard physiology estimates (Elia, 1992) attribute roughly a quarter of resting expenditure to the liver, about a fifth to the brain, about a tenth to the kidneys and a fifteenth or so to the heart — while skeletal muscle, the tissue people usually picture as “metabolism”, accounts for under a fifth of the burn at rest, because resting muscle is nearly idle muscle. Two consequences follow. First, your bmr is mostly the rent on your organs, which is why it scales with overall body size more than with gym habits. Second, the only tissue whose share you can meaningfully change is the muscle slice — a point the Katch-McArdle section below prices precisely.

For most adults living ordinary sedentary-to-moderate lives, resting expenditure is the largest single slice of the day’s burn — roughly 60–70% of total daily energy expenditure, ahead of digestion (about a tenth) and all movement combined. That is why this one number anchors every calorie calculation downstream of it.

Just as important is what BMR is not. It is not your maintenance calories — those include all your waking movement and are always higher. It is not a diet target: eating exactly at your BMR while living a normal upright life is a deficit diet by definition. And it is not a health score. A high reading mostly reports a larger body or more lean tissue; a low one mostly reports a smaller one. The equations that follow estimate the number for an average body of your stats — individual measured values scatter around the prediction by roughly ±10% — so read your result as the centre of a range, not as a verdict.

The tool above is, first of all, a mifflin st jeor calculator: the Mifflin-St Jeor equation is its primary method, the one named in the page title and the one modern practice reaches for first. It was published by Mark Mifflin, Sachiko St Jeor and colleagues in the American Journal of Clinical Nutrition in 1990, derived from indirect-calorimetry measurements on 498 healthy adults — 251 men and 247 women, aged 19 to 78, spanning normal-weight and obese bodies. The equations, exactly as published (weight in kg, height in cm, age in years):

Men: BMR = 10 × weight + 6.25 × height − 5 × age + 5
Women: BMR = 10 × weight + 6.25 × height − 5 × age − 161

Worked end to end on this page’s default man — 70 kg, 175 cm, 30 years: 10 × 70 = 700 for the weight term; 6.25 × 175 = 1,093.75 for height; −5 × 30 = −150 for age; then + 5. Total: 1,648.75 kcal/day, which the calculator displays rounded as 1,649. The matching woman — 60 kg, 165 cm, 30 years: 600 + 1,031.25 − 150 − 161 = 1,320.25 kcal/day. Notice the anatomy of the equation, because it explains most of what follows on this page: every kilogram of body weight adds 10 kcal/day, every centimetre of height adds 6.25, every year of age subtracts 5, and the only difference between the sexes is the constant — +5 for men against −161 for women, a 166 kcal/day gap that stands in for the average difference in body composition between the study’s men and women.

Any mifflin st jeor equation calculator is only as good as the evidence behind the equation, and this one carries the strongest validation record of the common formulas. In their 2005 systematic review for the Journal of the American Dietetic Association, Frankenfield, Roth-Yousey and Compher compared the predictive equations against measured resting metabolic rate and found Mifflin-St Jeor the most reliable of the set: it predicted within 10% of the measured value for 82% of non-obese adults, a higher hit rate than Harris-Benedict managed. Tellingly, the older equations had already overestimated measured expenditure by about 5% inside Mifflin’s own 1990 sample — modern bodies, measured with modern instruments, simply burn a little less at rest than the 1919 data implied. That is why this calculator leads with Mifflin-St Jeor, and why the Academy of Nutrition and Dietetics recommends it as the default estimate for healthy adults.

The harris benedict equation is where metabolic estimation begins. James Harris and Francis Benedict, working at the Carnegie Institution’s nutrition laboratory, published their biometric study of human basal metabolism in 1919, built from respiration measurements on 239 subjects — 136 men and 103 women. Their original equations, exactly as published:

Men (1919): BMR = 66.4730 + 13.7516 × weight + 5.0033 × height − 6.7550 × age
Women (1919): BMR = 655.0955 + 9.5634 × weight + 1.8496 × height − 4.6756 × age

In 1984, Roza and Shizgal re-analysed the original data with modern regression methods and issued the revision that nearly every harris benedict calculator — including the tool at the top of this page — actually runs:

Men (1984): BMR = 88.362 + 13.397 × weight + 4.799 × height − 5.677 × age
Women (1984): BMR = 447.593 + 9.247 × weight + 3.098 × height − 4.330 × age

Three generations of equation, run on the same two bodies:

The pattern is consistent. The 1919 original reads highest of the three — on the woman’s body it overshoots Mifflin by 74 kcal/day — because it was fitted on the leaner, smaller population of a century ago. The 1984 revision pulls it most of the way back: the gaps shrink to 47 kcal/day (2.8%) on the man and 63 kcal/day (4.8%) on the woman, which is why the calculator’s comparison bar shows the two modern results close together. Who still uses Harris-Benedict? Textbooks, much clinical software, and decades of published diet studies — so if you are comparing your number against a figure from a book, a clinic letter or an older app, it is very probably a Harris-Benedict figure, and the table above tells you how far apart the two dialects honestly are. Treat the pair as a range: on these bodies the truth, measured, would be expected to land near the Mifflin end.

The katch-mcardle formula, from Frank Katch, Victor Katch and William McArdle’s exercise-physiology literature, takes a different route entirely. Instead of estimating your burn from total weight, it prices your lean body mass — everything you weigh that is not fat:

BMR = 370 + 21.6 × lean body mass (kg), where lean mass = weight × (1 − body fat %)

One equation serves both sexes, because the sex difference is already inside the body-fat number. This page’s standing example: an 80 kg body at 20% body fat carries 64 kg of lean mass, so BMR = 370 + 21.6 × 64 = 1,752 kcal/day. When does the lean-mass route beat the weight-based equations? Run it at three body-fat readings for one stated man — 80 kg, 180 cm, 30 years, whose Mifflin result is 1,780 kcal/day:

At 15% body fat the lean-mass formula reads above Mifflin — correctly, because a lean 80 kg carries more working tissue than an average 80 kg, and Mifflin cannot see that. At 25% it reads below, for the mirror-image reason. For lean, muscular or otherwise composition-unusual bodies, Katch-McArdle is the most honest of the three estimates — provided the body-fat input is honest, and that proviso is the whole catch. The formula’s sensitivity is exact: on an 80 kg body, every single point of body-fat error moves lean mass by 0.8 kg and the result by 17.28 kcal/day. A five-point misread — well inside the error of a consumer smart scale — swings the answer by about 86 kcal/day, silently converting the most precise method into the least. A 65 kg woman at a measured 28% body fat, for instance, gets 1,381 kcal/day from Katch-McArdle against 1,339 from Mifflin — a believable gap for a leaner-than-average frame, if the 28% came from a real measurement. If your body-fat figure came from a bathroom scale’s guess, distrust the input before you trust the formula: get a tape-measure estimate from our Body Fat Calculator first, then bring the number back here.

Alongside BMR you will constantly meet its near-twin: resting metabolic rate, or RMR. The two terms describe the same resting engine under different measurement discipline. BMR is the strict laboratory quantity — overnight fast, full night’s sleep, measured on waking under controlled conditions. RMR relaxes the protocol so a clinic can actually run it: a few hours since the last meal, a short quiet rest, an ordinary room. Every relaxation lets a little residual digestion, alertness and muscle tone leak into the measurement, so RMR reads systematically higher — typically by about 10%.

Put the gap on this page’s own bodies. The default man’s BMR is 1,649 kcal/day; read as an RMR it would come back at roughly 1,814 kcal/day. The worked woman’s 1,320 becomes roughly 1,452. Neither figure is wrong — they are answers to slightly different questions — but the 150-odd kcal between them is exactly the discrepancy people find when a clinic printout or fitness app disagrees with a BMR result, and it is why the two terms should never be mixed inside one calorie plan.

Which number do apps and watches actually estimate? The RMR-style one. A watch cannot know whether you fasted for twelve hours, so its “resting calories” are an RMR estimate wearing a friendlier name — and because this calculator’s equations were themselves validated against resting measurements, the tool doubles as a practical rmr calculator: enter your stats, take the Mifflin-St Jeor result as your basal floor, and treat anything up to about a tenth higher as the same body read under resting conditions. Used that way — BMR as the floor, RMR as the lived resting figure — a resting metabolic rate calculator and a BMR calculator stop contradicting each other and start bracketing the truth.

Charts of “normal BMR” circulate everywhere, usually with no formula attached. This one is different: every cell is the Mifflin-St Jeor equation run on the stated height — 175 cm for the men’s table, 165 cm for the women’s, the same reference bodies used throughout this page — across six ages and five weights. Any cell can be checked against the calculator above in seconds; twelve of them were, at the time this page was built. Find your row by age, read across to your weight, and you have your estimate in the units the tool reports.

To adjust away from the printed heights: each centimetre of height is worth 6.25 kcal/day in the equation, so a man of 180 cm adds about 31 kcal/day to his row, and a woman of 160 cm subtracts the same. Weight steps are exactly 10 kcal/day per kilogram, which is why the columns march in clean hundreds.

Read across the two tables and the structure of resting metabolism shows itself. The average bmr for women across this grid is 1,345 kcal/day against 1,674 for men — a gap built from the smaller reference frame, the lighter weight columns and the −161 constant, in that order. Readers who calculate bmr female-side for the first time often expect the sex gap to be one mysterious constant; the table shows it is mostly frame and mass, with the constant contributing 166 kcal/day at every age. That is also the whole answer to how to calculate bmr in female terms by hand: identical weight, height and age terms, one different constant — the bmr calculator female and male searches land on the same arithmetic wearing two constants.

What does “low” or “high for your age” actually mean on this table? The equation’s own age term is gentle — exactly 50 kcal/day per decade at fixed stats — so most of the spread within a row is body size, not age. If your computed value sits far from the middle of your row’s band of neighbours, the likeliest explanation is composition the equation cannot see: more lean mass than average reads high, less reads low, and the Katch-McArdle section above is the tiebreaker. And a note on metabolic age, since devices love to report it: the term means nothing more than “the age in a table like this whose typical value equals yours” — a comparison label read off a grid like the one above, not a measurement of your tissues and not a health verdict. A “metabolic age” below your real age usually just reports a heavier or more muscular body than the table’s reference; treat it as trivia, not as a result.

BMR is the floor of your energy day, not the day itself. The standard bridge — the activity-factor table from the Harris-Benedict tradition — multiplies it by a factor between 1.2 and 1.9 chosen by lifestyle. Applied to this page’s two worked bodies:

Read the sedentary row carefully, because it settles the commonest confusion in this whole subject: even a desk-bound man of completely average stats maintains his weight near 1,979 kcal/day, not at his BMR of 1,649. The gap between the two — every step, chore and fidget of an ordinary day — is worth a fifth of the resting burn before any exercise is counted at all. It is also why eating “at BMR” is a deficit diet, as the FAQ below discusses.

This section is deliberately a bridge and not a destination. The full treatment — activity selection, goal calories for loss or gain, and the maintenance figure worked properly — lives on the TDEE Calculator and the Calorie Calculator, which take your BMR as their starting point. What this page owes you is the floor, computed three honest ways; what those pages owe you is everything built on top of it.