What basal metabolic rate is, and what it is not
Basal metabolic rate is the energy cost of staying alive at complete rest. Strictly defined, it is measured after an overnight fast, lying still, awake, in a thermoneutral room, having not exercised the day before. Almost all of it goes to organs you never think about: the liver, brain, heart and kidneys together account for the majority of resting expenditure while making up a small fraction of body weight.
Resting metabolic rate (RMR) is the number you will more often see quoted. It is measured under less strict conditions and typically comes out a few percent higher than true BMR. The distinction matters in a metabolic ward and almost nowhere else, and most published equations — Mifflin-St Jeor included — are used interchangeably for both.
What BMR is not is the number of calories you should eat. It is the baseline before any movement, digestion or exercise, and eating at BMR means eating in a substantial deficit. Multiply it by an activity factor to get total daily energy expenditure, which is what the TDEE calculator does, and set an intake from there.
Two people of the same age, sex, height and weight can have measured resting rates that differ by several hundred calories, driven mostly by how much of that weight is lean tissue. A prediction equation cannot see this. It gives you the average of everyone shaped like you.
Reading the Mifflin-St Jeor equation
The equation is a plain linear regression: BMR = 10·weight + 6.25·height − 5·age + s. Each coefficient is worth understanding, because together they tell you what actually moves your metabolic rate.
10 kcal per kilogram is the largest term. Body mass drives resting expenditure more than anything else, because there is simply more tissue to perfuse and maintain. Note that this is total weight, not lean mass, so the coefficient is an average blend of metabolically active tissue and much less active fat.
6.25 kcal per centimetre of height looks small until you multiply it out: at 175 cm the height term contributes 1,094 kcal, which is often more than the weight term. Height is a proxy for organ size and for the surface area you lose heat across.
Minus 5 kcal per year of age is the decline. Over forty years that is 200 kcal a day, which is the entire reason weight creeps up at a constant diet. Most of that decline is loss of lean mass rather than a change in the metabolic rate of tissue.
The sex constant — +5 for men, −161 for women — is a 166 kcal gap that persists after height, weight and age are accounted for. It reflects the difference in body composition at the same weight; men carry more muscle and less fat.
Because the equation is linear, the sensitivities are exact and easy to remember: one kilogram is worth 10 kcal/day, one centimetre is worth 6.25, and one year is worth 5.
Worked example: a woman aged 42, 165 cm, 68 kg
- Weight term. 10 × 68 = 680 kcal.
- Height term. 6.25 × 165 = 1,031.25 kcal.
- Age term. −5 × 42 = −210 kcal.
- Sex constant. −161 kcal.
- Add them. 680 + 1,031.25 − 210 − 161 = 1,340 kcal/day.
That is 1,340 ÷ 24 = 55.8 kcal an hour, or 1,340 × 4.184 = 5,608 kJ a day. Per kilogram it is 1,340 ÷ 68 = 19.7 kcal/kg/day, which is typical for a middle-aged adult.
Run the same body through revised Harris-Benedict: 447.593 + 9.247 × 68 + 3.098 × 165 − 4.330 × 42 = 447.593 + 628.796 + 511.17 − 181.86 = 1,406 kcal/day. That is 66 kcal higher, about 4.9%. Harris-Benedict was fitted in 1918 on a smaller and leaner sample, and it generally runs above Mifflin-St Jeor for modern bodies — which is precisely why the newer equation replaced it in dietetic practice.
Multiply the Mifflin figure by 1.375 for someone training three days a week and you get 1,340 × 1.375 = 1,843 kcal a day of total expenditure. Subtract 500 from that and you have a deficit of roughly half a kilogram a week, which is what the calorie deficit calculator works out properly, accounting for the floor below which intake should not fall.
What your number means and how to check it
Compare your result per kilogram rather than in absolute terms — that is the figure that tells you whether the number is plausible. Most adults land between about 18 and 24 kcal per kilogram per day, falling with age and with a higher body-fat percentage. A very high value per kilogram usually means you are light and young; a low one usually means you are older or carrying more fat.
The right way to validate the estimate is empirical, not theoretical. Eat at your predicted maintenance intake for two to three weeks, weighing yourself under the same conditions each morning, and track the trend rather than the daily figure. If your weight is stable, the estimate is good enough. If it drifts, adjust the intake by the drift: losing 0.25 kg a week means you are roughly 275 kcal a day below maintenance, so add that back. Two or three weeks of real data beats any equation.
Where prediction equations are least reliable is at the extremes. In obesity, total weight overstates metabolically active tissue, and equations driven by lean mass — Katch-McArdle in particular — usually fit better. In highly trained athletes the opposite bias appears, because they carry more lean mass than their weight implies. In both cases, if you know your body fat percentage, an equation that uses it will beat one that does not. Estimate it first with the Navy body fat calculator.
Predicted BMR at 170 cm by age, sex and weight
| Age | Man, 70 kg | Man, 85 kg | Woman, 60 kg | Woman, 75 kg |
|---|---|---|---|---|
| 20 | 1,668 | 1,818 | 1,402 | 1,552 |
| 30 | 1,618 | 1,768 | 1,352 | 1,502 |
| 40 | 1,568 | 1,718 | 1,302 | 1,452 |
| 50 | 1,518 | 1,668 | 1,252 | 1,402 |
| 60 | 1,468 | 1,618 | 1,202 | 1,352 |
| 70 | 1,418 | 1,568 | 1,152 | 1,302 |
Values are rounded to the nearest kilocalorie from a half-calorie exact result. Add 10 kcal for each extra kilogram and 6.25 kcal for each extra centimetre of height.
What the equation cannot account for
- Body composition. Two 80 kg bodies, one at 12% fat and one at 32%, have measurably different resting rates. Mifflin-St Jeor sees only the 80 kg.
- Thyroid status and medication. Hypothyroidism, beta blockers and stimulants all shift resting expenditure, and none appears in any prediction equation.
- Adaptive changes during dieting. Sustained energy restriction lowers resting expenditure by more than the weight loss alone predicts. The equation will keep returning the number for your new weight as though nothing else changed.
- Pregnancy and lactation. Both raise energy needs substantially and neither is in the model.
- Illness, injury and fever. Clinical nutrition applies separate stress factors on top of a predicted basal rate; this calculator does not.
- Age below 18. Children and adolescents have their own equations, because growth itself has an energy cost.
Never eat at your BMR
BMR is the cost of lying still all day, and you do not lie still all day. Intake set at BMR is already a deficit of several hundred calories before you have walked to the kitchen. Build your target from total daily expenditure instead, and keep intake above the commonly used clinical floors of roughly 1,200 kcal for women and 1,500 kcal for men unless you are being supervised.
Mifflin-St Jeor against the alternatives
Harris-Benedict (1918, revised by Roza and Shizgal in 1984) is the oldest in common use, and this calculator prints it beside the Mifflin result. It typically returns a higher figure for contemporary bodies because its original sample was leaner than today's population.
Katch-McArdle takes a different route entirely: BMR = 370 + 21.6 × lean body mass in kilograms. Because it ignores fat mass, it does not need separate constants for men and women, and it fits athletes and very lean people better than any total-weight equation. It requires a body-fat estimate, which is its only real cost.
Cunningham is similar in shape and is common in sports nutrition, returning slightly higher values than Katch-McArdle for the same lean mass.
Schofield equations, based on weight and age band, underpin much of the WHO/FAO/UNU energy requirement work and are still standard in some national dietary guidance.
The Academy of Nutrition and Dietetics' evidence analysis identified Mifflin-St Jeor as the most reliable of these for healthy adults, which is why it is the default here. If you know your body fat percentage, run Katch-McArdle as a cross-check; if the two disagree by more than about 10%, your body composition is far enough from average that the lean-mass equation is likely the better guide. Whichever you use, the multiplier you apply afterwards introduces far more uncertainty than the choice of equation — see the TDEE calculator for why.