BMR Calculator

What BMR measures, and what it does not

Basal metabolic rate is the energy a body spends on the work that never stops: breathing, circulation, cell repair, brain activity and keeping temperature steady. It is measured after an overnight fast, after eight hours of sleep, lying still in a room at a comfortable temperature. Those conditions are strict on purpose, because any movement, digestion or shivering adds to the number.

Resting metabolic rate is the looser cousin. It is measured at rest but not under the full set of basal conditions, so it usually comes out a little higher. Most online calculators, including this one, predict a resting value and call it BMR, which is fine for planning as long as you know the label is approximate.

The distinction matters when someone quotes a number. A BMR of 1,755 is not a calorie target. Eating 1,755 calories while walking 8,000 steps a day would put a 35 year old man in a deficit of roughly 950 calories, which is far more aggressive than most plans intend.

The Mifflin-St Jeor equation

The equation used here was published by Mifflin, St Jeor and colleagues in 1990 in the American Journal of Clinical Nutrition. It was derived from 498 healthy adults, 247 women and 251 men, aged 19 to 78, and the published model explained about 71 percent of the variation in measured resting energy expenditure. The original 1990 paper lists both the full and the simplified forms.

Men: BMR = 10 x weight in kg + 6.25 x height in cm - 5 x age in years + 5

Women: BMR = 10 x weight in kg + 6.25 x height in cm - 5 x age in years - 161

Weight is the strongest term. Every kilogram adds 10 calories a day. Height adds 6.25 per centimeter, which rewards a larger frame, and age subtracts 5 per year, which is why the same body at 55 needs less than it did at 25. The constant at the end carries the sex difference: plus 5 for men, minus 161 for women.

Units have to be metric inside the formula. Pounds divide by 2.20462 to give kilograms, and inches multiply by 2.54 to give centimeters. A 180 pound man is 81.65 kg, and 5 feet 10 inches is 177.8 cm. Entering pounds and inches directly returns a number that looks plausible and is wrong by hundreds of calories.

Worked example: 80 kg man, 180 cm, age 35

A man of 80 kg, 180 cm and 35 years. Work the three terms separately, then add the constant.

So his predicted BMR is 1,755 calories a day. Ten years later, at the same weight and height, the age term becomes -225 and the total drops to 1,705. Five extra kilograms at 35 would add 50 and bring it to 1,805. The equation is linear in each input, which makes those mental adjustments reliable.

Multiply by an activity factor of 1.55 for a moderately active week and the daily total is 2,720. That is the number a calorie target should be built from, not the 1,755.

Worked example: 65 kg woman, 165 cm, age 30

A woman of 65 kg, 165 cm and 30 years, using the female constant.

Rounded, 1,370 calories a day. The 161 point constant is what separates the two equations, and it is not a rounding detail: a man and a woman with identical weight, height and age differ by 166 calories in this model, mostly because of average differences in lean mass and fat distribution.

At a light activity multiplier of 1.375 her daily total is 1,884. At 1.55 it is 2,124. A common mistake is to treat 1,370 as the maintenance level and then wonder why energy drops on a 1,370 calorie diet. Maintenance is the total, not the basal rate.

Three equations, three answers

Three equations dominate consumer calculators, and they do not agree. The table runs the same two people through all three.

The Harris-Benedict revision is the older of the two general equations, refit by Roza and Shizgal in 1984. It tends to run 4 to 5 percent above Mifflin-St Jeor in these examples: 70 calories for the man and 60 for the woman. That is inside the noise of a single day, but it compounds over a month of tracking.

Katch-McArdle needs a body fat percentage and uses lean mass, so it lands closer to Mifflin here only because the assumed body fat figures are typical. Feed it a bad body fat estimate and it moves further than either of the other two. The dedicated Katch-McArdle calculator walks through that sensitivity, and it is the better route if you have a DEXA scan or a reliable caliper reading.

For most people the practical ranking is simple. Use Mifflin-St Jeor when you have a scale and a tape measure. Use Katch-McArdle when you know your body fat and carry unusual amounts of muscle. Use Harris-Benedict only to compare against older material that quotes it.

From BMR to daily calories

BMR becomes useful once it is multiplied by an activity factor. The multipliers below are the ones used across consumer calculators, and they are conventions rather than measured constants. Treat them as a starting bracket, then adjust from what the scale does over three to four weeks.

The spread between the top and bottom rows is 1,229 calories for the man, which is larger than the gap between any two BMR equations. Getting the activity level roughly right matters more than picking the perfect formula.

From there, a deficit or surplus is a separate decision. A 500 calorie daily deficit predicts roughly half a kilogram of loss a week, and the 3,500 calorie per pound figure behind that arithmetic is a rough rule, not a physiological constant. Loss slows as body mass falls, because a lighter body has a lower BMR. For a full daily burn figure with goal calories attached, the TDEE calculator carries the same math one step further. Two public tools use related approaches: the NIDDK Body Weight Planner models how intake and activity change weight over time, and the USDA DRI calculator returns estimated energy requirements from the national dietary reference equations.

What moves BMR up or down

BMR is not fixed. The terms in the equation hint at what moves it, and the list is longer than the formula suggests.

Lean mass is the biggest lever. Muscle and organs burn far more at rest than fat does, which is why two people at the same weight can differ by several hundred calories. In the Katch-McArdle model each kilogram of lean mass carries 21.6 calories a day, and fat carries almost none, so a body with more muscle at the same weight predicts a higher BMR.

Age subtracts 5 calories a year in the equation, which is 50 per decade. That term is partly a proxy for lost lean mass, so a 55 year old who lifts and keeps muscle will beat the prediction.

Thyroid output shifts the number in both directions. Untreated hypothyroidism lowers resting expenditure, and hyperthyroidism raises it. A formula cannot see either, which is one reason a predicted BMR and a measured one can disagree by more than the published error band.

Pregnancy, lactation, fever and some medications add to the total. So does a very low intake over months: adaptive thermogenesis lowers expenditure beyond what the lost weight alone would explain. The formula keeps returning the same number for the new weight, while the body spends less than that.

When a formula is the wrong tool

A prediction is not a measurement. Validation studies commonly describe individual error around 10 percent, so a predicted 1,755 has a plausible range of about 1,580 to 1,931 before any activity is added. Two people with identical inputs can sit at opposite ends of that band.

Some groups fall outside the population the equation was built from. Children and adolescents are still growing. Pregnant and breastfeeding women have needs the equation does not model. Highly muscular athletes carry lean mass that the weight term undercounts. People with a BMI above 40 are underrepresented in the original sample, and several validation studies report that the equation overpredicts for them. Critically ill patients need measured energy expenditure, not an estimate.

For those cases the honest answer is indirect calorimetry, which measures oxygen consumption and carbon dioxide production. A metabolic cart costs more than a formula, and it is the right tool when the number drives a clinical decision.

For everyone else, use the prediction as a starting point and let the scale correct it. Weigh in the morning under the same conditions, average the week, and compare that average with the average two weeks later. If weight is flat on a calculated maintenance intake, the calculation was right. If it drifts, adjust by 100 to 150 calories and watch another two weeks.

The calculator above returns Mifflin-St Jeor, the revised Harris-Benedict figure, Katch-McArdle when you supply a body fat percentage, the spread between them, and the daily total at your chosen activity level.