What BMR estimates
Basal metabolic rate, or BMR, is the energy your body uses just to stay alive while you are at rest: keeping the heart beating, the lungs working, the brain and organs running and the body at temperature. It is usually written in kilocalories per day, and in kilojoules per day in many countries. A strict BMR measurement is done in a laboratory, after fasting, lying still in a comfortably warm room. Almost nobody has that test, so calculators estimate the figure with an equation built from age, sex, height and weight, and in one case from body fat. This page gives you three of those equations side by side. For the default example, a 30-year-old male of 175 cm and 75 kg, the three results are 1,699, 1,763 and 1,666 kilocalories a day. They are close, but not identical, and that spread is useful information in itself.
Three named formulas
The Mifflin-St Jeor equation comes from a 1990 study of 498 healthy adults aged 19 to 78 whose resting energy was measured by indirect calorimetry. It reads 10 × weight in kg + 6.25 × height in cm − 5 × age in years, then plus 5 for males or minus 161 for females. The authors found that the older Harris-Benedict equations overestimated the measured values by about 5% in their group (Mifflin et al., 1990).
The revised Harris-Benedict equations date from a 1984 re-examination of the original data with additional subjects. For males they give 88.362 + 13.397 × kg + 4.799 × cm − 5.677 × age, and for females 447.593 + 9.247 × kg + 3.098 × cm − 4.330 × age. The re-examination put their precision at about plus or minus 14% for normally nourished people, which is a reminder of how wide the uncertainty is (Roza and Shizgal, 1984). The Katch-McArdle equation ignores age and sex and uses fat-free mass instead: 370 + 21.6 × fat-free mass in kg, where fat-free mass is weight × (1 − body fat ÷ 100). A 2012 study in the Journal of Clinical Endocrinology and Metabolism used the same coefficients and credited them to Cunningham (1991) (Johannsen et al., 2012). It needs a body-fat figure that you supply, so the calculator never guesses one.
BMR versus RMR and daily energy
You will often see resting metabolic rate, RMR, used as if it meant the same thing. Strictly, BMR is measured under tighter conditions than RMR, but the prediction equations above estimate resting expenditure in general, and most calculators use both labels for the result. The bigger distinction is between resting energy and everything you burn in a day. Daily energy includes movement, exercise and digesting food on top of the resting amount. The activity table on this page multiplies the selected equation by six conventional factors: 1.2, 1.375, 1.465, 1.55, 1.725 and 1.9. They are heuristic scenarios, not measurements of your own activity, and they are not food targets. For the default example, the sedentary scenario gives 2,039 kcal and the extra active scenario 3,228 kcal. To turn a daily figure into grams of protein, carbohydrate and fat, use the macro calculator.
Daily energy is not a fixed number either. It shifts with the season, with changes in body size and with how much you move from one week to the next. That is why the page shows activity scenarios side by side and never turns one of them into an intake target.
Worked 75 kg example
Take a 30-year-old male who is 175 cm tall and weighs 75 kg. Mifflin-St Jeor gives 10 × 75 + 6.25 × 175 − 5 × 30 + 5 = 1,698.8 kcal a day. Revised Harris-Benedict gives 88.362 + 13.397 × 75 + 4.799 × 175 − 5.677 × 30 = 1,762.7. With 20% body fat, fat-free mass is 75 × 0.8 = 60 kg, and Katch-McArdle gives 370 + 21.6 × 60 = 1,666.0. For a female of the same age, height and weight, the first two equations give 1,532.8 and 1,553.4, because the sex term and the coefficients differ. In kilojoules, the Mifflin result is 7,108 kJ, using 4.184 kJ per kcal. Changing any input and pressing Calculate shows how sensitive each equation is to it.
The highest and lowest of the three results differ by 97 kcal, which is 5.7% of the Mifflin-St Jeor figure. That spread is a quick way to see how much any one of them can be trusted.
Units and body-fat uncertainty
The equations use kilograms and centimeters. If you choose US units, the calculator converts exactly: 1 inch is 2.54 cm and 1 pound is 0.45359237 kg. Entering 5 ft 9 in and 165 lb gives 1,698.8 kcal with Mifflin-St Jeor, the same physical person as 175.26 cm and about 74.8 kg. Switching units keeps the exact values underneath, so toggling does not slowly change your numbers through rounding. Body fat is the weakest input, because home scales and calipers can be several percentage points off. The Katch-McArdle result moves by about 21.6 × weight ÷ 100 kcal for every percentage point of body fat, which is 16.2 kcal at 75 kg. If you leave the body-fat box empty, the Katch-McArdle row simply reads "Enter body fat to calculate", and the other two equations still work. In that case the example above would show only 1,699 and 1,763 kcal.
Limits and sources
This page is for healthy adults aged 19 to 80. It is not a diagnosis and it does not replace measurement by indirect calorimetry. It makes no allowance for pregnancy, breastfeeding, illness, medication, growth or very high or very low body weight, and the weight, height and age limits are limits of the tool, not statements about healthy ranges. Ages 79 and 80 sit above the age range of the Mifflin-St Jeor study, so that result is an extrapolation. Different equations suit different people, and a measured value can differ from every estimate. Nothing here is a calorie target or a plan to lose or gain weight. To compare weight-based protein references, use the protein calculator. Sources were read on 7 November 2026 and are reviewed at least once a year. If you have a medical condition or a nutrition question, ask a doctor or a registered dietitian.
BMR questions
Is my BMR the number of calories I should eat?
No. BMR is the resting part of your energy use. Your daily energy is higher once movement and digestion are included, and the amount that suits you depends on your health and goals, which this calculator does not assess. Eating more or less than your daily energy changes weight over time, so the right amount depends on your goals, your health and advice from a professional.
Why do the three formulas give different answers?
They were fitted to different groups of people with different methods, and Katch-McArdle uses fat-free mass instead of age and sex. The gap between them shows the uncertainty in any single estimate. No single equation is the right one for everyone, so comparing them is more honest than quoting one.
What does entering body fat change?
It switches on the Katch-McArdle row, which replaces age, sex and height with fat-free mass. A higher body-fat percentage lowers the result, so an inaccurate entry moves it directly.
How are pounds, feet and kilojoules handled?
Feet and inches become centimeters at 2.54 cm per inch, pounds become kilograms at 0.45359237 per pound, and kilocalories become kilojoules at 4.184 each. All three constants are exact.
Can this measure my metabolism?
No. It predicts from a formula. Only a test such as indirect calorimetry measures resting energy, and even then it reflects one day and one set of conditions.