What basal metabolic rate is
Basal metabolic rate is the amount of energy your body needs to stay alive at complete rest: breathing, circulating blood, maintaining body temperature and repairing cells. Strictly, BMR is measured lying down after a night’s sleep and a 12-hour fast in a warm room. Resting metabolic rate (RMR) is measured under slightly looser conditions and is usually a little higher, but the two terms are used interchangeably in everyday use.
For most adults BMR makes up the largest share of daily calorie burn, often 1,200 to 2,000 kcal. It is driven mainly by lean body mass. Muscle, the liver, brain, heart and kidneys are metabolically busy tissues, while fat burns relatively little energy. That is why BMR tends to be higher in taller, heavier and more muscular people and gradually declines with age.
The three BMR equations compared
Mifflin-St Jeor (1990) is the most widely recommended formula today: 10 × kg + 6.25 × cm − 5 × age, plus 5 for men or minus 161 for women. A 40-year-old man who weighs 80 kg and is 180 cm tall gets 800 + 1,125 − 200 + 5 = 1,730 kcal.
The revised Harris-Benedict equation (Roza and Shizgal, 1984) updates the original 1919 formula. For men it is 88.362 + 13.397 × kg + 4.799 × cm − 5.677 × age; for women 447.593 + 9.247 × kg + 3.098 × cm − 4.330 × age. It tends to give results a few percent higher than Mifflin-St Jeor.
Katch-McArdle uses lean body mass instead of sex, height and age: 370 + 21.6 × lean mass in kg. If you know your body fat percentage, from a DEXA scan, calipers or our body fat calculator, this formula can be more accurate for very lean or very muscular people. Enter a body fat percentage above and it appears automatically.
Why the formulas disagree
Each equation was developed from a different group of people, measured with the methods of its time. When your body is close to the average of those groups the formulas agree within about 100 kcal. When you are unusually muscular, very tall, older or carrying a lot of fat, they can differ by 200 kcal or more. No formula is perfect for everyone, so looking at all three side by side gives a realistic sense of the range.
Turning BMR into daily calories
BMR is not how much you should eat. Eating only your BMR would mean no energy for walking, working or exercise. To find your daily needs, multiply BMR by an activity factor between 1.2 and 1.9; the TDEE calculator does this for you and shows all five levels. Building muscle through strength training is one of the few ways to raise BMR over time, while crash diets can lower it slightly as the body adapts.
Frequently asked questions
What is a normal BMR?
Most adult women have a BMR between about 1,200 and 1,500 kcal per day and most men between about 1,500 and 1,900 kcal, depending on size, age and muscle mass.
Which BMR formula is most accurate?
For the general population, Mifflin-St Jeor is usually the most accurate. If you know your body fat percentage, Katch-McArdle can be better for lean or muscular people.
Can I increase my BMR?
Gaining muscle through resistance training raises BMR modestly. Adequate protein and sleep also help preserve it during weight loss.
Why does BMR decrease with age?
Mostly because people lose muscle mass as they age. Staying strong and active slows this decline.