The most widely recommended way to calculate your resting metabolic rate is the Mifflin-St Jeor equation, which uses your weight, height, age, and sex to estimate how many calories your body burns at rest. A systematic review in the Journal of the American Dietetic Association found it predicted RMR within 10% of lab-measured values in more people than any other formula, making it the best starting point for most adults.
The Mifflin-St Jeor Equation
This formula works in metric units, so you’ll need your weight in kilograms and height in centimeters. If you’re working in pounds and inches, multiply your weight by 0.4536 to get kilograms, and multiply your height in inches by 2.54 to get centimeters.
For men: (10 × weight in kg) + (6.25 × height in cm) − (5 × age in years) + 5
For women: (10 × weight in kg) + (6.25 × height in cm) − (5 × age in years) − 161
Here’s a worked example. A 35-year-old woman who weighs 150 pounds (68 kg) and stands 5’6″ (167.6 cm) would calculate: (10 × 68) + (6.25 × 167.6) − (5 × 35) − 161 = 680 + 1,047.5 − 175 − 161 = roughly 1,392 calories per day. That number represents what her body would burn lying still all day, before any movement or exercise is factored in.
What RMR Actually Measures
Your resting metabolic rate is the energy your body uses to keep itself alive: breathing, circulating blood, maintaining body temperature, and running your brain. It typically accounts for 60% to 75% of the calories you burn in a day, with physical activity and digesting food making up the rest.
What’s surprising is where that energy actually goes. Skeletal muscle accounts for about 20% of your resting energy expenditure, which is the same share as your brain. Your liver uses roughly 17%, your heart about 11%, and your kidneys around 6%. Fat tissue, despite sometimes making up a large portion of body weight, contributes only about 5% of resting energy expenditure. This is why two people at the same weight can have noticeably different metabolic rates: the person carrying more muscle burns more calories at rest.
RMR vs. BMR
You’ll see these terms used interchangeably, but they’re measured under different conditions. Basal metabolic rate (BMR) requires a 12-hour fast, measurement first thing in the morning immediately after waking, in a reclined position under temperature-controlled conditions. RMR relaxes those rules: the fast can be as short as 2 to 4 hours, and the measurement doesn’t have to happen in the morning. In practice, RMR runs about 3% to 10% higher than BMR because the conditions are less strict. Most online calculators and nutrition plans use RMR, even when they label the result “BMR.”
Formulas for People With More Muscle
The Mifflin-St Jeor equation uses total body weight, which means it doesn’t distinguish between muscle and fat. If you carry significantly more muscle than average, or if you know your body fat percentage from a DEXA scan, skinfold test, or bioelectrical impedance scale, two alternative formulas will give you a more accurate number.
The Katch-McArdle equation strips out fat entirely and works from lean body mass alone: 370 + (21.6 × lean body mass in kg). To find your lean mass, take your total weight in kilograms and subtract your fat mass. For example, a person weighing 80 kg at 20% body fat has 16 kg of fat and 64 kg of lean mass. Their RMR estimate: 370 + (21.6 × 64) = roughly 1,752 calories per day.
The Cunningham equation follows a similar logic and is especially useful for athletes. A study of male and female endurance athletes found it predicted measured RMR within 158 calories per day for men and 103 calories per day for women, making it the most accurate option for highly active people. If you train regularly and have a lean build, either of these body-composition-based formulas will outperform a weight-only equation.
Why Your Result May Be Off
Even the Mifflin-St Jeor equation, the most accurate general-purpose formula, has notable limitations when applied to individuals rather than groups. Several factors can push your true RMR away from the calculated estimate.
Age. The formulas include an age variable, but the decline in metabolic rate isn’t perfectly linear. Muscle mass tends to drop more steeply after 50 or 60, and hormonal shifts (particularly declining thyroid hormone levels and changes in sex hormones) can lower metabolic rate beyond what the age coefficient captures.
Dieting history. Prolonged calorie restriction can suppress RMR below what any formula predicts. Your body adapts to lower energy intake by reducing the energy it spends on non-essential processes. This effect can persist for months or even years after a diet ends, which is one reason weight regain is so common.
Body composition. Because lean tissue burns far more energy at rest than fat tissue, two people of the same weight, height, age, and sex can have metabolic rates that differ by hundreds of calories. If you’re unusually muscular or carry a high body fat percentage, the standard formula will over- or underestimate accordingly.
Ethnicity. The systematic review that endorsed Mifflin-St Jeor noted possible limitations when the equation is generalized to certain ethnic groups, since the original study population was predominantly white. Differences in body proportions and organ size can shift the result.
How to Use Your RMR Number
Your RMR is a baseline, not a daily calorie target. To estimate your total daily energy expenditure, you need to account for physical activity. The simplest approach is to multiply your RMR by an activity factor:
- Sedentary (desk job, little exercise): RMR × 1.2
- Lightly active (light exercise 1 to 3 days per week): RMR × 1.375
- Moderately active (moderate exercise 3 to 5 days per week): RMR × 1.55
- Very active (hard exercise 6 to 7 days per week): RMR × 1.725
- Extremely active (physical job plus training): RMR × 1.9
Using the earlier example, the 35-year-old woman with an RMR of 1,392 calories who exercises moderately would estimate her total daily burn at roughly 1,392 × 1.55 = 2,158 calories. From there, she could set a calorie target based on whether she wants to maintain, lose, or gain weight.
Keep in mind that all of these numbers are estimates. If you follow a calculated target for two to three weeks and your weight doesn’t move in the expected direction, adjust by 100 to 200 calories rather than assuming the math is broken. Real-world metabolism is messier than any equation, and small adjustments based on what actually happens on the scale will always beat recalculating with a different formula.
Getting a Lab Measurement
If you want a precise number rather than an estimate, indirect calorimetry is the gold standard. The test measures how much oxygen you consume and carbon dioxide you produce while resting, then converts those gas exchange rates into a calorie figure. You sit or recline while breathing into a mask or mouthpiece for 15 to 30 minutes. You need to stay still and awake throughout the test.
Many sports medicine clinics, university exercise science labs, and some dietitian offices offer this test, typically for $75 to $250. It’s worth considering if you’ve been dieting for a long time without results, if you’re an athlete fine-tuning performance nutrition, or if you suspect your metabolism runs significantly higher or lower than predicted. For everyone else, the Mifflin-St Jeor equation is a reliable and free starting point.