You can find your resting metabolic rate (RMR) in two ways: use a predictive equation based on your age, weight, height, and sex, or get a clinical test called indirect calorimetry that measures your actual oxygen consumption. The equation method takes about 30 seconds and gets most people within 10% of their true number. The lab test is more precise but costs money and requires an appointment. Here’s how both work and which one makes sense for you.
What RMR Actually Measures
Your resting metabolic rate is the number of calories your body burns just to keep you alive and handle basic daily functions: breathing, circulating blood, digesting food, regulating temperature, and even small activities like walking to the bathroom or sweating. It typically accounts for 60% to 75% of the total calories you burn each day, making it the single biggest piece of your energy budget.
You’ll sometimes see RMR used interchangeably with basal metabolic rate (BMR), but they’re slightly different. BMR is measured under stricter conditions (overnight sleep in a lab, 12-plus hours of fasting, no prior exercise) and captures only the bare minimum your body needs. RMR is measured while you’re resting but awake, without the overnight lab stay, and it includes the small energy cost of things like recent food digestion and light movement. RMR runs slightly higher than BMR as a result, and it’s the more practical number for estimating your daily calorie needs.
For reference, the average adult male has a resting metabolic rate around 1,700 calories per day, and the average adult female is around 1,400. But there’s wide variation depending on your body size, muscle mass, and age, so averages are just a starting point.
The Equation Method
The most widely recommended formula is the Mifflin-St Jeor equation. A systematic review in the Journal of the American Dietetic Association compared four commonly used equations and found Mifflin-St Jeor was the most reliable, predicting RMR within 10% of the lab-measured value in more people (both normal weight and obese) than any other equation. It also had the narrowest error range.
The formula uses your weight in kilograms, height in centimeters, and age in years:
- Men: (10 × weight) + (6.25 × height) – (5 × age) + 5
- Women: (10 × weight) + (6.25 × height) – (5 × age) – 161
So a 35-year-old woman who weighs 70 kg (154 lbs) and stands 165 cm (5’5″) would calculate: (10 × 70) + (6.25 × 165) – (5 × 35) – 161 = 700 + 1,031 – 175 – 161 = 1,395 calories per day. Dozens of free online calculators will do this math for you if you search “Mifflin-St Jeor calculator.”
If You Know Your Body Fat Percentage
Standard equations use total body weight, but lean tissue (muscle, organs, bone) burns far more calories at rest than fat tissue does. If you’re particularly muscular or you know your body fat percentage from a DEXA scan or other measurement, a lean-mass-based formula will be more accurate.
The Katch-McArdle equation works like this: first calculate your lean body mass (total weight minus fat weight), then plug it in. The formula is 370 + (21.6 × lean mass in kg). A person weighing 80 kg at 15% body fat has 68 kg of lean mass, giving an RMR of 370 + (21.6 × 68) = 1,839 calories. For athletes with significant muscle mass, the Cunningham formula (500 + 22 × lean mass in kg) tends to produce a slightly higher and often more accurate estimate.
The Lab Test: Indirect Calorimetry
If you want your actual measured RMR rather than an estimate, the gold standard is indirect calorimetry. The test measures the concentrations of oxygen you consume and carbon dioxide you produce, then calculates how many calories your body is burning at rest based on those gas exchanges.
The setup is straightforward. You lie down in a comfortable position while a clear plastic hood is placed over your head. The hood captures your exhaled air, and a machine (called a metabolic cart) analyzes the oxygen and carbon dioxide concentrations in real time. The whole process takes about 30 to 45 minutes, including a rest period before measurement begins. You can’t talk during the test, and some facilities play classical music or put a calming poster above the bed to help you relax.
The equipment needs to be properly calibrated and warmed up for at least 30 minutes before testing, and there’s typically a 10-minute delay at the start to let your breathing stabilize. The output includes your oxygen consumption rate, carbon dioxide production, respiratory quotient (which indicates whether you’re burning more fat or carbohydrate), and your total resting energy expenditure in calories.
How to Prepare
To get an accurate reading, you need to fast for 12 hours beforehand (water is fine) and avoid strenuous exercise for 24 hours before the test. Most facilities also ask you to skip caffeine the morning of, since it raises your metabolic rate and would inflate the result. Arrive in comfortable clothing and expect to lie still for the duration.
Where to Get Tested
Indirect calorimetry is available at university wellness centers, sports performance clinics, some dietitian offices, and hospital-based nutrition programs. Pricing typically falls between $75 and $250 depending on the facility and location. It’s rarely covered by insurance unless ordered for a specific medical reason. If you’re investing in one, it’s worth calling ahead to confirm the facility uses a ventilated hood system (the most accurate setup) rather than a handheld device, which can be less reliable.
What Affects Your RMR
Several factors determine where your number lands, and understanding them helps you interpret your result.
Muscle mass is the biggest controllable factor. Lean tissue is metabolically expensive to maintain, burning calories around the clock even when you’re doing nothing. Two people who weigh the same can have very different RMRs if one carries significantly more muscle. This is why strength training is often recommended for long-term metabolic health.
Age matters less than most people think, at least until your 60s. A large study analyzed in Harvard Health Publishing found that both total energy expenditure and basal metabolic rate remain stable from ages 20 to 60, regardless of sex. The common belief that metabolism crashes in your 30s or 40s doesn’t hold up. After age 60, however, RMR does begin declining at roughly 0.7% per year, and by age 90 it can be about 26% below middle-age levels. Much of this decline comes from losing muscle mass and overall body mass in older age.
Body size plays an obvious role: larger bodies require more energy to maintain. Height and weight are built into every predictive equation for this reason. Sex also factors in because men tend to carry more lean mass relative to body weight, which is why male RMR averages run about 300 calories higher than female averages.
Dieting history can temporarily lower RMR. Extended periods of significant calorie restriction cause your body to adapt by reducing energy expenditure, sometimes to a degree that exceeds what you’d expect from the weight lost alone. This adaptive response is one reason weight regain is common after aggressive diets.
Which Method to Choose
For most people, the Mifflin-St Jeor equation is a perfectly good starting point. It’s free, instant, and accurate enough to guide nutrition decisions. If you’re using the number to set calorie targets for weight loss or athletic performance, being within 10% is close enough to adjust from based on how your body responds over a few weeks.
A lab test makes more sense if you have an unusual body composition (very muscular, very lean, or very high body fat), if you’ve been dieting for a long time and suspect your metabolism has adapted, or if you’re working with a dietitian who needs a precise baseline. It’s also useful if you’ve tried calorie targets based on equations and they don’t seem to match your real-world results. The measured number removes the guesswork and gives you a verified starting line.
Whichever method you use, remember that RMR is just the resting component. Your total daily calorie burn also includes physical activity and the energy cost of digesting food, which together add anywhere from 25% to 40% on top of your RMR depending on how active you are.