You can find your resting metabolic rate (RMR) in two ways: estimate it using a validated equation that takes your age, weight, height, and sex, or get it measured directly at a clinic using a breathing test called indirect calorimetry. The equation method is free and takes two minutes. The clinical test costs $150 to $250 and gives you a personalized number based on your actual oxygen consumption. Which route makes sense depends on how precise you need the answer to be.
What Resting Metabolic Rate Actually Measures
Your resting metabolic rate is the number of calories your body burns just to stay alive while you’re awake and at rest. That includes keeping your heart beating, your lungs breathing, your brain running, and your cells repaired. For most people, this accounts for 60% to 75% of all the calories they burn in a day, making it the single largest piece of your daily energy budget.
You’ll sometimes see RMR used interchangeably with basal metabolic rate (BMR). They measure essentially the same thing, but BMR has stricter testing conditions: it’s taken first thing in the morning after an overnight fast, with no exercise in the previous 24 hours, in a completely rested and stress-free state. RMR is measured under slightly more relaxed conditions, typically at least 3 hours after eating and 8 hours after exercise. In practice, the two numbers land within a few percent of each other, and for everyday use they’re functionally the same.
Estimate It With the Mifflin-St Jeor Equation
The most widely recommended formula for estimating RMR is the Mifflin-St Jeor equation. You need four inputs: your weight in kilograms, height in centimeters, age in years, and sex. The formula works like this:
- Men: (9.99 × weight in kg) + (6.25 × height in cm) – (4.92 × age) + 5
- Women: (9.99 × weight in kg) + (6.25 × height in cm) – (4.92 × age) – 161
To convert pounds to kilograms, divide by 2.2. To convert inches to centimeters, multiply by 2.54. So a 35-year-old woman who weighs 150 pounds (68 kg) and stands 5’6″ (167.6 cm) would calculate: (9.99 × 68) + (6.25 × 167.6) – (4.92 × 35) – 161, which comes out to roughly 1,400 calories per day.
You can also find dozens of online calculators that run this equation for you. Just search “Mifflin-St Jeor calculator,” plug in your numbers, and you’ll get a result instantly. The older Harris-Benedict equation is still common in calculators too, but Mifflin-St Jeor is generally considered more accurate for modern populations.
If You Know Your Body Fat Percentage
Standard equations use your total body weight, but muscle tissue burns far more calories at rest than fat tissue does. Two people who weigh the same but carry very different amounts of muscle will have different metabolic rates. If you know your lean body mass (your total weight minus your fat mass), the Katch-McArdle formula accounts for this:
370 + (21.6 × lean body mass in kg)
This equation doesn’t need your sex, age, or height because lean mass already captures most of what those variables approximate. You can estimate your body fat percentage using skin-fold calipers, a bioelectrical impedance scale, or a DEXA scan for higher accuracy. If you carry significantly more or less muscle than average for your size, Katch-McArdle will likely give you a better estimate than Mifflin-St Jeor.
How Accurate Are These Equations?
Here’s the honest limitation: the Mifflin-St Jeor equation predicts RMR within 10% of the true measured value for only about 50% of people. That means for half the population, the estimate could be off by more than 10%, which on a 1,500-calorie RMR translates to an error of 150 calories or more in either direction. That’s enough to meaningfully affect weight management plans over weeks and months.
The equations work by averaging across large populations, but your individual metabolism is shaped by factors no simple formula can capture. Research analyzing the variance in metabolic rate found that lean body mass explains about 63% of the differences between people, with fat mass accounting for another 6% and age about 2%. Thyroid hormone levels also play a role, explaining as much as 25% of the remaining variation in men. These are all things a four-variable equation can only approximate.
Get It Measured With Indirect Calorimetry
The gold standard for finding your actual RMR is a test called indirect calorimetry. You breathe into a mask or under a clear canopy hood while lying still for 15 to 20 minutes. The device measures how much oxygen you consume and how much carbon dioxide you produce. Because your body uses oxygen to burn fuel, the ratio of these two gases reveals precisely how many calories you’re burning at rest.
The experience is straightforward. You lie on a table or recline in a chair, a lightweight hood or mask is placed over you, and you breathe normally. There are no needles, no exertion, and it’s not uncomfortable. The machine does the math, and you walk out with a specific calorie number tailored to your body.
Where to Get Tested and What It Costs
Indirect calorimetry tests are offered at sports performance centers, longevity clinics, some dietitian offices, university exercise science labs, and certain hospitals. A standalone RMR test typically costs around $150 to $250. Some facilities bundle it with other assessments: a combined RMR and body composition scan (DEXA) might run $400 to $425, while a package adding a VO2 max fitness test can reach $500 to $700. If cost is a concern, check whether local universities with exercise physiology programs offer testing at reduced rates.
How to Prepare for the Test
Getting accurate results requires some preparation. Food, alcohol, caffeine, and nicotine all temporarily raise your metabolic rate, so you’ll need to avoid them for a set period before testing. Most facilities ask you to fast for at least 3 to 4 hours beforehand, though some prefer an overnight fast. You should also skip moderate or vigorous exercise in the hours before the appointment, since intense activity can keep your metabolism elevated well after you stop. Light daily activities are fine, but you’ll want to rest quietly for 10 to 20 minutes once you arrive to let your body settle before the measurement begins. The testing room should be a comfortable temperature, since being too cold or too hot will shift your results.
What Affects Your RMR Over Time
Your resting metabolic rate isn’t fixed. It shifts with changes in your body composition, age, and hormonal status. Gaining muscle raises it. Losing muscle, which naturally happens with aging, lowers it. Crash dieting can temporarily suppress it as your body adapts to fewer calories, a phenomenon sometimes called metabolic adaptation.
Thyroid function has a measurable impact. Your thyroid hormones regulate how fast your cells convert fuel to energy. If your thyroid is underactive, your RMR drops. If it’s overactive, your RMR rises. This is one reason two people of the same age, size, and body composition can have noticeably different metabolic rates.
Age itself has a smaller direct effect than most people assume. The 4.92-calorie-per-year reduction built into the Mifflin-St Jeor equation reflects what happens on average, but much of that decline traces back to losing lean mass over time rather than aging itself. Maintaining or building muscle through resistance training is one of the most effective ways to keep your RMR from dropping as you get older.
Choosing the Right Method for You
If you’re looking for a general starting point to plan your nutrition, the Mifflin-St Jeor equation is a reasonable first step. It’s free, instant, and close enough for many people. If you’ve been following a calorie target based on an equation and your results don’t match your expectations, or if you have an unusual body composition, a history of significant weight changes, or a thyroid condition, a measured RMR from indirect calorimetry will give you a much more reliable number to work from.
Keep in mind that your RMR is just the baseline. To estimate your total daily calorie needs, you still need to factor in physical activity and the calories your body uses digesting food. Most people multiply their RMR by an activity factor ranging from 1.2 for sedentary lifestyles to 1.9 for very active ones. Starting with an accurate RMR makes that final number more trustworthy.