RMR: What Is Resting Metabolic Rate?

Have you ever wondered how dietitians estimate how many calories their clients should eat in a day? While the science is far from exact, several useful calculations can help determine how many calories you should eat for weight loss, gain, or maintenance.

Part of the calculation determines your resting metabolic rate (RMR), which is how many calories your body burns while at rest. You can calculate your RMR to see how many calories your body needs to perform basic functions like breathing and circulation. The amount of activity you do also determines how many calories you may need each day. Read on to learn some tips and tricks for estimating your energy needs.

What Is Metabolism and Metabolic Rate?

Your metabolism is the reactions that occur within each cell of your body and provide it with energy. Metabolism is how cells convert food into energy for daily functioning—from breathing and blood circulation to chewing and walking.

Food contains nutrients such as protein, vitamins, and minerals that your body absorbs and converts into units of heat, or calories. The energy—the calories—provided by the food is either used immediately or stored for your body to use later. Extra calories are usually stored as fat.

Metabolic rate measures the energy we use in a given time. It can be affected by age, diet, gender, race, disease, and activity level.

What Is RMR?

RMR rates vary from person to person, and several factors can be measured:

• Basal metabolic rate (BMR): Energy expenditure when you're still and awake. It is the minimum metabolic rate needed to keep your lungs, heart, brain, and circulation working.
• Resting metabolic rate (RMR): RMR is similar to BMR but can include some low-effort tasks, such as going to the bathroom.

If it's being measured clinically in a lab, BMR is assessed first in the morning when a patient is at rest after an overnight fast and has had no exercise for the previous 24 hours. RMR is measured after at least 15 minutes of rest with few other restrictions and does not need to be measured before getting out of bed.

If you are not measuring BMR or RMR in a lab setting and are using a simple calculation instead, the time of day doesn't matter. Studies show that RMR may better indicate daily energy needs than BMR. Once you measure your RMR, the answer will give you the approximate number of calories your body burns daily while at rest.

How to Calculate RMR

There are many ways to calculate RMR and BMR. The simplest is by plugging numbers into a calculation that takes your height, weight, age, and gender into account, but the accuracy of this method is questionable.

A lab-based test called indirect calorimetry is the most reliable method to measure RMR, but this method is expensive and time-consuming.

Using an Equation to Calculate Your Own BMR

If you enjoy math, you can also calculate BMR on your own. The 100-year-old Harris-Benedict Equation is still used to help estimate BMR.

Just how accurate is the Harris-Benedict equation? It's said to have an accuracy of no more than 70%, which means it can lead to major errors in estimating your true calorie needs. Of the equations for measuring metabolic rate, the Harris-Benedict is still the best choice (no equation is more accurate than 70%).

RMR Calculators

You can use an online calculator to measure your RMR if you know your height and weight. Use these links for an online RMR calculator:

• Orange Regional Medical Center Basal Metabolic Rate Calculator
• The Bariatric and Metabolic Center of Colorado Resting Metabolic Rate Calculator
• MyFitnessPal BMR Calculator

Calculate RMR in a Lab

Some medical facilities offer indirect calorimetry to provide a more reliable metabolic rate than a calculation. The test is non-invasive and usually takes about an hour.

For the test, you will wear a mask for a short period (around 15 minutes) while resting. The mask measures the exchange of gasses to determine the calories you burn at rest. The test is often used in critically ill patients to determine their nutritional needs, but some non-medical settings (like gyms) may also offer it.

Average RMR

No one RMR value is appropriate for all adults, but some still like to know the average RMR. When the Harris-Benedict equation was set in the 1920s, the average RMR for women was 1,400 calories per day and just over 1,600 calories for men.

A recent reference found that RMR in sedentary adults can range from less than 1,200 to more than 3,000 calories per day in both men and women meaning there is a large range for average RMR. These RMR estimates are the calorie levels at rest, which do not account for activity levels.

Factors Affecting RMR

Your weight, height, age, and gender are used to calculate RMR so these factors can impact the results. Race, diet, and activity level can all impact your RMR or BMR.

Interestingly, about 80% of the variability can be explained by how much lean and fat tissue a person has.

Factors Related to RMR

You can add one more layer to your results in the Harris-Benedict calculation, which accounts for your total daily energy expenditure (TDEE), including activity. There are five possible numbers based on your activity level:

• Sedentary (little or no exercise): calories = BMR × 1.2
• Lightly active (light exercise/sports 1-3 days/week): calories = BMR × 1.375
• Moderately active (moderate exercise/sports 3-5 days/week): calories = BMR × 1.55
• Very active (hard exercise/sports 6-7 days a week): calories = BMR × 1.725
• If you are extra active (rigorous exercise/sports and a physical job): calories = BMR × 1.9

Other factors that can be involved in determining RMR include:

• Thermic effect of food: We use energy to chew and digest food. Researchers call this the thermic effect of food (TEF). It makes up a small part of your total energy requirements. It's calculated by multiplying the total calories consumed/day x 0.1 (example: 2000-calorie (kcal) diet. TEF = 2000 x 0.1 = 200 kcal/day).
• Non-exercise: Non-exercise activity thermogenesis (NEAT) describes all calories you burn doing basic movements throughout the day like carrying groceries, walking to your desk at work, or cooking dinner. A low level of NEAT is associated with obesity.

Using RMR for Your Weight Goals

RMR calculations can be a basic tool to estimate your calorie needs but remember that calculations like the Harris-Benedict are only about 70% accurate. That means it would be easy to overestimate or underestimate your daily calorie needs by using this calculation. It's not a reliable method to determine calorie needs.

A lab test such as indirect calorimetry is a more reliable measure, but it's also a costly method and is still a best guess at your actual calorie needs.

A Word From Verywell

Estimating your calorie needs using a calculation that considers your activity level is a quick way to get a general estimate of your calorie needs. But remember, the number is not completely reliable and is just a rough estimate.

Meticulously counting every calorie you eat (or burn with exercise) based on a calculation is an exercise in futility because it's all based on estimates. A better idea? Listen to your hunger cues. Eat when you feel hungry, and stop when you feel full. Enjoy movement and stay active. And put the calculator away.