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 RMRto 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?
Metabolism refers to all of the reactions that occur within each cell of the body and provide the body with energy. Metabolism is how our cells change the food we eat into energy for our daily functioning—from breathing to circulation to chewing to walking.
Each food you consume contains nutrients, such as protein, vitamins, and minerals. Your body absorbs the nutrients and converts them into units of heat—or calories. The energy—thecalories—that are provided by the food are either used right away 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 period. It can be affected by age, diet, sex, race, disease, and activity level.
What Is Resting Metabolic Rate?
Resting Metabolic Rate is the number of calories that your body burns while at rest.
Metabolic rates vary from person to person, and there are several factors that can be measured:
- Basal metabolic rate (BMR):The amount of energy that is used when you're lying 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.
The Difference Between BMR and RMR
BMR is measured when fully at rest, while RMR can have a small bit of movement.
If it's being measured clinically in a lab, BMR is assessed first in the morning. It is done 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 be a better indicator of 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.
Harris-Benedict Equation for BMR:
- Men:BMR = 88.362 + (13.397 x weight in kg) + (4.799 x height in cm) - (5.677 x age in years)
- Women:BMR = 447.593 + (9.247 x weight in kg) + (3.098 x height in cm) - (4.330 x age in years)
You can also use this equation online at Cornell University.
Just how accurate is the Harris-Benedict equation? It's said to have an accuracy of no more than 70 percent, which means it can lead to major errors in estimating your true calorie needs. Of the equations that exist for measuring metabolic rate, the Harris-Benedict is still the best choice (no equation is more accurate than 70 percent).
You can use an online calculator to measure your RMR as long as you know your height and weight. You can use these links to find 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 you with a metabolic rate that's more reliable than using 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 of time (around 15 minutes) while resting. The mask measures the exchange of gasses to determine the number of calories you burn when your body is at complete rest. The test is most often used in critically ill patients to determine their nutritional needs, but some non-medical settings (like gyms) may also offer it.
There is no single RMR value that is appropriate for all adults. But some people still like to know what the average RMR is for fellow humans. When the Harris-Benedict equation was set in the 1920s, the average RMR for women was 1400 calories per day and just over 1600 calories for men.
A more recent reference found that RMR in sedentary adults can range from less than 1200 to more than 3000 calories per day in both men and women. So, there's a huge range for what's deemed an 'average' RMR. And remember, these RMR estimates are the calorie levels at rest, which does not take activity levels into account.
Factors that Affect RMR
Your weight, height, age and gender all are used to calculate your RMR, so these factors can impact the results. Race, diet, and activity level can all have an impact on your RMR or BMR too.
- Ethnic descent
- Activity Level
Interestingly, about 80 percent 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 (above), which accounts for your total daily energy expenditure (TDEE), including activity. There are five possible numbers, based on how active you are:
- 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 (very hard exercise/sports & a physical job):calories = BMR × 1.9
Some 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 very 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 movement: Non-exercise activity thermogenesis (NEAT) describes all of the calories that 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.
How to Use RMR to Work Towards Your Weight Goals
RMR calculations can be used as a very basic tool to estimate your calorie needs, but remember that calculations like the Harris-Benedict are only about 70 percent accurate. That means it would be very easy to overestimate or underestimate your daily calorie needs by using this calculation. It's not a very 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 takes your actively level into account is a quick way to get a vague 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 off 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.
Frequently Asked Questions
What is an average RMR?
RMR in sedentary adults can range from less than 1200 to more than 3000 calories per day.
What is the difference between RMR and BMR?
BMR is the amount of energy used when you're lying still and awake. RMR is similar but can include some low-effort tasks. BMR is measured when fully at rest, while RMR can have a small bit of movement.
How do I use RMR to lose weight?
In an ideal world, RMR calculations would be 100 percent accurate and would let us know exactly how many calories our bodies need each day. That would allow us to cut calories for weight loss. However, RMR calculations are not very reliable (only 70% accurate), so they are not necessarily the only calculation you should use for your daily caloric intake.