Why Metabolic Rate Calculators Aren’t Always Accurate

Why Metabolic Rate Calculators Aren’t Always Accurate

(Last Updated On: April 4, 2019)


Why Metabolic Rate Calculators Aren’t Always Accurate

You’re exercising, watching what you eat and how much and still not losing weight. After so much work and so little results, you’re convinced that you have a slow metabolic rate – but how do you know? Short of getting your resting metabolic rate tested in a fitness or health care center, how can you be sure that you really DO have a “pokey” metabolism?

What does it mean to have a slow metabolism?  Your basal metabolic rate is the number of calories you burn over a 24- hour period when your body is resting and you’re not eating or moving around. Since measuring basal metabolic rate isn’t very practical, resting metabolic rate is a close approximation of your basal metabolic rate. Resting metabolic rate is when your body is in a relaxed state but not completely immobilized.

Your Body is Never Completely at Rest 

When you lie in bed all day, not moving, your body still burns calories. It does this to maintain bodily functions like your heart beating, breathing, blinking your eyes, etc. It’s true that you burn more calories when you exercise than when you rest, but you only spend a certain amount of time pumping iron, jumping, and running around. Unless you have a very active job, you spend the greatest part of your day at rest. That’s why resting metabolic rate accounts for 60 to 75% of the total calories most people expend every day. Exercise accounts for between 15% to 30% of daily energy expenditure. The third component is the thermic effect of food, the extra calories your body burns digesting food amounts to around 10% of the total calories you burn in a 24-hour period.

Since resting metabolic rate accounts for most of a person’s daily calorie expenditure, you’d probably like to know what your resting metabolic rate is. By knowing your resting metabolic rate, how many calories you burn in 24 hours, you would know how many calories you need to consume per day to maintain your current body weight. You could then adjust your calories to help with weight loss.

Metabolic Rate Calculators: Online Calculators 

One of the easiest ways to estimate your resting metabolic rate is to use one of the many online metabolic rate calculators that do the calculations for you. These calculators use various equations to estimate your daily energy expenditure. Resting metabolic rate varies with factors like age, sex, height, and how much muscle you have on your body. With these metabolic rate calculators, you input these values to get your resting metabolic rate and how many calories you burn at rest in a 24-hour period.

Unfortunately, these calculators aren’t accurate for everyone. They don’t take into account factors that affect your resting metabolic rate, like hormones and thyroid function. For example, if you have an under-active thyroid, you burn significantly fewer calories at rest each day than the formula suggests.

Metabolic Rate Calculators: Is There a More Accurate Way to Measure Your Resting Metabolic Rate?

Fortunately, an inexpensive test that uses the principle of indirect calorimetry is available. This test gives a more accurate measure of your resting metabolic rate. The test costs about $60.00 in most areas and only takes about 10 minutes. Many fitness centers, health clubs, and medical facilities offer this simple test that requires little preparation.

To do the test, you would avoid exercise, food, caffeine, or smoking for at least four hours beforehand. After resting for a period of time, the technician places a mouthpiece around your face for you to breathe through. This mouthpiece blocks air from entering your nose. Now, all you have to do is sit quietly and breathe in and out. As you breathe, a computer records the amount of oxygen you’re using at rest. The amount of oxygen you use is proportional to how many calories you’re burning. The machine can then extrapolate your oxygen usage over 24 hours and tell you how many calories you burn at rest in a day. This value doesn’t take into account the thermic effect of food or exercise. It’s simply the number of calories you need every 24 hours to maintain your body at rest. If you’re active, they can adjust the value by using a multiplier based on how active you are. This would give you a good idea of how many calories you need each day for weight maintenance.

Boosting Your Resting Metabolic Rate 

How can you maximize your resting metabolic rate? Unfortunately, your resting metabolic rate declines with age, partially because you lose muscle as you age. The more muscle you carry on your frame, the higher your resting metabolic rate. So, now you know one way to boost or at least prevent the slowdown in your metabolic rate that happens with age – build muscle by pumping iron.

Unfortunately, many people are short-sighted and spend too much time on cardio. Yes, cardio may burn more calories while you’re doing it but you only spend a certain amount of time engaged in it. When you build muscle, it’s a longer term investment in your resting metabolism. The additional lean body mass you build burns more calories even when you’re resting.

When you do cardio, the best choice for your metabolism is high-intensity interval training. With more intense exercise, your resting metabolism gets a bump up even after you’ve finished due to the after-burn, or EPOC, effect.

The thermic effect of food is a relatively minor component of your resting metabolism (around 10%). So, increasing that component won’t have a major impact on how many calories you burn daily. One thing you can do to subtly boost this component of resting metabolic rate is to eat whole foods and foods higher in protein. Your body expends more energy processing whole foods and foods high in protein than it does processed foods.

There are still some factors you can’t control. As mentioned, resting metabolic rate slows with age, which you can partially compensate for with weight training. Gender is another factor you have no control over. Men, are generally larger than women and have more lean body mass. Again, you do have SOME control in that you can build muscle through weight training.

The Bottom Line 

Metabolic rate calculators that measure your resting metabolic rate aren’t extremely accurate but they give you a rough idea of how many calories you burn at rest over a 24-hour period. If you want a more accurate assessment, you can get your resting metabolic rate measured at a fitness center.



Proc Nutr Soc. 2003 Aug;62(3):621-34.

Exercise Physiology for Health, Fitness, and Performance. Second edition. 2003.


Related Articles By Cathe:

5 Numbers That Impact Your Metabolic Health That You Need to Know

Why Your TDEE is Important for Weight Loss

5 Most Important Factors That Affect Total Daily Energy Expenditure

Is Muscle Loss the Only Reason Your Metabolism Slows with Age?


2 thoughts on “Why Metabolic Rate Calculators Aren’t Always Accurate

  1. Great post, I love how you mentioned ‘the body is never at rest’ this is so true, I think its an important thing to keep in mind when trying to loose weight.

  2. This is why you shouldn’t take what these calculators tell you as gospel, and instead track your calories and weight and figure out what works for you. If the calculator tells you to eat 2000 calories to lose a pound a week and you find that you have to eat 1800 to lose that, then eat 1800!

Leave a Reply

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.