How Accurate Are RMR Calculators? A Quick Guide!

Some Background

Whether you’re looking to shed body fat or put on lean mass, you’ll need to know your total daily energy expenditure (TDEE). With that number in hand, it’s a lot simpler to achieve your goal. Just by subtracting, or adding, you’ll shed fat, or gain muscle. All you need now is the number.

So you search for an online calculator, plug in your stats and figure you’re set.

Unfortunately, it’s not so simple as online calculators offer estimates which may be off the mark. The margin of error onthat number can be as high as 200 to 300 calories.

So few calories may not sound like a lot, but they can be the difference between gaining fat or losing fat. This applies especially to those who are trying to lose the stubborn lower belly fat.

Naturally, it’s discouraging when you’ve applied so much discipline for weeks only to see no results because of an online calculator error. So how can you ensure that your numbers are right, before committing to them?

The RMR Difference

Figuring out your total daily energy expenditure is an imperative part of any fitness goal. Of the three components making up that number, your resting metabolic rate (RMR) is the main one as it’s responsible for around 70% of your TDEE.

With such a vital role, it’s imperative that you get this number right. Otherwise, the discipline and sacrifice may be for nothing.

Subsequently, it’s crucial that you understand how online calculators come up with your RMR, and how much they may be off by.

After all, web-based estimates don’t offer concrete numbers. Rather, they provide an approximate range. This doesn’t only apply to your RMR however.

Any online derived figure which calculates how many calories you burn, whether RMR, or from lifestyle and exercise, will generally be off.

Too Many Formulas

Most often, people turn to online calculators to figure your how many calories they burn at rest. With so much trust placed in web-based estimates, it’s important to know how these calculators arrive at their numbers.

Every online calculator uses a formula to determine your total daily energy expenditure, and subsequently your resting metabolic rate.

So what are the formulas, and which ones are the most accurate? 

The Original Harris-Benedict Formula

Originally published in 1919 by the Carnegie Institute of Washington, this original of the Harris-Benedict Formula is the oldest of the set, and is no longer used.

  • Men – 66.4730 + (13.7516 x weight in kg) + (5.033 x height in cm) – (6.7550 x age in years)
  • Women – 655.0955 + (9.5634 x weight in kg) + (1.8496 x height in cm) – (4.6756 x age in years)

The Revised Harris-Benedict Formula

 A 1984 update to the original, this version of the Harris-Benedict Formula overshoots modern energy expenditure requirements. Even though some online calculators still apply this formula, it’s best to skip either version.

  • Men – 88.362 + (13.397 x weight in kg) + (4.799 x height in cm) – (5.677 x age in years)
  • Women – 447.593 + (9.247 x weight in kg) + (3.098 x height in cm) – (4.330 x age in years)

The Mifflin St. Jeor Equation

Originally published in 1990, this equation was considered superior to the Harris-Benedict formula and received endorsements from numerous associations, including the American Journal of Nutrition.

While it provided a better idea of the small difference between men and women, the formula was body weight, which does not take into account the difference in metabolic activity between lean body mass and body fat.

  • Men – (10 x weight in kg) + (6.25 x height in cm) – (5 x age in years) + 5 (measured in K/cal a day)
  • Women – (10 x weight in kg) + (6.25 x height in cm) – (5 x age in years) – 161 (measured in K/cal a day)

The Katch-McArdle Formula

Unlike the earlier formulas, Katch-McArdle calculates RMR based on lead body mass. Hence, it distinguishes between total body weight and lean body mass. So if you know your body fat percentage and lean body mass, this is the formula for you.

The formula is considered to be the most accurate by fitness industry insiders as it takes into account more than total body weight.

  • 370 + (21.6 + LBM) – LBM

The Cunningham Formula

The Cunningham Formula should be used for more athletic clients, where muscle mass is significantly higher than average. In most cases, it provides a slightly higher average than the Katch-McArdle Formula.

There is some evidence of the Cunningham Formula being the most accurate. Specifically, this 19991 recommends it as the best estimation formula available.

  • 500 + (22 x LBM) – LBM

* LBM = Lean Body Mass

For Real Though

Fortunately, there are concrete alternatives to these formulas and the online calculators that use them.

By far, the most accurate method involves measuring and analyzing the composition of your breath. Obviously, the process is more involved than plugging in a couple numbers into a calculator. But as a result, you get an accurate measurement of how your metabolism functions and how many calories you burn at rest.

The hardest part of achieving any fitness goals is the willpower and discipline. Putting forth the effort just to get the wrong numbers can be devastating. So make the effort count, if you’re applying yourself, don’t half-ass it and do it right from the get-go.

The process is simple. While analyzing the composition of your breath, the measures the rate at which your body consumes energy, thereby determining how many calories your body burns at complete rest.

Formulas Versus Breathing Test

To give you an accurate idea of the difference between and RMR Breathing Test and online calculators, I calculated my RMR using each and every method and then compared it to my Breathing Test results.

For the aforementioned formulas, I plugged in my relevant stats to determine my calorie expenditure at rest.

For the RMR Breathing Test, I did a KORR Medical Technologies’ MetaCheck. In the interest of time, I’m not going to explain how the KORR MetaCheck determines your RMR, so here’s a link to their site.


  • The Harris-Benedict (Original)  – 1,515
  • The Harris-Benedict (Revised)  – 1,511
  • The Mifflin St Jeor Equation  – 1,507
  • The Katch-McArdle Formula  – 1,435
  • The Cunningham Formula – 1585
  • KORR RMR Breathing Test – 1325


So there is almost a 200 calorie difference between the formula derived average and the KORR test.

Over time, 200 calories may translate to your lean bulk being ineffective, or your weight loss plan effectuating at a slower than expected rate. When chasing fitness goals, these perceived setbacks can seem especially draining.

You should always pursue the most efficient method in order to maximize your efforts. Whether you’re pursuing fat loss or lean gains, every number counts. So make sure your numbers are right! Spend a little extra effort and opt for a breathing test instead of an online calculator.

These numbers are the foundation of your hard work. Make sure you’re building on the right base!

Leave a Reply

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