How to Calculate BMR: The Most Accurate Method Explained (2024)

By: Jeremy Fox, CNC, CPT, BSEng – Published: September 9, 2024

The Issue With BMR Calculators

BMR calculators can help you find your calorie intake for maintaining, losing, or gaining weight. However, some have limitations that could lead you down the wrong path.

As a certified nutritionist and engineer, I’ve checked the math and found some glaring problems. I’ve also developed a more accurate way to calculate BMR for weight loss or muscle gain.

So, if you’re stuck in your fitness journey and want better results from your diet, this is for you.

Why Accuracy Matters

Basal metabolic rate, or BMR, is the number of calories your body burns daily, excluding activity and exercise. Think of it as the minimum energy required to keep the lights on.

Surprisingly, your metabolism typically burns 60-80% of your daily calories. But everybody’s metabolic rate differs depending on your body size, gender, and genetics.

Understanding BMR is vital to determining the number of calories you need for a diet uniquely tailored to your metabolic rate.

How to Calculate BMR

The easiest way to calculate BMR is by using an estimation formula. You can do the calculations by hand or use an online BMR calculator to do the math.

One option is the Mifflin St-Jeor equation. It estimates BMR using gender, height, weight, and age. This one is the easiest to use but usually the least accurate.

Mifflin St-Jeor Equation

Men: BMR = ((10 x weight in kg) + (6.25 x height in cm) – (5 x age)) + 5

Women: BMR = ((10 x weight in kg) + (6.25 x height in cm) – (5 x age)) – 161

Another option is the Katch-McArdle equation, which uses your weight and body fat percentage. It then uses your lean body mass to estimate BMR.

Katch-McArdle Equation

Men & Women: BMR = (21.6 x lean mass in kg) + 370

Lean Mass = weight x ( 1 – (body fat percent / 100))

The Katch-McArdle equation is usually more accurate despite being simpler and not gender-specific. That’s because muscle mass strongly correlates with metabolic rate.

However,studies show that differences in lean mass can’t explain 27% of BMR variation. That means the remaining deviation must be due to other genetic factors.

Therefore, conventional BMR equations can overestimate or underestimate the number of calories burned because they don’t account for variations outside of gender, body mass, and age.

They assume everyone fits the same predictive model. Your actual BMR could be higher or lower than the calculation if you’re not on the linear trendline.

In other words, these equations don’t work if your metabolism is faster or slower than average. So, I developed a new BMR formula that accounts for genetic variation in metabolic rate.

In another article, I defined metabolic types as a spectrum from slow to fast metabolism. Applying this concept to BMR calculations allows me to shift the trendline and expand the model to a broader range of people.

I then created a new BMR formula that adjusts for individual metabolic differences. Let me show you how it works.

BMR Calculator Examples

Next, I’ll show you how BMR calculations can be off for individuals with slower or faster metabolisms and demonstrate how adjusting for your unique metabolic type provides a more accurate BMR estimation.

Calculating BMR for Weight Loss

In this example, Sarah is a 25-year-old woman who weighs 170 pounds and is 5’4″ tall.

With these inputs, a BMR calculator using the Mifflin St-Jeor equation estimates her BMR to be about 1,500 calories.

Now, let’s add that she has 40% body fat. Using the Katch-McArdle equation, her estimated BMR is significantly lower at 1,369 calories.

In this case, the Katch-McArdle equation is more accurate because she has less lean mass than the average person. Sarah would have difficulty losing weight if she used the Mifflin-St Jeor equation.

However, Sarah also struggles to lose weight due to a slower-than-average metabolism. In terms of metabolic types, she would be considered an endomorph.

Using the Nutritioneering BMR formula, I can adjust for Sarah’s naturally slow metabolism, and her BMR goes down to 1,219 calories. That’s almost 300 calories less than most BMR calculators would show!

This precision in calculating BMR could result in a calorie target that finally helps her lose weight.

Calculating BMR for Muscle Gain

Let’s look at the other end of the spectrum, where most calculators underestimate BMR. This time, I’ll use myself as an example.

If I put my numbers in the Mifflin St-Jeor equation, it says my BMR is 1,691. But the Katch-McArdle equation is higher, at 1,853.

Again, the KM equation is closer to reality because I have slightly more lean mass than the average person.I also have a relatively fast metabolism and other traits associated with an ecto-meso metabolic type.

When I plug that in, my true BMR is 1,927 calories. That’s about 240 calories higher than the MSJ equation. Maintaining or gaining muscle is much easier if I use this BMR to find my calorie intake.

Get this BMR Calculator Excel Spreadsheet

Calculating Calorie Intake from BMR

It’s important to understand that BMR is different from target calorie intake. To determine how many calories you should eat, we need to know your total daily energy expenditure or TDEE.

TDEE is all the calories you burn, including your BMR, everyday activities, and exercise. In another article, I covered the equations for calculating TDEE based on your BMR and activity level, which you can refer to for more information.

Understanding TDEE establishes your maintenance calorie intake. From there, you can reduce calories for weight loss or increase calories for muscle gain.

To lose weight, you must create a calorie deficit by eating fewer calories than you burn. However, consistently eating at or below your BMR can result in excessive muscle loss, metabolic slowdown, and weight loss plateaus.

Therefore, eating more than your BMR but less than your TDEE is generally best for weight loss. The cookie-cutter recommendation is to consume 500 calories less than you burn.

But a 500-calorie deficit could be too much for some and too little for others relative to the number of calories you burn.

So, I recommend setting a calorie target as a percentage of your TDEE. Specifically, a 10-20% calorie deficit should result in weight loss with minimal muscle loss for most people.

If you want to lose weight faster, adding activity is often better than cutting calories because you increase the deficit with less strain on your metabolism.

On the other hand, gaining muscle requires creating a calorie surplus by eating more calories than you burn. But again, it’s better to start with a conservative approach to avoid excess fat gain and other complications.

I recommend starting with as little as a 5-10% calorie surplus for lean muscle gain and gradually increasing if you’re not gaining weight.

Next Steps

Ultimately, finding your BMR and calorie intake is an excellent first step. These calculations empower you to create a healthy meal plan to reach your fitness goals.

For more help, click the link below to create your custom meal plan. You can also check out the related resources for calculating your body fat percentage or finding your metabolic type.

Create Your Custom Meal Plan Based on the Nutritioneering BMR Calculation

Find Your Body Fat Percentage for the Katch-McArdle Equation

Take the Metabolic Type Quiz to See If Your BMR is High or Low