BMR Calculator

Understanding Basal Metabolic Rate (BMR)

These days we talk a lot about health, well-being, and a balanced lifestyle. We are concerned with nutrition, exercise, sleeping routine, stress relief, and metabolism. But we tend to do it all ourselves, reading self-help books and relying on the internet for information, which may be overwhelming and a bit confusing. That’s why we need SMART goals, especially when we're trying to lose weight. Basal Metabolic Rate, aka BMR, can help us with the measurable (M) and achievable (A) parts of our SMART approach. Here is everything you need to know about BMR and how to calculate it.

What is BMR?

BMR represents the amount of energy your body needs at rest to support basic functions, such as blood circulation, breathing, body temperature management, brain activity, and so on. It is the minimum amount of energy your body needs to survive, so it’s essential that you receive it. BMR is measured in calories.

However, because you don’t spend all day in bed, resting, your body needs more energy than BMR to function properly and sustain your daily activities. Usually, BMR accounts for 60% to 75% of your total calorie burn. Digestion, movement, and other activities account for the rest.

Understanding BMR means understanding what your body needs to function, what’s negotiable and what’s not, and how to manage your weight in a healthy, sustainable way. Of course, BMR is not the only measure that matters. Other factors should be considered, and discussing with a specialist is often required.

Factors that Affect BMR

BMR is not the same for everybody, so no need to compare with your peers or data gathered from the internet. Although we may label a few categories, the truth is our BMR is as personal as our genes. The main factors that affect BMR are:

• Sex: Males tend to have a larger BMR than females because they have larger bodies and more lean muscle mass.
• Age: Because you lose muscle mass as you age, your BMR usually decreases.
• Overall body size: The larger you are, the higher the BMR because your body has more cells and tissue to sustain.
• Body structure: Muscles burn more energy than fat cells; therefore, the amount of muscles and the amount of fat cells in your body affect BMR as well.
• Race and genetics: Although it is not fully researched, it seems that race and genetics have something to say about your BMR.

There are also temporary factors that can affect your BMR, some of which are changeable, some of which are not. For instance, you may find yourself in one or more of the following situations:

• Growth: Children require more energy to grow, so they may have higher BMRs than adults.
• Pregnancy and lactation: A pregnant woman or one who’s lactating will probably have a higher BMR because her body is either growing new cells or producing breast milk.
• Menopause: Menopausal women register a fast decline in muscle mass and, if they don’t do anything about it, may have a lower BMR.
• Menstrual cycle: For women, BMR varies throughout the menstrual cycle by about 3% to 8%. BMR is lowest during menstruation and highest during the luteal phase.
• Stress: Stress has a high impact on our bodies, including on BMR. Short-term acute stress increases metabolism, but only temporarily, to allow the body to trigger the fight-or-flight response. Long-term chronic stress, on the other hand, decreases metabolism due to elevated cortisol levels that signal the body to store fat.

This takes us to the factors that affect BMR but aren’t written in stone. They are temporary or changeable, such as:

• Fasting or eating less: Your body is smart. It will be noticed that food doesn’t come in as much or as frequently as it used to, which slows down your metabolism. BMR will decrease.
• Stimulants: Nicotine and caffeine, for instance, speed up your metabolism and increase your BMR, although it is a temporary effect.
• Extreme temperatures: When your body works harder to regulate body temperature, it consumes more energy, and BMR increases. For instance, if you plan a trip to the Arctic Circle, you must plan for an intake of 5,000 - 6,000 calories per day just to survive.
• Thyroid hormone levels: They directly play havoc with your metabolism. Elevated levels speed up your metabolism (you tend to eat more, high BMR, but don’t gain weight), while low levels slow down your metabolism (you tend to eat less, low BMR, but still gain weight).
• Medical issues: Illnesses and injuries may also affect your metabolism. They usually increase your BMR because the body needs extra energy to heal.
• Activity level: Just like with temperature, BMR depends on your daily activities. A sedentary person has a lower BMR than an athlete who trains daily. Also, you burn more energy on the day you exercise than on the day you sit at your desk.

How to Calculate BMR?

1. Mifflin-St Jeor Equation

In 1990, researchers M.D. Mifflin and S.T. St Jeor developed a more accurate formula that improves the Harris-Benedict results by up to 5%. To calculate BMR using the Mifflin-St Jeor Equation, apply these formulas:

For women:

BMR = 10 x Weight(kg) + 6.25 x Height(cm) - 5 x Age(years) - 161

For men:

BMR = 5 + 10 x Weight(kg) + 6.25 x Height(cm) - 5 x Age(years)

2. Harris-Benedict Equation

Many BMR calculators still use the Harris-Benedict Equation, developed in 1919 by botanist James Arthur Harris and nutritionist Francis Gano Benedict. The formula requires four variables: gender, weight, height, and age. However, it doesn’t consider body composition.

To calculate BMR using the Harris-Benedict Equation, apply these formulas:

For women:

BMR = 447.593 + 9.247 x Weight(kg) + 3.098 x Height(cm) - 4.330 x Age(years)

For men:

BMR = 88.362 + 13.397 x Weight(kg) + 4.799 x Height(cm) - 5.677 x Age(years)

3. Mifflin-St Jeor Equation

If you know your exact body fat percentage (which you might if you use an intelligent scale), you may want to use the Katch-McArdle formula developed in 1975 by Frank I. Katch and William D. McArdle. However, it is more accurate for people with low body fat percentage and doesn’t take gender and age into consideration. It was tested and therefore recommended for young adults with low body mass. To calculate BMR using the Katch-McArdle formula, follow these steps:

1. Calculate lean body mass:
   LeanBodyMass = Weight(kg) x (1-BodyFat(%) 100)
2. Apply the Katch-McArdle equation:
   BMR = 370 + (21.6 x LeanBodyMass)

BMR vs. TDEE

BMR represents the energy your body burns at rest, in other words, the fuel it needs to survive. However, a more accurate measure for us is the Total Daily Energy Expenditure (TDEE), which reflects how many calories you burn on a regular day and accounts for your level of activity, not just your basal metabolic rate.

To calculate TDEE starting from your BMR, all you need to do is multiply the latter by the corresponding coefficient for your activity level:

• Sedentary - no exercise: TDEE = BMR x 1.2
• Light activity - low intensity exercise 1-2 times per week: TDEE = BMR x 1.375
• Moderate activity - low/moderate intensity exercise 3-4 times per week: TDEE = BMR x 1.55
• High activity - high intensity exercise 5-6 times per week: TDEE = BMR x 1.725
• Intense activity - very high intensity daily training: TDEE = BMR x 1.9

Related: TDEE Calculator

Is Calculating BMR Accurate Enough?

The differences among various BMR formulas are small enough not to cause concern. However, for athletes and people with serious metabolic disorders, formulas may not be enough. If that’s your case, you can take a lab BMR test, which will cost you somewhere between $70 and $200 in the U.S. There are two types of tests available:

• Breath Test (Direct Resting Metabolic Rate(RMR) Testing) - measures the oxigen and CO2 exchange
• Blood Panels for Metabolic Markers - tests metabolic-related hormones

Lab BMR tests are taken at rest, after a good night's sleep, in a calm state, and in a room with a comfortable temperature.

Examples for Men and Women

Let’s go through a few examples to clarify how to calculate BMR, RMR, and TDEE. It will help you understand better your SMART goals and the importance of data accuracy and adaptability in achieving them.

FAQs

BMR meaning?

BMR means Basal Metabolic Rate and represents the amount of calories your body needs to stay alive.

What is a good BMR for my age?

BMR varies with so many factors that it’s impossible to pin a single value per age. Consider your body structure, including total body mass, lean muscle percentage, fat percentage, and lifestyle, and monitor your BMR constantly. If you notice a significant change, check if it’s age-related or lifestyle-related and take appropriate measures. You may need to adjust your training routine, reduce stress, consult a nutritionist, or have your hormone levels checked.

Do I need to know my BMR to lose weight?

No, you don’t need to. But it may help you set SMART goals and lose weight in a healthier, more sustainable way. You can use BMR to calculate TDEE and create a calorie deficit that still provides the body with the energy it needs to survive. However, constantly adapt your diet, training routine, and lifestyle, and consult a professional rather than relying on formulas.

Are online BMR calculators accurate?

Online BMR calculators use the same formulas we provide in this article and, as far as calculating goes, are accurate. However, they don’t take into account factors such as body composition, hormone levels, stress levels, menstrual cycle, and dietary habits. Therefore, use BMR calculators as a benchmark and personalize the result to fit your body and habits.

Related: BMI Calculator