BMR Calculator

Understanding Your Basal Metabolic Rate (BMR)

What is Basal Metabolic Rate (BMR)?

Your basal metabolic rate (BMR) represents the minimum amount of energy your body needs while at complete rest in a comfortable environment, when your digestive system is inactive. Think of it like the fuel your car burns just idling while parked. In this resting state, your body uses energy solely to maintain vital organs such as your heart, brain, kidneys, nervous system, intestines, liver, lungs, sex organs, muscles, and skin. For most individuals, a significant portion – around 70% – of the total calories burned each day is dedicated to these essential upkeep processes. Physical activity accounts for approximately 20% of daily energy expenditure, and about 10% is used for digesting food, a process known as thermogenesis.

BMR is measured under very specific and controlled conditions while you’re awake. An accurate BMR measurement requires your sympathetic nervous system to be inactive, meaning you must be completely rested. Basal metabolism typically forms the largest component of your overall daily calorie needs. To estimate your total daily caloric need, your BMR value is multiplied by an activity factor, which ranges from 1.2 to 1.9 depending on your activity level.

In most practical situations, BMR is estimated using equations derived from statistical data. The Harris-Benedict Equation was one of the earliest formulas developed for this purpose. It was revised in 1984 to improve accuracy and remained in use until 1990, when the Mifflin-St Jeor Equation was introduced. Research has indicated that the Mifflin-St Jeor Equation tends to be more accurate than the revised Harris-Benedict Equation for most people. The Katch-McArdle Formula takes a slightly different approach by calculating resting daily energy expenditure (RDEE), which considers lean body mass – a factor not included in either the Mifflin-St Jeor or the Harris-Benedict Equations. Among these equations, the Mifflin-St Jeor Equation is generally considered the most reliable for calculating BMR, with the exception that the Katch-McArdle Formula can be more precise for individuals with a lower body fat percentage who know their body composition. You can select which equation to use in our calculator settings.

BMR Calculation Equations

Here are the three equations our calculator uses:

Mifflin-St Jeor Equation:

• For men: $BMR=(10\times \text{weight in kg})+(6.25\times \text{height in cm})-(5\times \text{age in years})+5$
• For women: $BMR=(10\times \text{weight in kg})+(6.25\times \text{height in cm})-(5\times \text{age in years})-161$

Revised Harris-Benedict Equation:

• For men: $BMR=(13.397\times \text{weight in kg})+(4.799\times \text{height in cm})-(5.677\times \text{age in years})+88.362$
• For women: $BMR=(9.247\times \text{weight in kg})+(3.098\times \text{height in cm})-(4.330\times \text{age in years})+447.593$

Katch-McArdle Formula:

$BMR=370+(21.6\times (1-\text{body fat percentage}))\times \text{weight in kg}$

Where:

• W = Body weight in kilograms (kg)
• H = Body height in centimeters (cm)
• A = Age in years
• F = Body fat percentage

Factors Influencing BMR

Several factors can affect an individual’s BMR:

• Muscle Mass: While aerobic exercises like running don’t directly impact BMR, anaerobic exercises such as weightlifting can indirectly increase it by building muscle mass. Muscle tissue burns more calories at rest than fat tissue, so a higher muscle mass leads to a higher BMR.
• Age: As individuals age, their BMR tends to decrease. This means that older adults generally require fewer calories at rest to maintain basic bodily functions.
• Genetics: Inherited traits passed down through families can influence an individual’s BMR. Some people are naturally predisposed to having a higher or lower metabolic rate.
• Weather: Exposure to cold environments can elevate BMR as the body expends more energy to maintain a stable internal temperature. Similarly, excessive external heat can also increase BMR as the body works to cool down. For every 1.36 degrees Fahrenheit increase in internal body temperature, BMR can rise by approximately 7%.
• Diet: Consuming small, regular meals throughout the day can help to slightly increase BMR. Conversely, prolonged starvation or very restrictive diets can significantly reduce BMR (by as much as 30%) as the body attempts to conserve energy.
• Pregnancy: Supporting the growth and development of a fetus internally increases a pregnant woman’s BMR, often leading to increased food intake. Hormonal changes during menopause can also influence BMR, causing it to either increase or decrease.
• Supplements: Certain supplements and drugs, often marketed for weight loss, can increase BMR. Caffeine is a common example of a substance that can have this effect.

Limitations of Online BMR Tests

It’s important to understand that online BMR calculators, while convenient, provide an estimate based on general formulas and may not be the most accurate way to determine your individual BMR. For a more precise measurement, consulting a certified specialist or undergoing a calorimetry test is recommended. Calorimetry devices, which measure the amount of heat your body produces, are available in many health and fitness centers, doctor’s offices, and weight-loss clinics.

Resting Metabolic Rate (RMR) vs. BMR

While the terms BMR and resting metabolic rate (RMR), also sometimes referred to as resting energy expenditure (REE), are often used interchangeably, there is a subtle but key difference. BMR is measured under very strict conditions of complete rest and physiological equilibrium. RMR, on the other hand, measures energy expenditure in a relaxed state but not necessarily under the same rigorous conditions as BMR. The measurement conditions for RMR can be less controlled and defined by practical limitations.

The Complexity of Metabolic Rate

A 2005 meta-analysis study on BMR indicated that even when accounting for known factors influencing metabolic rate, there remains a significant (around 26%) unexplained variation between individuals. This highlights that while average BMR values can be predicted for someone with a typical diet and lifestyle, there are still aspects of human metabolism that are not fully understood and can influence BMR in ways we don’t yet fully grasp.

Therefore, it’s crucial to remember that all BMR calculations, even those performed using sophisticated methods, are still estimates and not perfectly precise measurements. Because our understanding of all human bodily functions is still evolving, total daily energy expenditure (TDEE) derived from BMR estimates is also an approximation. While BMR can be a helpful starting point for health and fitness goals, it’s just one piece of the puzzle. Keeping a daily log of exercise, food intake, and other relevant factors can provide valuable insights into what truly works for your individual body and help you make necessary adjustments over time to achieve your personal goals. Tracking your progress and making adjustments based on your own experiences is often the best way to see results.

Reference:

• Johnstone AM, Murison SD, Duncan JS, Rance KA, Speakman JR, Factors influencing variation in basal metabolic rate include fat-free mass, fat mass, age, and circulating thyroxine but not sex, circulating leptin, or triiodothyronine1. Am J Clin Nutr 2005; 82: 941-948.