BMR Calculator

What is Basal Metabolic Rate?

The basal metabolic rate (BMR) is the amount of energy needed while resting in a temperate environment when the digestive system is inactive. It is the equivalent of figuring out how much gas an idle car consumes while parked.

In such a state, energy will be used only to maintain vital organs, which include the heart, brain, kidneys, nervous system, intestines, liver, lungs, sex organs, muscles, and skin. For most people, upwards of ~70% of total energy (calories) burned each day is due to upkeep. Physical activity makes up ~20% of expenditure and ~10% is used for the digestion of food, also known as thermogenesis.

The BMR is measured under very restrictive circumstances while awake. An accurate BMR measurement requires that a person's sympathetic nervous system is inactive, which means the person must be completely rested. Basal metabolism is usually the largest component of a person's total caloric needs. The daily caloric need is the BMR value multiplied by a factor with a value between 1.2 and 1.9, depending on activity level.

How is BMR Calculated?

In most situations, the BMR is estimated with equations summarized from statistical data. The Harris-Benedict Equation was one of the earliest equations introduced. It was revised in 1984 to be more accurate and was used up until 1990, when the Mifflin-St Jeor Equation was introduced.

The Mifflin-St Jeor Equation has been shown to be more accurate than the revised Harris-Benedict Equation. The Katch-McArdle Formula is slightly different in that it calculates resting daily energy expenditure (RDEE), which takes lean body mass into account, something that neither the Mifflin-St Jeor nor the Harris-Benedict Equation does.

The three equations used by calculators are listed below (W = body weight in kg, H = body height in cm, A = age, F = body fat percentage):

Variables that affect BMR

• Muscle Mass – Aerobic exercises have no effect on BMR. However, anaerobic exercises, such as weight-lifting, indirectly lead to a higher BMR because they build muscle mass, increasing resting energy consumption.
• Age – The more elderly and limber an individual, the lower their BMR, or the lower the minimum caloric intake required to sustain the functioning of their organs.
• Genetics – Hereditary traits passed down from ancestors influence BMR.
• Weather – Cold environments raise BMR because of the energy required to create a homeostatic body temperature. Likewise, too much external heat can raise BMR as the body expends energy to cool off.
• Diet – Small, routinely dispersed meals increase BMR. Starvation can reduce BMR by as much as 30%.
• Pregnancy – Ensuring the livelihood of a separate fetus internally increases BMR.
• Supplements – Certain supplements or drugs raise BMR, mostly to fuel weight loss (e.g., Caffeine).

BMR Tests vs. Resting Metabolic Rate (RMR)

Online BMR tests with rigid formulas are not the most accurate method of determining an individual's BMR. It is better to consult a certified specialist or measure BMR through a calorimetry device.

While the two are used interchangeably, there is a key difference. Resting metabolic rate (RMR) is the rate at which the body burns energy in a relaxed, but not fully inactive state. BMR measurements must meet total physiological equilibrium while RMR conditions of measurement can be altered.

Modern Wisdom

A 2005 meta-analysis study on BMR showed that when controlling all factors of metabolic rate, there is still a 26% unknown variance between people. Therefore, all BMR calculations will not be perfectly accurate in their measurements. Tracking progress in a daily journal and making adjustments over time is generally the best indication of progress towards reaching personal fitness goals.

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.