The basal metabolic rate (BMR) represents the energy required to sustain basic bodily functions while at rest in a neutral temperature environment with a non-active digestive system. Think of it as the fuel consumed by an idle car when parked. In this state, energy is primarily allocated to maintaining essential organs such as the heart, brain, kidneys, nervous system, intestines, liver, lungs, reproductive organs, muscles, and skin. Typically, over 70% of a person’s daily energy expenditure is devoted to these basic functions, while physical activity accounts for roughly 20% and the digestion of food (thermogenesis) about 10%.

BMR is measured under very strict conditions—while the individual is awake but completely at rest with an inactive sympathetic nervous system. Since basal metabolism usually forms the largest portion of one’s total calorie needs, daily caloric requirements are estimated by multiplying the BMR by a factor that generally ranges from 1.2 to 1.9, depending on the individual’s level of physical activity.

In practice, BMR is most often estimated using equations derived from statistical data. One of the earliest methods was the Harris-Benedict Equation, which was updated in 1984 for improved accuracy. However, in 1990 the Mifflin-St Jeor Equation was introduced and has since been recognized as more accurate for most individuals. Another formula, the Katch-McArdle Formula, calculates the resting daily energy expenditure (RDEE) by considering lean body mass—a factor not addressed by the Mifflin-St Jeor or Harris-Benedict equations. In general, the Mifflin-St Jeor Equation is the preferred method unless an individual is very lean and has precise body fat measurements, in which case the Katch-McArdle Formula might offer a better estimate. Users can select their preferred equation in the settings of the calculator.

The three formulas are as follows:

Mifflin-St Jeor Equation
For men:
 BMR = 10 × W + 6.25 × H – 5 × A + 5
For women:
 BMR = 10 × W + 6.25 × H – 5 × A – 161

Revised Harris-Benedict Equation
For men:
 BMR = 13.397 × W + 4.799 × H – 5.677 × A + 88.362
For women:
 BMR = 9.247 × W + 3.098 × H – 4.330 × A + 447.593

Katch-McArdle Formula
 BMR = 370 + 21.6 × (1 – F) × W

where:

• W is body weight in kilograms
• H is height in centimeters
• A is age in years
• F is the body fat percentage

Several factors influence BMR:

• Muscle Mass: Anaerobic activities like weight lifting increase muscle mass, which in turn raises BMR, since more muscle requires more energy at rest.
• Age: BMR tends to decrease with age, meaning older individuals generally require fewer calories for basic bodily functions.
• Genetics: Inherited traits also play a role in determining BMR.
• Temperature: In colder environments, the body uses more energy to maintain its temperature, increasing BMR. Similarly, extreme heat can elevate energy expenditure as the body works to cool itself. In fact, a rise of about 1.36°F in internal body temperature can boost BMR by roughly 7%.
• Diet: Frequent, small meals can slightly increase BMR, while prolonged starvation may reduce it by as much as 30%, as the body shifts to a more energy-conserving state.
• Pregnancy: Energy needs rise during pregnancy to support the developing fetus, leading to an increased BMR. Hormonal changes during menopause can also influence BMR, either increasing or decreasing it.
• Supplements: Some substances, such as caffeine, are known to boost BMR and are sometimes used to support weight loss.

Measuring BMR

While online tests using these formulas can provide estimates, they are not the most accurate method. For precision, it’s best to consult a specialist or use a calorimetry device—commonly available in fitness centers, medical offices, and weight-loss clinics.

Resting Metabolic Rate vs. BMR

Although BMR and resting metabolic rate (RMR, sometimes called resting energy expenditure or REE) are often used interchangeably, they are not identical. BMR is measured under conditions of complete physiological equilibrium, whereas RMR is taken under less stringent conditions when the body is relaxed but not entirely inactive.

Modern Perspectives

A 2005 meta-analysis revealed that even after accounting for known factors, about 26% of the variance in metabolic rate remains unexplained. This means that while average BMR values can be estimated for most people, individual differences are influenced by factors not yet fully understood. Consequently, BMR and the resulting total daily energy expenditure (TDEE) calculations should be viewed as approximations. For those pursuing health or fitness goals, monitoring exercise, diet, and overall progress over time remains essential to making effective adjustments.