BMR Calculator

Understanding Basal Metabolic Rate (BMR)

BMR is defined as the rate of energy expenditure by the body at complete rest in a neutral environment, while not digesting food. Consider that it is analogous to determining how much fuel a car uses while sitting idling in the driveway. At rest, energy usage is minimal and only required to sustain vital organs, which include: your heart, brain, kidneys, liver, lungs, intestines, muscles, skin, and even your reproductive organs.

For most people, about 70% of daily calories are being spent just for this basic maintenance. About 20% is used for physical activity, while the digestion process (thermogenesis) of food takes up the remaining 10%.

BMR Measurement

BMR is measured under very controlled conditions while a person is awake but at complete rest. To measure accurately the sympathetic nervous system, your body’s “fight or flight” system must not be activated as energy will be used for functions other than basic ones.

Since BMR is the largest factor in determining daily energy requirements, it serves as the base for estimating Total daily energy expenditure (TDEE). To calculate TDEE multiply BMR by an activity factor that ranges from 1.2 to 1.9 depending upon your activity level.

How BMR is calculated

BMR is estimated using formulas based on large population studies.
Three of the major equations commonly used are:

1. Mifflin-St Jeor Equation

• Men: BMR = 10 × weight + 6.25 × height – 5 × age + 5
• Women: BMR = 10 × weight + 6.25 × height – 5 × age – 161

2. Revised Harris-Benedict Equation

• Men: BMR = 13.397 × weight + 4.799 × height – 5.677 × age + 88.362
• Women: BMR = 9.247 × weight + 3.098 × height – 4.330 × age + 447.593

3. Katch-McArdle Formula

• BMR = 370 + 21.6 × (1 − % body fat) × weight

where:

• W is body weight in kg
• H is body height in cm
• A is age
• F is body fat in percentage

The weight is in kilograms, height in centimeters, age in years, and body fat as a decimal.

The Mifflin-St Jeor Equation is generally believed to be the most accurate for the average person.

The Katch-McArdle Formula can be more accurate for leaner individuals who know their body fat percentage.

Factors That Affect BMR

Your BMR is not set in stone, and multiple factors influence it:

Muscle Mass
The more muscle a person has, the higher the BMR, since muscle burns more calories at rest. Anaerobic exercises, such as weightlifting, can increase BMR over time.

Age
The basal metabolic rate decreases with age. Older individuals need fewer calories to maintain basic functions.

Genetics

Certain individuals naturally have higher or lower BMRs.

Temperature
Cold environments increase BMR because your body has to work harder to keep you warm. Very hot temperatures can also raise BMR.

Diet
Frequent but small meals can enhance BMR slightly, while long-term starvation reduces BMR by as much as 30%.

Pregnancy & Hormonal
Pregnancy increases BMR to support fetal development. Menopause and other hormonal changes can either raise or lower BMR.

Supplements & Drugs
Substances like caffeine can raise the BMR temporarily.

BMR vs RMR

You may also come across Resting Metabolic Rate (RMR) or Resting Energy Expenditure (REE). While similar, there is a subtle difference./

BMR needs complete physiological rest and strict conditions.
RMR is measured under conditions that are slightly more flexible and is usually somewhat higher than BMR.

Accuracy and Practical Use

Even using precise formulas, calculations of BMR are estimates, 26 percent of the variation in metabolic rate between individuals could not be explained.

BMR provides a good starting point when you try to work out your diet or exercise regimen, but that is not all there is, Pay attention to what you eat, how you move, and what actually happens as a result. Track it all and make tweaks as you go. Honestly, those day to day results tell you way more than any formula ever will.