BMR Calculator

 The Basal Metabolic Rate (BMR) Calculator estimates how many calories your body burns at complete rest in a neutral environment. This measurement reflects the energy needed to sustain basic bodily functions—such as breathing, circulation, and cell production—while in a fasting state (typically after 12 hours without food).

Basal Metabolic Rate (BMR) Calculator

Gender: Male Female

Age: years

Weight: kg lbs

Height: cm feet inches

Calculation Settings

Formula: Mifflin-St Jeor Equation (Recommended) Revised Harris-Benedict Equation Katch-McArdle Formula (Requires body fat %)

Body Fat %: %

Results

Your BMR is: calories/day

Your daily calorie needs based on activity level:

• Sedentary: calories/day
• Light exercise: calories/day
• Moderate exercise: calories/day
• Heavy exercise: calories/day
• Athlete: calories/day

Understanding Basal Metabolic Rate (BMR)

What is BMR?
Basal Metabolic Rate (BMR) is the number of calories your body burns at complete rest to maintain basic physiological functions—such as breathing, circulation, and cell production—in a neutral environment when the digestive system is inactive (after ~12 hours of fasting). Think of it like the fuel an idling car consumes just to keep the engine running.

For most people, BMR accounts for ~70% of daily calorie expenditure, while physical activity makes up ~20%, and digestion (thermogenesis) contributes ~10%.

How is BMR Measured?

An accurate BMR measurement requires the body to be in a fully rested state, with the sympathetic nervous system inactive. Since strict lab conditions are impractical for most, BMR is often estimated using predictive equations.

Common BMR Formulas

1. Mifflin-St Jeor Equation (Most Accurate for General Use)

    Men: BMR = (10 × weight in kg) + (6.25 × height in cm) – (5 × age) + 5

    Women: BMR = (10 × weight in kg) + (6.25 × height in cm) – (5 × age) – 161

2. Revised Harris-Benedict Equation (Older, Slightly Less Accurate)

    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 (Best for Lean Individuals with Known Body Fat %)

    BMR = 370 + (21.6 × lean mass in kg)

    Lean mass = Weight × (1 – body fat %)*

Factors Affecting BMR

- Muscle Mass: More muscle = higher BMR (anaerobic exercise like weightlifting helps).
- Age: BMR decreases with age due to muscle loss and slower organ function.
- Genetics: Hereditary factors play a role.
- Climate: Cold environments raise BMR (to maintain body heat); extreme heat can also increase it.
- Diet: Frequent small meals may slightly boost BMR, while severe calorie restriction lowers it.
- Pregnancy & Hormones: BMR increases during pregnancy; menopause may alter it unpredictably.

- Supplements/Drugs: Stimulants like caffeine can temporarily raise BMR.

BMR vs. RMR (Resting Metabolic Rate)

While often used interchangeably, RMR is measured under less strict conditions (not fully fasted or at complete rest) and tends to be slightly higher than BMR.

How Accurate Are BMR Calculations?

Even the best equations are estimates. A 2005 study found that ~26% of BMR variation between individuals remains unexplained, meaning factors beyond current understanding influence metabolism. For greater accuracy, specialized tests (like indirect calorimetry) are recommended.

Practical Takeaways

- BMR provides a starting point for estimating daily calorie needs (TDEE = BMR × activity factor).
- Tracking food intake, exercise, and progress in a journal helps fine-tune adjustments.

- No formula is perfect—listen to your body and adjust based on results.

Reference:
Johnstone et al. (2005). Factors influencing variation in basal metabolic rate... Am J Clin Nutr.