How BMR Calculator (Mifflin-St Jeor) works

This calculator estimates your Basal Metabolic Rate (BMR) using the Mifflin-St Jeor equation, which predicts the number of calories your body requires at rest to maintain vital functions—breathing, circulation, cell production, and temperature regulation. The tool accepts weight in kilograms, height in centimeters, age in years, and biological sex, then applies sex-specific constants to produce a daily caloric estimate. The Mifflin-St Jeor method is widely regarded as one of the more accurate predictive equations for BMR in clinical and research settings, often replacing older models like the Harris-Benedict formula in contemporary practice.

The formula

The calculator implements the Mifflin-St Jeor equation exactly as published in 1990:
Males: BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age in years) + 5
Females: BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age in years) − 161

The sex-specific constants (+5 for males, −161 for females) account for physiological differences in lean body mass and hormonal profiles that affect resting energy expenditure. The tool also displays per-hour and per-week estimates by dividing or multiplying the daily BMR accordingly.

Where this method is most accurate

The Mifflin-St Jeor equation was derived from indirect calorimetry measurements in adults with a range of body compositions. It tends to perform well for individuals of average to moderately overweight body mass within the age ranges studied (roughly 19–78 years in the original validation sample). Accuracy may decline in populations with very high lean mass (competitive bodybuilders), very low body fat (elite endurance athletes), or extreme obesity. The equation does not account for individual metabolic variation, thyroid function, adaptive thermogenesis, or medication effects, all of which can shift true BMR away from predicted values by several hundred calories per day.

What this tool does not do

This calculator provides an estimate of resting energy expenditure only. It does not calculate Total Daily Energy Expenditure (TDEE), account for activity level, prescribe caloric intake targets, or offer weight-loss or weight-gain recommendations. It does not diagnose metabolic conditions, replace indirect calorimetry measurement, or adjust for factors like pregnancy, lactation, illness, or specific medical treatments. The output is a mathematical prediction, not a personalized nutrition or training plan.

Disclaimer

This tool is for educational and informational purposes only. It does not constitute medical, nutritional, or fitness advice. Consult a qualified healthcare provider, registered dietitian, or certified exercise professional before making decisions based on calculator outputs, especially if you have a medical condition or are taking medication that affects metabolism.