The BMR (Basal Metabolic Rate) calculator determines the number of calories your body needs to perform basic life functions at rest. Here’s a detailed explanation of how it works:

Input Collection

1. Basic Information:
   • Age: Accepts values between 15-100 years
   • Gender: Male or female selection (affects formula calculations)
   • Height: In centimeters (metric) or feet/inches (imperial)
   • Weight: In kilograms (metric) or pounds (imperial)
   • Formula Selection: Choice between three scientific formulas
2. Unit Conversion:
   • When using imperial units, the calculator automatically converts:
     • Feet and inches to centimeters (1 foot = 30.48 cm, 1 inch = 2.54 cm)
     • Pounds to kilograms (1 pound = 0.453592 kg)
3. Input Validation:
   • Age must be between 15-100 years
   • In metric: height must be 100-250 cm, weight must be 30-250 kg
   • In imperial: height must be 3-8 feet, weight must be 66-550 pounds

Formula Calculations

The calculator offers three different formulas:

1. Mifflin-St Jeor Formula (Default):
   • For men: BMR = 10W + 6.25H – 5A + 5
   • For women: BMR = 10W + 6.25H – 5A – 161
   • Where W = weight (kg), H = height (cm), A = age (years)
2. Harris-Benedict Formula (Revised):
   • For men: BMR = 13.397W + 4.799H – 5.677A + 88.362
   • For women: BMR = 9.247W + 3.098H – 4.330A + 447.593
   • Where W = weight (kg), H = height (cm), A = age (years)
3. Katch-McArdle Formula:
   • BMR = 370 + 21.6(1 – F)W
   • Where F = body fat percentage (as decimal), W = weight (kg)
   • Note: This calculator estimates body fat percentage based on age and gender:
     • For men: ~20% + 0.2% for each year over 20
     • For women: ~27% + 0.2% for each year over 20
     • Limited between 5% and 45% for realistic values

Activity Level Calculations

After calculating the base BMR, the calculator applies activity multipliers to estimate Total Daily Energy Expenditure (TDEE):

1. Sedentary (little/no exercise): BMR × 1.2
2. Light Exercise (1-3 days/week): BMR × 1.375
3. Moderate Exercise (3-5 days/week): BMR × 1.55
4. Active (6-7 days/week): BMR × 1.725
5. Very Active (hard exercise & physical job): BMR × 1.9

Technical Implementation

1. UI Functions:
   • switchUnits(unit): Toggles between metric and imperial input fields
   • isFormValid(): Validates all inputs, displays error messages if needed
   • calculateBMR(): Main calculation function that:
     • Retrieves and validates all inputs
     • Performs unit conversions if needed
     • Applies the selected formula
     • Calculates activity-based calorie needs
     • Displays results
2. Result Display:
   • Shows the calculated BMR rounded to the nearest whole number
   • Shows five different daily calorie targets based on activity levels
   • Results section is hidden until calculation is performed
3. Data Flow:
   • User inputs → Validation → Unit conversion → Formula application → Activity level multiplication → Results display

Practical Application

The calculator helps users in several ways:

1. Weight Management:
   • For weight loss: Consume fewer calories than your TDEE
   • For weight maintenance: Consume equal to your TDEE
   • For weight gain: Consume more calories than your TDEE
2. Fitness Planning:
   • Provides baseline for creating nutrition plans
   • Helps determine appropriate calorie targets based on activity level
3. Metabolic Awareness:
   • Shows how factors like age, gender, and body composition affect calorie needs
   • Educates users about different scientifically validated BMR calculation methods

The calculator provides a solid starting point for understanding your body’s energy needs, though individual metabolic factors can cause actual needs to vary somewhat from the calculated values.