Body Mass Index Calculator

Calculate BMI using WHO standards with automatic category detection. Includes age-adjusted percentiles for children and athletes. Provides health risk assessment and weight status classification.

Measurement Guide

  1. Select metric or imperial units
  2. Enter height and weight
  3. Provide age for percentile calculations
  4. Review BMI classification chart
  5. Compare to healthy weight ranges

Understanding Body Mass Index

Body Mass Index (BMI) is a mathematical formula that correlates body mass with height:

BMI = weight(kg) / height²(m)

BMI = weight(lb) × 703 / height²(in)

This formula, developed by Belgian mathematician Adolphe Quetelet in the 1830s, provides a simple numeric measure of body composition based on mass distribution relative to height.

Physiological Basis of BMI

BMI's theoretical foundation rests on several key physiological principles:

  • Square-Cube Law: As height increases, body mass should increase cubically while surface area increases quadratically
  • Body Composition: Distribution of muscle, fat, and bone mass affects overall body density
  • Allometric Scaling: Relationship between body dimensions follows predictable mathematical patterns
  • Mass Distribution: Vertical distribution of mass influences health outcomes independent of total mass

Mathematical Analysis

Dimensional Analysis:

  • • Weight dimension: [M]
  • • Height dimension: [L]
  • • BMI dimension: [M/L²]

Scale Invariance: BMI maintains consistency across different measurement systems through conversion factors:

1 kg/m² = 703 lb/in²

Body Composition Analysis

BMI interpretation requires understanding of body composition factors:

Tissue Density Variations:

  • • Muscle tissue: ~1.06 g/cm³
  • • Adipose tissue: ~0.9 g/cm³
  • • Bone tissue: ~1.5 g/cm³

These density differences explain why BMI may not accurately reflect body composition in athletes or elderly individuals with significant muscle mass variations.

Alternative Assessment Methods

BMI should be considered alongside other body composition metrics:

  • Waist Circumference: Measures central adiposity distribution
  • Body Fat Percentage: Direct measure of adipose tissue proportion
  • Waist-to-Height Ratio: Accounts for body frame variations
  • Bioelectrical Impedance: Measures tissue composition through electrical conductivity

These methods provide complementary data for comprehensive body composition assessment.