Building a Body Mass Index Calculator: Input Validation, Edge Cases, and Ethical Design

When I set out to build a BMI calculator for zovo.one, I thought it would take thirty minutes. The formula is two lines of code. The UI is a form with two inputs and a result. How complicated could it be? Three days later, I was deep in research about ethnic-specific thresholds, reading W3C accessibility guidelines for health tools, and writing input validation for edge cases I had never considered. Building a BMI calculator taught me more about responsible software design than any enterprise project I have worked on. The Deceptively Simple Formula In imperial units: BMI = (weight in pounds x 703) / (height in inches)^2 In metric: BMI = weight in kg / (height in meters)^2 Implementation in JavaScript: function calculateBMI ( weight , height , system = ' metric ' ) { if ( system === ' imperial ' ) { return ( weight * 703 ) / ( height * height ); } return weight / ( height * height ); } That function works. But it is nowhere close to production-ready. Input Validation: More Than You Thin