How I Built a BMI Calculator

#webdev #javascript #sideprojects #programming

💡 Introduction

As part of my web development learning journey, I created a fun and useful project — a BMI (Body Mass Index) Calculator. This helped me practice HTML form inputs, JavaScript logic, and CSS styling in a real-world mini-project.

In this blog, I'll walk you through the complete HTML, CSS, and JavaScript code I used to build it — and share what I learned along the way.

⚙️ What is BMI?

BMI stands for Body Mass Index. It is a simple formula used to check if a person has a healthy weight.

📌 Formula:

BMI = weight (kg) / [height (m)]²

Based on your BMI, you can be categorized as underweight, normal, overweight, or obese.

🧱 HTML Structure

Here’s the complete HTML I used:

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>BMI Calculator</title>
  <link rel = "stylesheet" href = "style.css">
</head>
<body>
  <div class="container">
    <h1 class="heading">Body Mass Index(BMI) Calculator</h1>
    Your Height (cm):
    <input type="number" class="input" id="height" value="180" placeholder="Enter your height in cm...">
    Your Weight (kg):
    <input type="number" class="input" id="weight" value="80" placeholder="Enter your weight in kg...">
    <button class="btn" id="btn">Calculate BMI</button>
    <input disabled type="text" class="input" id="bmi-result">
     <h4 class="info-text">Weight Condition: <span id ="weight-condition"></span></h4>
  </div>
  <script src = "index.js"></script>

</body>
</html>

🎨 CSS Styling
Here’s how I styled the calculator using CSS:

body{
  margin:0;
  background:  linear-gradient(to left bottom, lightgreen , lightblue);
  display : flex;
  height: 100vh;
  justify-content: center;
  align-items: center;
  font-family: 'Courier New', Courier, monospace;
}

.container{
  background: rgba(255,255,255,.3);
  padding: 20px;
  display: flex;
  flex-direction: column;
  border-radius: 5px;
  box-shadow: 0 10px 10px rgba(0,0,0,.3);
  margin: 5px;
}

.heading {
  font-size: 30px;
}

.input {
  padding: 10px 20px;
  font-size: 18px;
  background: rgba(255,255,255,.4);
  border-color: rgba(255,255,255,.5);
  margin: 10px;
}

.btn{
  background-color: lightgreen;
  border: none;
  padding: 10px 20px;
  border-radius: 5px;
  margin: 10px;
  font-size: 20px;
  box-shadow: 0 0 4px rgba(0,0,0,.3);
  cursor: pointer;
}

.btn:hover{
  box-shadow: 0 0 8px rgba(0,0,0,.3);
  transition: all 300ms ease;
}

.info-text{
  font-size: 20px;
  font-weight: 500;
}

🧠 JavaScript Logic
This is the JavaScript code that powers the BMI calculation and updates the result on the screen:

const btnEl = document.getElementById("btn");
const bmiInputEl = document.getElementById("bmi-result");
const weightConditionEl = document.getElementById("weight-condition")

function calculateBMI(){
    const heightValue = document.getElementById("height").value / 100;
    const weightValue = document.getElementById("weight").value;


    const bmiValue = weightValue / (heightValue * heightValue);

    bmiInputEl.value = bmiValue;

    if(bmiValue < 18.5){
             weightConditionEl.innerText = "Under Weight";
    } else if ( bmiValue >= 18.5 && bmiValue <= 24.9){
      weightConditionEl.innerText = "Normal Weight";
    } else if ( bmiValue >= 25 && bmiValue <= 29.9){
      weightConditionEl.innerText = "Over Weight";
    } else if (bmiValue >= 30 ){
      weightConditionEl.innerText  = "Obesity";
    }

}

btnEl.addEventListener("click", calculateBMI)

** Sample Output 🎉**