Design a BMI Calculator using JavaScript

Last Updated : 20 Dec, 2023

The Body Mass Index (BMI) Calculator can be used to calculate BMI values based on height and weight. BMI is a fairly reliable indicator of body fatness for most people.

Formula:

BMI = (weight) / (height * height) 
// height in cms and weight in kgs

Output Preview:

Image Preview

Approach

BMI is a number calculated from an individual’s weight and height. To find out BMI we will take input from the user (both height and weight) which will be stored in height and weight variables for further calculation. The calculation process is simple, we will simply divide weight in kilograms by the square of the height. Now as per the BMI calculated, it will execute the respective if-else statement. We are also checking if the user is pressing the submit button without entering the inputs, in that case, we are printing provide height or provide weight.
Using HTML we are giving the desired structure, an option for the input, and a submit button. With the help of CSS, we are beautifying our structure by giving colors and desired font, etc.

In the JavaScript section, we are processing the taken input and after calculating, the respective output is printed.

Example: In this example, we will structure in the index.html file and implement the logic in the app.js file

Javascript

window.onload = () => {
    let button = document.querySelector("#btn");
 
    // Function for calculating BMI
    button.addEventListener("click", calculateBMI);
};
 
function calculateBMI() {
 
    /* Getting input from user into height variable.
    Input is string so typecasting is necessary. */
    let height = parseInt(document
            .querySelector("#height").value);
 
    /* Getting input from user into weight variable. 
    Input is string so typecasting is necessary.*/
    let weight = parseInt(document
            .querySelector("#weight").value);
 
    let result = document.querySelector("#result");
 
    // Checking the user providing a proper
    // value or not
    if (height === "" || isNaN(height)) 
        result.innerHTML = "Provide a valid Height!";
 
    else if (weight === "" || isNaN(weight)) 
        result.innerHTML = "Provide a valid Weight!";
 
    // If both input is valid, calculate the bmi
    else {
 
        // Fixing upto 2 decimal places
        let bmi = (weight / ((height * height) 
                            / 10000)).toFixed(2);
 
        // Dividing as per the bmi conditions
        if (bmi < 18.6) result.innerHTML =
            `Under Weight : <span>${bmi}</span>`;
 
        else if (bmi >= 18.6 && bmi < 24.9) 
            result.innerHTML = 
                `Normal : <span>${bmi}</span>`;
 
        else result.innerHTML =
            `Over Weight : <span>${bmi}</span>`;
    }
}

HTML

<!-- index.html file  -->
<!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>
    </head>
    <body style="background-color: rgb(244, 255, 244)">
        <div class="container">
            <h1>BMI Calculator</h1>
 
            <!-- Option for providing height 
            and weight to the user-->
            <p>Height (in cm)</p>
 
            <input type="text" id="height" />
 
            <p>Weight (in kg)</p>
 
            <input type="text" id="weight" />
 
            <button id="btn">Calculate</button>
 
            <div id="result"></div>
        </div>
    </body>
</html>