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Mohammed mhanna
Mohammed mhanna

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⚖️ Build a BMI Calculator in Java (Step by Step)

#java #beginners #coding #programming

Ever wondered how apps calculate your Body Mass Index (BMI)? Let’s build one in Java!
This project is simple but teaches you important programming skills:

Taking user input

Doing calculations

Using conditions (if-else)

Printing results

📝 1. What is BMI?

BMI = weight (kg) ÷ height² (m²)

It’s a simple formula doctors use to check if your weight is in a healthy range.

Categories:

BMI < 18.5 → Underweight

18.5 ≤ BMI < 24.9 → Normal weight

25 ≤ BMI < 29.9 → Overweight

BMI ≥ 30 → Obese

💻 2. The Java Program

Here’s the full code:

import java.util.Scanner;

public class BMICalculator {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);

        // Get weight
        System.out.print("Enter your weight in kilograms: ");
        double weightInKg = scanner.nextDouble();

        // Get height
        System.out.print("Enter your height in meters: ");
        double heightInMeters = scanner.nextDouble();

        // Calculate BMI
        double bmi = weightInKg / (heightInMeters * heightInMeters);
        bmi = Math.round(bmi * 100.0) / 100.0; // round to 2 decimal places

        // Determine category
        String bmiCategory;
        if (bmi < 18.5) {
            bmiCategory = "Underweight";
        } else if (bmi >= 18.5 && bmi < 24.9) {
            bmiCategory = "Normal weight";
        } else if (bmi >= 25 && bmi < 29.9) {
            bmiCategory = "Overweight";
        } else {
            bmiCategory = "Obese";
        }

        // Print result
        System.out.println("\nYour BMI is: " + bmi);
        System.out.println("BMI Category: " + bmiCategory);

        scanner.close();
    }
}
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🔍 3. Key Parts Explained
✅ User Input with Scanner

We use Scanner to ask the user for weight and height.

Scanner scanner = new Scanner(System.in);
double weightInKg = scanner.nextDouble();
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👉 Takeaway: This is how Java reads numbers typed by the user.

✅ BMI Calculation

The formula is applied directly:

double bmi = weightInKg / (heightInMeters * heightInMeters);
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We then round it:

bmi = Math.round(bmi * 100.0) / 100.0;
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👉 Takeaway: Multiplying by 100 and dividing back is a common trick to round to 2 decimals.

✅ Using if-else for Categories

if (bmi < 18.5) {
    bmiCategory = "Underweight";
} else if (bmi >= 18.5 && bmi < 24.9) {
    bmiCategory = "Normal weight";
}
// ...
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👉 Takeaway: Conditions allow your program to make decisions.

🚀 4. Try It Out

Example Run:

Enter your weight in kilograms: 68
Enter your height in meters: 1.75

Your BMI is: 22.2
BMI Category: Normal weight

💡 5. Possible Improvements

If you want to expand this project:

Validate input (what if the user types text instead of numbers?).

Convert units (accept pounds and inches).

Add health tips based on BMI result.

Build a GUI version with JavaFX or Swing.

🎯 Final Thoughts

This project shows how you can take a real-world formula and turn it into working code.

You practiced:
✔ Reading user input
✔ Doing math in Java
✔ Making decisions with if-else
✔ Formatting output

💬 Question for you:
👉 What would you add next — unit conversion, GUI, or health tips?

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