How to Build a Mortgage Calculator in Java (with Replit + GitHub)

Introduction

Calculating mortgage payments is one of the most common financial operations — yet understanding how it works under the hood can be enlightening. In this article, you’ll learn how to build a Mortgage Calculator using pure Java, complete with user input, validation, error handling, and formatted currency output.

This project is beginner-friendly and can be run locally or directly on Replit, a cloud-based IDE.

Prerequisites

To follow along, you should know:

• Basic Java syntax (if, while, Scanner, etc.)
• How to run Java code using javac and java
• Basic arithmetic and formula usage

The Mortgage Formula

We use the standard amortised loan formula:

M=P×r(1+r)n(1+r)n−1M = P \times \frac{r(1 + r)^n}{(1 + r)^n — 1}M=P×(1+r)n−1r(1+r)n​

Where:

• M = Monthly payment
• P = Principal (loan amount)
• r = Monthly interest rate (annual rate / 12 / 100)
• n = Total number of payments (years * 12)

Project Structure

Here’s the layout:

MortgageCalculator/
├── src/
│   └── com/
│       └── kshitij/
│           ├── MortgageCalculator.java
│           └── MortgageCalculator_ErrorHandling.java
├── README.md

MortgageCalculator.java (Basic Version)

This version simply takes user input and calculates the mortgage:

public class MortgageCalculator {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        final byte MONTHS_IN_YEAR = 12;
        final byte PERCENT = 100;
        System.out.print("Enter Principal: ");
        int principal = scanner.nextInt();
        System.out.print("Enter Annual Interest Rate: ");
        double annualInterestRate = scanner.nextDouble();
        System.out.print("Enter Period (Years): ");
        double years = scanner.nextDouble();
        double monthlyInterestRate = annualInterestRate / MONTHS_IN_YEAR / PERCENT;
        int numberOfPayments = (int) (years * MONTHS_IN_YEAR);
        double mortgage = principal * (monthlyInterestRate * Math.pow(1 + monthlyInterestRate, numberOfPayments)) /
                          (Math.pow(1 + monthlyInterestRate, numberOfPayments) - 1);
        String formattedMortgage = NumberFormat.getCurrencyInstance(Locale.CANADA).format(mortgage);
        System.out.println("Monthly Payment: " + formattedMortgage);
    }
}

MortgageCalculator_ErrorHandling.java (Enhanced Version)

This version improves the input process with validation and error messages using try-catch and while loops.

public class MortgageCalculator_ErrorHandling {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        final byte MONTHS_IN_YEAR = 12;
        final byte PERCENT = 100;
        double principal = 0;
        double annualInterestRate = 0;
        double years = 0;
        // Validate Principal
        while (true) {
            try {
                System.out.print("Enter principal ($1K - $1M): ");
                principal = scanner.nextDouble();
                if (principal < 1000 || principal > 1_000_000) throw new InputMismatchException();
                break;
            } catch (InputMismatchException e) {
                System.out.println(" Enter a number between 1,000 and 1,000,000");
                scanner.nextLine();
            }
        }
        // Validate Interest Rate
        while (true) {
            try {
                System.out.print("Annual Interest Rate (0 - 30): ");
                annualInterestRate = scanner.nextDouble();
                if (annualInterestRate <= 0 || annualInterestRate > 30) throw new InputMismatchException();
                break;
            } catch (InputMismatchException e) {
                System.out.println(" Enter a value greater than 0 and less than or equal to 30");
                scanner.nextLine();
            }
        }
        // Validate Period
        while (true) {
            try {
                System.out.print("Period (Years): ");
                years = scanner.nextDouble();
                if (years <= 0 || years > 30) throw new InputMismatchException();
                break;
            } catch (InputMismatchException e) {
                System.out.println(" Enter a value between 1 and 30");
                scanner.nextLine();
            }
        }
        double monthlyInterestRate = annualInterestRate / MONTHS_IN_YEAR / PERCENT;
        int numberOfPayments = (int) (years * MONTHS_IN_YEAR);
        double mortgage = principal * (monthlyInterestRate * Math.pow(1 + monthlyInterestRate, numberOfPayments)) /
                          (Math.pow(1 + monthlyInterestRate, numberOfPayments) - 1);
        String formattedMortgage = NumberFormat.getCurrencyInstance(Locale.CANADA).format(mortgage);
        System.out.println("\n Monthly Payment: " + formattedMortgage);
    }
}

Try It Online (Replit)

If you don’t want to install anything, you can run both versions directly in your browser using Replit:

Run Mortgage Calculator on Replit

• Click Run to launch
• Enter values as prompted
• View output in the console

If you get a Main class not found error, ensure your .replit file contains:

run = "javac src/com/kshitij/MortgageCalculator.java && java -cp src com.kshitij.MortgageCalculator"

GitHub Source Code

All source code is open and available here:

GitHub Repo — MortgageCalculator

Includes:

• Clean code with input validation
• README instructions
• Two working calculator versions
• Compatible with Replit, IntelliJ, or terminal

References

This project uses the standard mortgage formula from WikiHow

Final Thoughts

This project is a great way to practice real-world Java concepts:

• User input and validation
• Loops and error handling
• Math formulas and formatting
• Modular project structure

If you’re learning Java and want a hands-on project with clear logic and utility — this is a great place to start.