Loan Amortization Calculator
Understanding Loan Amortization
A loan amortization schedule is a table that shows each periodic payment on an amortizing loan. It details how much of each payment goes towards interest and how much goes towards the principal balance, and the remaining balance after each payment.
Amortizing loans are common for mortgages, auto loans, and personal loans. The key characteristic is that each payment consists of both interest and principal repayment. In the early stages of the loan, a larger portion of your payment goes towards interest. As the loan matures, more of your payment is applied to reducing the principal balance.
How This Calculator Works
This calculator helps you understand your monthly payments, the total interest paid, and the total amount repaid over the life of the loan. It uses the following standard loan payment formula:
M = P [ i(1 + i)^n ] / [ (1 + i)^n – 1]
Where:
- M = Monthly Payment
- P = Principal Loan Amount
- i = Monthly Interest Rate (Annual Rate / 12)
- n = Total Number of Payments (Loan Term in Years * 12)
It then uses these monthly payments to generate an amortization schedule, breaking down each payment into principal and interest components.
Example Calculation
Let's consider a loan of $200,000 with an annual interest rate of 5% over 30 years.
- Principal (P): $200,000
- Annual Interest Rate: 5%
- Monthly Interest Rate (i): 5% / 12 = 0.0041667
- Loan Term: 30 years
- Total Number of Payments (n): 30 * 12 = 360
Using the formula, the estimated monthly payment (M) would be approximately $1,073.64. Over 30 years, you would pay a total of $386,510.40, with $186,510.40 of that amount being interest.
function calculateAmortization() {
var principal = parseFloat(document.getElementById("loanAmount").value);
var annualRate = parseFloat(document.getElementById("annualInterestRate").value);
var years = parseFloat(document.getElementById("loanTermYears").value);
var resultsDiv = document.getElementById("amortizationResults");
resultsDiv.innerHTML = ''; // Clear previous results
if (isNaN(principal) || isNaN(annualRate) || isNaN(years) || principal <= 0 || annualRate < 0 || years <= 0) {
resultsDiv.innerHTML = 'Please enter valid positive numbers for all fields.';
return;
}
var monthlyRate = annualRate / 100 / 12;
var numberOfPayments = years * 12;
// Calculate monthly payment
var monthlyPayment = principal * (monthlyRate * Math.pow(1 + monthlyRate, numberOfPayments)) / (Math.pow(1 + monthlyRate, numberOfPayments) - 1);
if (isNaN(monthlyPayment) || !isFinite(monthlyPayment)) {
resultsDiv.innerHTML = 'Calculation resulted in an invalid number. Please check your inputs.';
return;
}
var totalInterestPaid = (monthlyPayment * numberOfPayments) - principal;
var totalAmountPaid = monthlyPayment * numberOfPayments;
var htmlOutput = '
Loan Payment Summary
';
htmlOutput += '
Estimated Monthly Payment: $' + monthlyPayment.toFixed(2) + '';
htmlOutput += '
Total Principal Paid: $' + principal.toFixed(2) + '';
htmlOutput += '
Total Interest Paid: $' + totalInterestPaid.toFixed(2) + '';
htmlOutput += '
Total Amount Paid: $' + totalAmountPaid.toFixed(2) + '';
// Optional: Generate amortization schedule table
htmlOutput += '
Amortization Schedule (First Few Payments)
';
htmlOutput += '
';
htmlOutput += '| Payment # | Payment Date | Principal | Interest | Remaining Balance |
|---|
';
htmlOutput += '';
var remainingBalance = principal;
var currentDate = new Date(); // Use current date as a starting point
for (var i = 1; i remainingBalance) {
principalPayment = remainingBalance;
monthlyPayment = interestPayment + principalPayment; // Adjust monthly payment for the last payment if needed
}
remainingBalance -= principalPayment;
// Format date - simple example, could be more robust
var paymentDate = new Date(currentDate);
paymentDate.setMonth(currentDate.getMonth() + i -1);
var formattedDate = (paymentDate.getMonth() + 1) + '/' + paymentDate.getFullYear();
htmlOutput += '';
htmlOutput += '| ' + i + ' | ';
htmlOutput += '' + formattedDate + ' | ';
htmlOutput += '$' + principalPayment.toFixed(2) + ' | ';
htmlOutput += '$' + interestPayment.toFixed(2) + ' | ';
htmlOutput += '$' + remainingBalance.toFixed(2) + ' | ';
htmlOutput += '
';
}
if (numberOfPayments > 12) {
htmlOutput += '| ... and so on ... |
';
}
htmlOutput += '
';
resultsDiv.innerHTML = htmlOutput;
}
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}
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}
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}