Please enter valid positive numbers for Loan Amount, Rate, and Term.
Monthly Principal & Interest:$0.00
Total Interest Paid:$0.00
Total Cost of Loan:$0.00
Payoff Date:–
Interest Saved (via Extra Payments):$0.00
function calculateMortgage() {
// Clear errors
document.getElementById('errorMsg').style.display = 'none';
document.getElementById('results').style.display = 'none';
// Get Inputs
var amount = parseFloat(document.getElementById('loanAmount').value);
var rateInput = parseFloat(document.getElementById('interestRate').value);
var termYears = parseFloat(document.getElementById('loanTerm').value);
var extra = parseFloat(document.getElementById('extraPayment').value);
// Validation
if (isNaN(amount) || amount <= 0 || isNaN(rateInput) || rateInput <= 0 || isNaN(termYears) || termYears <= 0) {
document.getElementById('errorMsg').style.display = 'block';
document.getElementById('errorMsg').innerText = "Please enter valid positive values for Amount, Rate, and Term.";
return;
}
if (isNaN(extra) || extra 0.01 && monthsPassed < maxMonths) {
var interestForMonth = balance * monthlyRate;
var principalForMonth = actualMonthlyPayment – interestForMonth;
// If the remaining balance is less than the principal payment, adjust payment
if (balance 0 && interestSavings > 0) {
document.getElementById('savingsRow').style.display = 'flex';
document.getElementById('interestSaved').innerText = formatter.format(interestSavings);
} else {
document.getElementById('savingsRow').style.display = 'none';
}
document.getElementById('results').style.display = 'block';
}
Understanding Mortgage Amortization
When you take out a home loan, your monthly payments are split into two distinct buckets: principal and interest. Mortgage amortization is the process of paying off your debt over time through regular payments. An amortization schedule shows exactly how much of each payment goes toward paying down the loan balance versus paying the lender for the privilege of borrowing the money.
How the Math Works
In the early years of a typical 30-year fixed-rate mortgage, the majority of your monthly payment goes toward interest. As time passes and the loan balance decreases, the interest portion of the payment shrinks, and the principal portion grows. This allows you to build equity faster in the later years of the loan.
Key Concept: Even though your total monthly payment remains constant with a fixed-rate mortgage, the allocation changes every month. Month 1 might be 80% interest, while Month 300 might be 80% principal.
The Power of Extra Payments
Using the calculator above, you can see how making an "Extra Monthly Payment" affects your loan. Because extra payments are applied directly to the principal balance, they reduce the amount of interest charged in all future months.
For example, adding just $100 a month to the principal can often shave several years off a 30-year loan term and save tens of thousands of dollars in interest. This is because you are effectively shortening the "life" of the loan, giving interest less time to accrue.
Frequently Asked Questions
What is the Principal?
The principal is the amount of money you originally borrowed to buy the home. As you pay this down, your equity (ownership stake) in the home increases.
Why does the interest amount decrease over time?
Interest is calculated based on your current outstanding balance. As you pay down the principal every month, the balance gets smaller. Therefore, the interest calculation (Balance × Rate) results in a smaller number each subsequent month.
Does this calculator include taxes and insurance?
No. This calculator focuses strictly on Principal and Interest (often abbreviated as P&I). Property taxes, homeowners insurance, and private mortgage insurance (PMI) are usually held in an escrow account and added on top of this figure by your servicer. To get your full "out-of-pocket" cost, you would need to add those estimated monthly costs to the result shown above.