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Mortgage Amortization Calculator

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function calculateMortgageAmortization() { // 1. Get inputs var loanAmountStr = document.getElementById('mortgageLoanAmount').value; var interestRateStr = document.getElementById('mortgageInterestRate').value; var loanTermStr = document.getElementById('mortgageLoanTerm').value; var resultsArea = document.getElementById('mortgageResultsArea'); // 2. Validate and parse numbers var P = parseFloat(loanAmountStr); // Principal var annualRate = parseFloat(interestRateStr); var years = parseInt(loanTermStr); if (isNaN(P) || isNaN(annualRate) || isNaN(years) || P <= 0 || annualRate < 0 || years <= 0) { resultsArea.style.display = 'block'; resultsArea.innerHTML = 'Please enter valid positive values for Loan Amount and Interest Rate.'; return; } // 3. Calculate variables for formula var n = years * 12; // Total number of payments (months) var i = (annualRate / 100) / 12; // Monthly interest rate // 4. Fixed Monthly Payment Calculation var monthly Payment; if (i === 0) { monthlyPayment = P / n; // Handle 0% interest edge case } else { // M = P [ i(1 + i)^n ] / [ (1 + i)^n – 1 ] var discountFactor = (Math.pow(1 + i, n) – 1) / (i * Math.pow(1 + i, n)); monthlyPayment = P / discountFactor; } // 5. Calculate Summary Totals var totalPaid = monthlyPayment * n; var totalInterest = totalPaid – P; // 6. Generate Amortization Schedule Loop var scheduleRowsHTML = ''; var currentBalance = P; for (var month = 1; month currentBalance) { principalPayment = currentBalance; interestPayment = monthlyPayment – principalPayment; currentBalance = 0; } else { currentBalance = currentBalance – principalPayment; } // Formatting rows scheduleRowsHTML += ''; scheduleRowsHTML += '' + month + ''; scheduleRowsHTML += '$' + monthlyPayment.toFixed(2) + ''; scheduleRowsHTML += '$' + principalPayment.toFixed(2) + ''; scheduleRowsHTML += '$' + interestPayment.toFixed(2) + ''; scheduleRowsHTML += '$' + currentBalance.toFixed(2) + ''; scheduleRowsHTML += ''; if (currentBalance <= 0) break; // Safety break } // 7. Construct Output HTML var outputHTML = '
'; outputHTML += '

Loan Summary

'; outputHTML += '
'; outputHTML += '
'; outputHTML += '$' + monthlyPayment.toFixed(2) + ''; outputHTML += 'Monthly P&I Payment'; outputHTML += '
'; outputHTML += '
'; outputHTML += '$' + totalInterest.toFixed(2) + ''; outputHTML += 'Total Interest Paid'; outputHTML += '
'; outputHTML += '
'; outputHTML += '$' + totalPaid.toFixed(2) + ''; outputHTML += 'Total Cost of Loan'; outputHTML += '
'; outputHTML += '
'; outputHTML += '

Amortization Schedule

'; outputHTML += '
'; outputHTML += ''; outputHTML += ''; outputHTML += '' + scheduleRowsHTML + ''; outputHTML += '
MonthTotal PaymentPrincipalInterestRemaining Balance
'; outputHTML += '
'; // 8. Display results resultsArea.innerHTML = outputHTML; resultsArea.style.display = 'block'; }

Understanding Mortgage Amortization

Mortgage amortization is the process of paying off a home loan over time through regular, scheduled payments. While your total monthly payment amount (for Principal and Interest) generally remains fixed for the life of a fixed-rate mortgage, the composition of that payment changes significantly over time.

In the early years of your loan, the majority of your payment goes toward paying off interest, with only a small portion reducing your principal balance. As the principal balance decreases over time, the interest charged each month also decreases, meaning more of your fixed payment is applied to the principal. This is why building equity is slow at first and accelerates later in the loan term.

How This Calculator Helps

This specific Mortgage Amortization Calculator allows you to input your loan amount, annual interest rate, and loan term (e.g., 15 or 30 years) to generate a detailed view of your loan repayment. It calculates your required monthly Principal & Interest (P&I) payment and provides a complete amortization schedule. This schedule breaks down every single payment, showing you exactly how much money is going toward interest versus principal every month until the loan is paid in full.

Example Calculation

Consider a common scenario: You borrow $350,000 to purchase a home with a fixed 30-year term at an interest rate of 6.5%. By entering these figures into the calculator above, you will see that your fixed monthly P&I payment is approximately $2,212.24. The amortization schedule will reveal that in the very first month, over $1,895 of that payment goes purely to interest, while only about $316 goes toward lowering your $350,000 debt. It takes many years before the principal portion of your payment exceeds the interest portion.

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