function calculateMortgage() {
// 1. Get Elements
var homePriceInput = document.getElementById("mc_home_price");
var downPaymentInput = document.getElementById("mc_down_payment");
var interestRateInput = document.getElementById("mc_interest_rate");
var loanTermInput = document.getElementById("mc_loan_term");
var propertyTaxInput = document.getElementById("mc_property_tax");
var homeInsuranceInput = document.getElementById("mc_home_insurance");
var hoaInput = document.getElementById("mc_hoa");
var resultDiv = document.getElementById("mc-results");
var errorDiv = document.getElementById("mc-error");
// 2. Parse Values
var homePrice = parseFloat(homePriceInput.value);
var downPayment = parseFloat(downPaymentInput.value);
var interestRate = parseFloat(interestRateInput.value);
var loanTerm = parseFloat(loanTermInput.value);
var propertyTax = parseFloat(propertyTaxInput.value);
var homeInsurance = parseFloat(homeInsuranceInput.value);
var hoa = parseFloat(hoaInput.value);
// 3. Validation
if (isNaN(homePrice) || isNaN(downPayment) || isNaN(interestRate) || isNaN(loanTerm)) {
errorDiv.style.display = "block";
resultDiv.style.display = "none";
return;
}
// Handle optional fields if empty
if (isNaN(propertyTax)) propertyTax = 0;
if (isNaN(homeInsurance)) homeInsurance = 0;
if (isNaN(hoa)) hoa = 0;
errorDiv.style.display = "none";
// 4. Calculations
var loanAmount = homePrice – downPayment;
if (loanAmount Home price
loanAmount = 0;
}
var monthlyRate = (interestRate / 100) / 12;
var numberOfPayments = loanTerm * 12;
var monthlyPrincipalInterest = 0;
// Standard Amortization Formula: M = P [ i(1 + i)^n ] / [ (1 + i)^n – 1 ]
if (monthlyRate === 0) {
monthlyPrincipalInterest = loanAmount / numberOfPayments;
} else {
monthlyPrincipalInterest = loanAmount * ( (monthlyRate * Math.pow(1 + monthlyRate, numberOfPayments)) / (Math.pow(1 + monthlyRate, numberOfPayments) – 1) );
}
// Monthly Taxes and Insurance
var monthlyTax = propertyTax / 12;
var monthlyInsurance = homeInsurance / 12;
// Total Monthly Payment
var totalMonthlyPayment = monthlyPrincipalInterest + monthlyTax + monthlyInsurance + hoa;
// Total Interest and Cost
var totalPaidOverTerm = monthlyPrincipalInterest * numberOfPayments;
var totalInterest = totalPaidOverTerm – loanAmount;
// 5. Update DOM
document.getElementById("res_monthly_total").innerText = formatMoney(totalMonthlyPayment);
document.getElementById("res_principal_interest").innerText = formatMoney(monthlyPrincipalInterest);
document.getElementById("res_tax").innerText = formatMoney(monthlyTax);
document.getElementById("res_insurance").innerText = formatMoney(monthlyInsurance);
document.getElementById("res_hoa").innerText = formatMoney(hoa);
document.getElementById("res_loan_amount").innerText = formatMoney(loanAmount);
document.getElementById("res_total_interest").innerText = formatMoney(totalInterest);
document.getElementById("res_total_cost").innerText = formatMoney(totalPaidOverTerm);
resultDiv.style.display = "block";
}
function formatMoney(amount) {
return "$" + amount.toFixed(2).replace(/\d(?=(\d{3})+\.)/g, '$&,');
}
How to Calculate Your Mortgage Payment
Understanding your monthly mortgage payment is the first step toward responsible homeownership. While the sticker price of a home tells part of the story, your actual monthly obligation involves several moving parts: principal, interest, taxes, and insurance (often referred to as PITI).
Using a reliable Mortgage Payment Calculator allows prospective buyers to estimate their budget accurately before making an offer. By inputting the home price, down payment, and expected interest rate, you can visualize how different factors impact your wallet.
Key Factors That Affect Your Payment
Principal & Interest: This is the core of your loan repayment. The principal pays down the debt balance, while interest is the cost of borrowing money from the lender. In the early years of a 30-year mortgage, a larger portion of your payment goes toward interest.
Down Payment: Putting more money down reduces your loan amount, which lowers your monthly payment. Additionally, a down payment of 20% or more typically eliminates the need for Private Mortgage Insurance (PMI).
Property Taxes: These are levied by local governments and are usually based on the assessed value of the property. They can vary significantly by location and are often bundled into your monthly mortgage bill via an escrow account.
Homeowner's Insurance: Lenders require insurance to protect the asset against damage from fire, theft, or natural disasters. This cost is also frequently paid through escrow.
Interpreting the Results
When you use the calculator above, you will see a breakdown of "Total Monthly Payment." It is crucial to look beyond just the Principal & Interest. High property taxes or HOA fees can push a seemingly affordable home out of your budget range. The "Total Cost of Loan" metric highlights the long-term impact of interest rates; even a fractional difference in rate can save or cost you tens of thousands of dollars over the life of a 30-year loan.
Tips for Lowering Your Mortgage Costs
If the estimated payment is higher than you are comfortable with, consider extending the loan term (e.g., from 15 to 30 years) to lower monthly payments, though this increases total interest paid. Alternatively, improving your credit score before applying can secure a lower interest rate, significantly reducing both your monthly payment and total loan cost.