This calculator provides an estimate based on common mortgage formulas. It does not reflect Zillow's specific algorithms or proprietary data.
20%
Monthly Principal & Interest: $0.00
Understanding Your Estimated Monthly Mortgage Payment
This calculator helps you estimate the principal and interest (P&I) portion of your monthly mortgage payment. This is a crucial component of home affordability and is often the largest part of your housing expense.
How the Calculation Works (The P&I Formula)
The monthly payment for a mortgage is calculated using the standard annuity formula. Zillow and other real estate platforms use this fundamental mathematical principle, though they may incorporate additional factors or proprietary data for their instant estimates.
The formula for calculating the monthly payment (M) is:
M = P [ i(1 + i)^n ] / [ (1 + i)^n – 1]
M = Your total estimated monthly mortgage payment (Principal & Interest)
P = The principal loan amount (Home Value – Down Payment)
i = Your monthly interest rate (Annual Interest Rate / 12)
n = The total number of payments (Loan Term in Years * 12)
Key Inputs Explained:
Estimated Home Value: The price you expect to pay for the property or its current estimated market value.
Down Payment: The upfront amount of cash you pay towards the home's purchase price. A larger down payment reduces the loan amount and your monthly payments.
Loan Term: The number of years you have to repay the loan. Common terms are 15, 20, and 30 years. Longer terms generally result in lower monthly payments but more interest paid over the life of the loan.
Annual Interest Rate: The yearly percentage charged by the lender for borrowing money. This rate significantly impacts your monthly payment and the total interest paid.
What This Calculator Doesn't Include:
It's important to note that this calculator focuses solely on the Principal and Interest (P&I) portion of your mortgage. Your actual total monthly housing cost will likely be higher and may include:
Property Taxes: Annual taxes assessed by your local government.
Homeowner's Insurance: Protection against damage or loss to your home.
Private Mortgage Insurance (PMI): Often required if your down payment is less than 20%.
Homeowner Association (HOA) Fees: Dues for living in a community with shared amenities or services.
Closing Costs: Fees paid at the time of closing the loan.
Zillow's estimates often attempt to incorporate property taxes and insurance, making their figures more comprehensive. Always consult with a mortgage lender for a precise breakdown of all costs associated with your specific situation.
How to Use This Calculator:
Enter the estimated value of the home you're interested in.
Specify your desired down payment percentage. You can use the slider or type in the exact percentage. The calculator will automatically show the resulting loan amount.
Select the loan term in years (e.g., 30 years).
Input the current annual interest rate you anticipate.
Click "Calculate Monthly Payment" to see your estimated P&I cost.
Use this tool to compare different home prices, down payment scenarios, and interest rates to better understand your potential mortgage obligations.
function calculateLoan() {
var homePrice = parseFloat(document.getElementById("homePrice").value);
var downPaymentPercent = parseFloat(document.getElementById("downPaymentPercentInput").value);
var loanTerm = parseInt(document.getElementById("loanTerm").value);
var interestRate = parseFloat(document.getElementById("interestRate").value);
var resultDiv = document.getElementById("result");
var resultParagraph = resultDiv.querySelector("p");
if (isNaN(homePrice) || homePrice <= 0) {
resultParagraph.innerText = "Please enter a valid home value.";
return;
}
if (isNaN(downPaymentPercent) || downPaymentPercent 100) {
resultParagraph.innerText = "Please enter a valid down payment percentage (0-100%).";
return;
}
if (isNaN(loanTerm) || loanTerm <= 0) {
resultParagraph.innerText = "Please enter a valid loan term (in years).";
return;
}
if (isNaN(interestRate) || interestRate < 0) {
resultParagraph.innerText = "Please enter a valid annual interest rate.";
return;
}
var loanAmount = homePrice * (1 – (downPaymentPercent / 100));
var monthlyInterestRate = (interestRate / 100) / 12;
var numberOfPayments = loanTerm * 12;
var monthlyPayment;
if (monthlyInterestRate === 0) {
monthlyPayment = loanAmount / numberOfPayments;
} else {
monthlyPayment = loanAmount * (monthlyInterestRate * Math.pow(1 + monthlyInterestRate, numberOfPayments)) / (Math.pow(1 + monthlyInterestRate, numberOfPayments) – 1);
}
if (isNaN(monthlyPayment) || !isFinite(monthlyPayment)) {
resultParagraph.innerText = "Calculation error. Please check your inputs.";
} else {
resultParagraph.innerText = "Monthly Principal & Interest: $" + monthlyPayment.toFixed(2);
}
}
// Initialize the display for down payment percentage and trigger calculation on load
document.addEventListener('DOMContentLoaded', function() {
var downPaymentSlider = document.getElementById('downPaymentPercent');
var downPaymentInput = document.getElementById('downPaymentPercentInput');
var downPaymentValueSpan = document.getElementById('downPaymentPercentValue');
// Sync slider and input value on load
downPaymentInput.value = downPaymentSlider.value;
downPaymentValueSpan.innerText = downPaymentSlider.value + '%';
calculateLoan(); // Calculate initial value on page load
});