Your Estimated Monthly Payment (Principal & Interest)
$0.00
Total Interest Paid:$0.00
Total Amount Paid:$0.00
This calculator provides an estimate for principal and interest payments only. It does not include property taxes, homeowners insurance, or Private Mortgage Insurance (PMI), which are often required in Washington State and will increase your actual monthly housing expense. Consult with a mortgage professional for precise figures.
Understanding Your Washington Mortgage Calculation
Purchasing a home in Washington State involves several financial considerations, and understanding your mortgage payment is crucial. This calculator helps estimate your Principal and Interest (P&I) payment, which is the core component of your monthly mortgage obligation.
The Mortgage Payment Formula
The standard formula used to calculate the monthly mortgage payment (M) is as follows:
M = P [ i(1 + i)^n ] / [ (1 + i)^n – 1]
Where:
M = Your total estimated monthly mortgage payment (Principal & Interest)
P = The principal loan amount (the total amount you borrow)
i = Your monthly interest rate. This is calculated by dividing your annual interest rate by 12 (e.g., a 6.5% annual rate becomes 0.065 / 12 = 0.005417).
n = The total number of payments over the loan's lifetime. This is calculated by multiplying the loan term in years by 12 (e.g., a 30-year loan has 30 * 12 = 360 payments).
How the Calculator Works
This calculator takes your inputs for:
Loan Amount: The total sum you need to borrow for the home purchase.
Annual Interest Rate: The yearly interest rate charged by the lender.
Loan Term: The duration of the loan in years (commonly 15, 20, or 30 years).
It then applies the formula above to calculate your estimated monthly P&I payment. Additionally, it computes the total interest paid over the life of the loan and the total amount repaid (principal + total interest).
Important Considerations for Washington Homebuyers
While this calculator is a valuable tool, remember that your actual total monthly housing cost in Washington will likely be higher. Lenders typically require you to pay for:
Property Taxes: Washington State has varying property tax rates depending on the county and city. These are usually paid monthly into an escrow account managed by your lender.
Homeowners Insurance: Required to protect against damage to your property. This also goes into your escrow account.
Private Mortgage Insurance (PMI): If your down payment is less than 20% of the home's value, you'll likely need PMI. This protects the lender.
HOA Dues: If the property is part of a Homeowners Association, these fees will be an additional monthly cost.
Always factor these additional costs when budgeting for your home purchase in Washington. For the most accurate picture, speak with a qualified mortgage broker or lender who can provide a Loan Estimate detailing all associated costs.
function calculateMortgage() {
var loanAmount = parseFloat(document.getElementById("loanAmount").value);
var annualInterestRate = parseFloat(document.getElementById("annualInterestRate").value);
var loanTermYears = parseInt(document.getElementById("loanTermYears").value, 10);
var monthlyPaymentResultElement = document.getElementById("monthlyPaymentResult");
var totalInterestResultElement = document.getElementById("totalInterestResult");
var totalPaymentResultElement = document.getElementById("totalPaymentResult");
// Clear previous results
monthlyPaymentResultElement.textContent = "$0.00";
totalInterestResultElement.textContent = "$0.00";
totalPaymentResultElement.textContent = "$0.00";
// Validate inputs
if (isNaN(loanAmount) || loanAmount <= 0 ||
isNaN(annualInterestRate) || annualInterestRate 100 ||
isNaN(loanTermYears) || loanTermYears <= 0) {
alert("Please enter valid numbers for all fields. Loan amount must be positive, interest rate between 0 and 100, and term must be positive.");
return;
}
// Calculate monthly interest rate
var monthlyInterestRate = annualInterestRate / 100 / 12;
// Calculate the number of payments
var numberOfPayments = loanTermYears * 12;
var monthlyPayment = 0;
var totalInterestPaid = 0;
var totalPayment = 0;
// Handle the edge case of 0% interest rate
if (monthlyInterestRate === 0) {
monthlyPayment = loanAmount / numberOfPayments;
} else {
// Calculate monthly payment using the mortgage formula
var numerator = loanAmount * monthlyInterestRate * Math.pow(1 + monthlyInterestRate, numberOfPayments);
var denominator = Math.pow(1 + monthlyInterestRate, numberOfPayments) – 1;
monthlyPayment = numerator / denominator;
}
// Calculate total payment and total interest
totalPayment = monthlyPayment * numberOfPayments;
totalInterestPaid = totalPayment – loanAmount;
// Format results to two decimal places and add currency symbol
monthlyPaymentResultElement.textContent = "$" + monthlyPayment.toFixed(2);
totalInterestResultElement.textContent = "$" + totalInterestPaid.toFixed(2);
totalPaymentResultElement.textContent = "$" + totalPayment.toFixed(2);
}