A mortgage is a significant financial commitment, and understanding how your monthly payment is calculated is crucial for making informed decisions. This calculator helps you estimate your principal and interest (P&I) payment for a home loan.
The Math Behind the Mortgage Payment
The standard formula for calculating the monthly payment (M) of a fixed-rate mortgage is:
M = P [ i(1 + i)^n ] / [ (1 + i)^n – 1]
Where:
P = Principal loan amount (the amount you borrow).
i = Monthly interest rate. This is the annual interest rate divided by 12. For example, if the annual rate is 6%, the monthly rate (i) is 0.06 / 12 = 0.005.
n = Total number of payments over the loan's lifetime. This is the loan term in years multiplied by 12. For a 30-year mortgage, n = 30 * 12 = 360.
How the Calculator Works:
Our calculator takes your inputs and applies this formula:
It converts your Loan Amount directly to 'P'.
It converts your Annual Interest Rate (%) into the monthly interest rate ('i') by dividing by 100 and then by 12.
It converts your Loan Term (Years) into the total number of monthly payments ('n') by multiplying by 12.
It then plugs these values into the formula to compute your estimated Monthly Payment (principal and interest only).
Important Considerations:
This calculator estimates the Principal & Interest (P&I) portion of your mortgage payment only. Your actual total monthly housing expense will likely be higher and may include:
Property Taxes: Paid to your local government.
Homeowner's Insurance: Required by lenders to protect against damage.
Private Mortgage Insurance (PMI): Often required if your down payment is less than 20%.
Homeowner Association (HOA) Fees: If applicable to your property.
Factors like your credit score, down payment amount, loan type (e.g., FHA, VA, Conventional), and current market conditions can significantly affect the actual interest rate you receive. This calculator serves as an educational tool to help you understand the basic relationship between loan amount, interest rate, term, and your monthly P&I payment.
var loanAmountInput = document.getElementById('loanAmount');
var interestRateInput = document.getElementById('interestRate');
var loanTermInput = document.getElementById('loanTerm');
var interestRateValueSpan = document.getElementById('interestRateValue');
var loanTermValueSpan = document.getElementById('loanTermValue');
var resultDiv = document.getElementById('result');
function updateSliderValue(input, spanId) {
var span = document.getElementById(spanId);
if (spanId === 'interestRateValue') {
span.textContent = parseFloat(input.value).toFixed(1) + '%';
} else {
span.textContent = input.value + ' years';
}
}
interestRateInput.oninput = function() {
updateSliderValue(this, 'interestRateValue');
};
loanTermInput.oninput = function() {
updateSliderValue(this, 'loanTermValue');
};
function calculateMortgage() {
var principal = parseFloat(loanAmountInput.value);
var annualInterestRate = parseFloat(interestRateInput.value);
var loanTermYears = parseInt(loanTermInput.value);
if (isNaN(principal) || principal <= 0) {
resultDiv.innerHTML = 'Please enter a valid loan amount.';
return;
}
if (isNaN(annualInterestRate) || annualInterestRate <= 0) {
resultDiv.innerHTML = 'Please enter a valid annual interest rate.';
return;
}
if (isNaN(loanTermYears) || loanTermYears 0) {
monthlyPayment = principal * (monthlyInterestRate * Math.pow(1 + monthlyInterestRate, numberOfPayments)) / (Math.pow(1 + monthlyInterestRate, numberOfPayments) – 1);
} else {
// Handle case of 0 interest rate (though unlikely for mortgages)
monthlyPayment = principal / numberOfPayments;
}
resultDiv.innerHTML = 'Monthly Payment: $' + monthlyPayment.toFixed(2) + 'Principal & Interest Only';
}
// Initialize slider values on load
updateSliderValue(interestRateInput, 'interestRateValue');
updateSliderValue(loanTermInput, 'loanTermValue');