Mortgage Payment Calculator
Understanding Your Mortgage Payment
A mortgage is a significant financial commitment, and understanding how your monthly payment is calculated is crucial for budgeting and financial planning. The monthly mortgage payment typically consists of four main components, often referred to as PITI:
- Principal: This is the actual amount of money you borrowed to purchase your home. Each payment gradually reduces the outstanding principal balance.
- Interest: This is the cost of borrowing money. Lenders charge interest, and a portion of your monthly payment goes towards paying this fee.
- Taxes: This refers to property taxes levied by your local government. These are often collected by your lender as part of your monthly payment and held in an escrow account to be paid on your behalf when they are due.
- Insurance: This includes homeowner's insurance, which protects against damage to your property, and potentially Private Mortgage Insurance (PMI) if your down payment was less than 20% of the home's value. Like taxes, these are often collected in escrow.
The Calculation Explained
While taxes and insurance (the 'TI' in PITI) can vary, the core of your monthly payment – the principal and interest (P&I) – is determined by a standard formula. This calculator focuses on the P&I portion. The formula used is:
M = P [ i(1 + i)^n ] / [ (1 + i)^n – 1]
Where:
- M = Your total monthly mortgage payment (Principal & Interest)
- P = The principal loan amount (the amount you borrowed)
- i = Your monthly interest rate (annual interest rate divided by 12)
- n = The total number of payments over the loan's lifetime (loan term in years multiplied by 12)
Example Calculation
Let's say you take out a mortgage for $200,000 with an annual interest rate of 3.5% for a term of 30 years.
- Principal (P) = $200,000
- Annual Interest Rate = 3.5%
- Monthly Interest Rate (i) = 3.5% / 12 = 0.035 / 12 ≈ 0.00291667
- Loan Term = 30 years
- Total Number of Payments (n) = 30 years * 12 months/year = 360
Plugging these values into the formula results in a principal and interest payment of approximately $898.09 per month.
Remember that this calculator provides an estimate for the principal and interest portion only. Your actual total monthly housing expense will likely be higher once property taxes and homeowner's insurance are factored in.
function calculateMortgage() {
var principal = parseFloat(document.getElementById("principal").value);
var interestRate = parseFloat(document.getElementById("interestRate").value);
var loanTerm = parseFloat(document.getElementById("loanTerm").value);
var resultDiv = document.getElementById("result");
if (isNaN(principal) || isNaN(interestRate) || isNaN(loanTerm) || principal <= 0 || interestRate < 0 || loanTerm <= 0) {
resultDiv.innerHTML = "Please enter valid positive numbers for all fields.";
return;
}
var monthlyInterestRate = interestRate / 100 / 12;
var numberOfPayments = loanTerm * 12;
// Mortgage Payment Formula: M = P [ i(1 + i)^n ] / [ (1 + i)^n – 1]
var numerator = monthlyInterestRate * Math.pow((1 + monthlyInterestRate), numberOfPayments);
var denominator = Math.pow((1 + monthlyInterestRate), numberOfPayments) – 1;
var monthlyPayment = principal * (numerator / denominator);
if (isNaN(monthlyPayment) || !isFinite(monthlyPayment)) {
resultDiv.innerHTML = "Calculation resulted in an invalid number. Please check your inputs.";
return;
}
resultDiv.innerHTML = "
Your Estimated Monthly Payment (Principal & Interest):
$" + monthlyPayment.toFixed(2) + "";
}
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#result {
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}
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}
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