function calculateAutoLoan() {
var vehiclePrice = parseFloat(document.getElementById('vehiclePrice').value) || 0;
var downPayment = parseFloat(document.getElementById('downPayment').value) || 0;
var tradeIn = parseFloat(document.getElementById('tradeIn').value) || 0;
var interestRate = parseFloat(document.getElementById('interestRate').value) || 0;
var loanTerm = parseFloat(document.getElementById('loanTerm').value) || 0;
var salesTaxRate = parseFloat(document.getElementById('salesTax').value) || 0;
// Calculate tax on the vehicle price
var taxAmount = vehiclePrice * (salesTaxRate / 100);
// Principal calculation: Price + Tax – Down Payment – Trade-In
var principal = vehiclePrice + taxAmount – downPayment – tradeIn;
if (principal <= 0) {
document.getElementById('monthlyPaymentDisplay').innerText = "$0.00";
document.getElementById('totalDetails').innerHTML = "Your down payment and trade-in cover the full cost of the vehicle.";
document.getElementById('auto-calc-result').style.display = 'block';
return;
}
var monthlyInterest = (interestRate / 100) / 12;
var monthlyPayment = 0;
if (monthlyInterest === 0) {
monthlyPayment = principal / loanTerm;
} else {
var x = Math.pow(1 + monthlyInterest, loanTerm);
monthlyPayment = (principal * x * monthlyInterest) / (x – 1);
}
var totalCost = monthlyPayment * loanTerm;
var totalInterest = totalCost – principal;
document.getElementById('monthlyPaymentDisplay').innerText = "$" + monthlyPayment.toLocaleString(undefined, {minimumFractionDigits: 2, maximumFractionDigits: 2});
var detailText = "Total Loan Amount: $" + principal.toLocaleString(undefined, {minimumFractionDigits: 2, maximumFractionDigits: 2}) + "" +
"Total Interest Paid: $" + totalInterest.toLocaleString(undefined, {minimumFractionDigits: 2, maximumFractionDigits: 2}) + "" +
"Total Cost of Loan: $" + totalCost.toLocaleString(undefined, {minimumFractionDigits: 2, maximumFractionDigits: 2});
document.getElementById('totalDetails').innerHTML = detailText;
document.getElementById('auto-calc-result').style.display = 'block';
}
How to Use the Auto Loan Payment Calculator
Buying a car is one of the most significant financial decisions you'll make. Our Auto Loan Payment Calculator is designed to help you navigate this process with transparency. By inputting a few key details, you can determine exactly how much a new or used vehicle will cost you on a monthly basis.
Key Inputs Explained
Vehicle Price: The sticker price of the car or the negotiated price before any additions.
Down Payment: The cash you pay upfront. A higher down payment reduces your monthly obligation and total interest.
Trade-In Value: The amount a dealer offers for your current vehicle, which acts like an additional down payment.
Interest Rate (APR): The Annual Percentage Rate charged by the lender. This is heavily influenced by your credit score.
Loan Term: The duration of the loan. Common terms are 60 or 72 months. While longer terms lower monthly payments, they increase the total interest paid.
The Math Behind Your Car Loan
Our calculator uses the standard amortization formula to calculate your fixed monthly payments:
M = P [ i(1 + i)^n ] / [ (1 + i)^n – 1 ]
Where:
M: Total monthly payment
P: Principal loan amount (Price + Tax – Down Payment)
i: Monthly interest rate (Annual Rate / 12)
n: Number of months in the loan term
Practical Examples
Example 1: The Budget Commuter
Imagine you are purchasing a used sedan for $15,000. You have $2,000 for a down payment and no trade-in. With a 48-month term and an interest rate of 6%, your monthly payment would be approximately $305.99. Over the life of the loan, you would pay $1,687.52 in total interest.
Example 2: The New Family SUV
You decide on a new SUV priced at $45,000. You trade in your old car for $10,000 and put down $5,000 in cash. With a 72-month term at a 5% interest rate, your monthly payment would be roughly $483.15. While the monthly cost is manageable, the 6-year term means you'll pay $4,786.80 in interest.
Tips for Lowering Your Monthly Payment
If the calculated result is higher than your budget allows, consider these strategies:
Improve Your Credit Score: Even a 1% drop in your APR can save you thousands over several years.
Increase Down Payment: Aim for at least 20% down on new cars to avoid "gap" issues where you owe more than the car is worth.
Shorten the Term: While it raises monthly costs, a 36 or 48-month loan significantly reduces interest expenses.
Shop for Financing Early: Get pre-approved by a credit union or bank before heading to the dealership to ensure you're getting a competitive rate.