function calculateAutoLoan() {
var priceInput = document.getElementById('vehiclePrice');
var downInput = document.getElementById('downPayment');
var tradeInInput = document.getElementById('tradeInValue');
var rateInput = document.getElementById('interestRate');
var termInput = document.getElementById('loanTerm');
var price = parseFloat(priceInput.value) || 0;
var down = parseFloat(downInput.value) || 0;
var tradeIn = parseFloat(tradeInInput.value) || 0;
var rate = parseFloat(rateInput.value) || 0;
var term = parseInt(termInput.value) || 0;
var errorMsg = "";
if (price < 0) errorMsg += "Vehicle price cannot be negative. ";
if (down < 0) errorMsg += "Down payment cannot be negative. ";
if (tradeIn < 0) errorMsg += "Trade-in value cannot be negative. ";
if (rate < 0) errorMsg += "Interest rate cannot be negative. ";
if (term <= 0) errorMsg += "Loan term must be greater than zero. ";
if (errorMsg !== "") {
document.getElementById('calc-error-msg').innerHTML = errorMsg;
document.getElementById('calc-results').style.display = 'none';
return;
} else {
document.getElementById('calc-error-msg').innerHTML = "";
}
var loanAmount = price – down – tradeIn;
if (loanAmount 0) {
var mathPow = Math.pow(1 + monthlyRate, term);
monthlyPayment = loanAmount * (monthlyRate * mathPow) / (mathPow – 1);
totalCost = monthlyPayment * term;
totalInterest = totalCost – loanAmount;
} else {
monthlyPayment = loanAmount / term;
totalCost = loanAmount;
totalInterest = 0;
}
document.getElementById('monthlyPaymentResult').innerHTML = "$" + monthlyPayment.toFixed(2).replace(/\d(?=(\d{3})+\.)/g, '$&,');
document.getElementById('totalPrincipalResult').innerHTML = "$" + loanAmount.toFixed(2).replace(/\d(?=(\d{3})+\.)/g, '$&,');
document.getElementById('totalInterestResult').innerHTML = "$" + totalInterest.toFixed(2).replace(/\d(?=(\d{3})+\.)/g, '$&,');
document.getElementById('totalCostResult').innerHTML = "$" + totalCost.toFixed(2).replace(/\d(?=(\d{3})+\.)/g, '$&,');
document.getElementById('calc-results').style.display = 'block';
}
Understanding Your Auto Loan Financing
Purchasing a vehicle is a significant financial commitment for most people. Unless you are paying entirely in cash, you will likely need an auto loan to finance the purchase. This calculator is designed to help you estimate your monthly payments and understand the total cost of borrowing over the life of the loan. By inputting the vehicle's price, your down payment, trade-in value, interest rate, and loan term, you can see a breakdown of your potential financial obligation.
How the Calculator Works
The core of an auto loan calculation determines the principal loan amount first. This is calculated by taking the vehicle's selling price and subtracting any down payment you make upfront, as well as the value of any vehicle you trade in. For example, if you buy a car for $35,000, make a $5,000 down payment, and get $2,000 for your trade-in, you only need to borrow $28,000.
Once the principal is known, the calculator uses the Annual Percentage Rate (APR) and the loan term in months to determine your fixed monthly payment. A portion of each payment goes toward paying down the principal, while the rest pays the interest accrued that month. In the beginning, a larger share of your payment goes toward interest.
Key Factors Affecting Your Loan
Interest Rate (APR): This is the cost of borrowing money. A lower score typically qualifies you for a lower interest rate, which can significantly reduce your monthly payment and the total interest paid over the life of the loan. Even a 1% difference can save you hundreds or thousands of dollars.
Loan Term: This is the length of time you have to repay the loan, usually expressed in months (e.g., 36, 48, 60, 72 months). Longer terms lower your monthly bill but increase the total amount of interest you pay because you are borrowing the money for a longer period. Shorter terms have higher monthly payments but save you money on interest in the long run.
Down Payment and Trade-in: Putting more money down upfront or having a higher-value trade-in reduces the amount you need to borrow. A smaller loan principal means lower monthly payments and less total interest paid.
Use this tool to experiment with different scenarios. See how increasing your down payment or choosing a shorter loan term affects your bottom line to make the most informed decision for your budget.