function calculateCarLoan() {
// 1. Retrieve and Parse Inputs
var price = parseFloat(document.getElementById('cl_vehiclePrice').value);
var down = parseFloat(document.getElementById('cl_downPayment').value);
var trade = parseFloat(document.getElementById('cl_tradeIn').value);
var taxRate = parseFloat(document.getElementById('cl_salesTax').value);
var interestRate = parseFloat(document.getElementById('cl_interestRate').value);
var months = parseInt(document.getElementById('cl_loanTerm').value);
// 2. Validate Inputs
if (isNaN(price) || price < 0) price = 0;
if (isNaN(down) || down < 0) down = 0;
if (isNaN(trade) || trade < 0) trade = 0;
if (isNaN(taxRate) || taxRate < 0) taxRate = 0;
if (isNaN(interestRate) || interestRate < 0) interestRate = 0;
if (isNaN(months) || months <= 0) months = 60; // Default to 60 if invalid
// 3. Calculate Tax Amount
var taxAmount = price * (taxRate / 100);
// 4. Calculate Amount to Finance (Principal)
// Formula: Price + Tax – Down Payment – Trade In
var principal = price + taxAmount – down – trade;
// Handle edge case where down payment covers the car
if (principal <= 0) {
document.getElementById('cl_resultSection').style.display = 'block';
document.getElementById('cl_monthlyPayment').innerHTML = "$0.00";
document.getElementById('cl_loanAmount').innerHTML = "$0.00";
document.getElementById('cl_totalInterest').innerHTML = "$0.00";
document.getElementById('cl_totalCost').innerHTML = "$" + (price + taxAmount).toLocaleString('en-US', {minimumFractionDigits: 2, maximumFractionDigits: 2});
document.getElementById('cl_payoffDate').innerHTML = "Paid in full";
return;
}
// 5. Calculate Monthly Payment
// Formula: P * (r(1+r)^n) / ((1+r)^n – 1)
// where r = monthly interest rate, n = total months
var monthlyRate = (interestRate / 100) / 12;
var monthlyPayment = 0;
if (monthlyRate === 0) {
// Zero interest case
monthlyPayment = principal / months;
} else {
monthlyPayment = principal * (monthlyRate * Math.pow(1 + monthlyRate, months)) / (Math.pow(1 + monthlyRate, months) – 1);
}
// 6. Calculate Totals
var totalPayments = monthlyPayment * months;
var totalInterest = totalPayments – principal;
var totalCost = price + taxAmount + totalInterest;
// 7. Calculate Payoff Date
var today = new Date();
today.setMonth(today.getMonth() + months);
var options = { month: 'long', year: 'numeric' };
var payoffDateString = today.toLocaleDateString('en-US', options);
// 8. Display Results
document.getElementById('cl_resultSection').style.display = 'block';
document.getElementById('cl_monthlyPayment').innerHTML = "$" + monthlyPayment.toLocaleString('en-US', {minimumFractionDigits: 2, maximumFractionDigits: 2});
document.getElementById('cl_loanAmount').innerHTML = "$" + principal.toLocaleString('en-US', {minimumFractionDigits: 2, maximumFractionDigits: 2});
document.getElementById('cl_totalInterest').innerHTML = "$" + totalInterest.toLocaleString('en-US', {minimumFractionDigits: 2, maximumFractionDigits: 2});
document.getElementById('cl_totalCost').innerHTML = "$" + totalCost.toLocaleString('en-US', {minimumFractionDigits: 2, maximumFractionDigits: 2});
document.getElementById('cl_payoffDate').innerHTML = payoffDateString;
}
Understanding Your Car Loan Options
Financing a new or used vehicle involves more than just looking at the sticker price. Our Car Loan Payment Calculator helps you estimate your monthly payments, total interest costs, and the final price of the vehicle once the loan is fully paid off. By adjusting the down payment, interest rate, and loan term, you can find a financing plan that fits your monthly budget.
How Auto Loan Interest Works
The interest rate, often expressed as an Annual Percentage Rate (APR), significantly impacts the total cost of your car. A lower credit score may result in a higher APR, increasing your monthly payment and the total interest paid over the life of the loan. Even a 1% difference in APR can save or cost you hundreds of dollars over a 60-month term.
The Impact of Down Payments and Trade-Ins
Putting money down or trading in an old vehicle reduces the Principal Amount—the money you need to borrow. A larger down payment has two main benefits:
Lower Monthly Payments: Borrowing less means you pay less each month.
Less Interest Paid: Interest is calculated based on the principal balance. Starting with a lower balance reduces the total interest accrual.
Choosing the Right Loan Term
Auto loans typically range from 36 to 84 months. While a longer term (e.g., 72 or 84 months) lowers your monthly bill, it often results in higher total interest costs. Conversely, a shorter term (e.g., 36 or 48 months) increases the monthly payment but allows you to own the car outright sooner and pay less in interest.
What is the 20/4/10 Rule?
Financial experts often recommend the 20/4/10 rule for buying a car to ensure affordability:
20% Down Payment: Aim to put at least 20% down to avoid "gap" insurance issues and lower your payments.
4-Year Term: Try to finance for no more than 4 years (48 months) to prevent the car's depreciation from outpacing your equity.
10% of Income: Your total monthly transportation costs (payment, insurance, gas) should not exceed 10% of your gross monthly income.
Don't Forget Sales Tax
State and local sales taxes can add a significant amount to the "out-the-door" price of a vehicle. This calculator allows you to input your local sales tax rate to see how it affects your loan amount if you choose to roll these taxes into your financing rather than paying them upfront.