function calculateAutoLoan() {
// 1. Get DOM elements
var priceInput = document.getElementById("vehiclePrice");
var downInput = document.getElementById("downPayment");
var tradeInput = document.getElementById("tradeInValue");
var taxInput = document.getElementById("salesTax");
var rateInput = document.getElementById("interestRate");
var termInput = document.getElementById("loanTerm");
var resultArea = document.getElementById("clc-results-area");
var errorMsg = document.getElementById("clc-error-msg");
var paymentDisplay = document.getElementById("monthlyPaymentResult");
var financedDisplay = document.getElementById("financedAmountResult");
var taxDisplay = document.getElementById("totalTaxResult");
var interestDisplay = document.getElementById("totalInterestResult");
var costDisplay = document.getElementById("totalCostResult");
// 2. Parse values
var price = parseFloat(priceInput.value) || 0;
var down = parseFloat(downInput.value) || 0;
var trade = parseFloat(tradeInput.value) || 0;
var taxRate = parseFloat(taxInput.value) || 0;
var interestRate = parseFloat(rateInput.value) || 0;
var months = parseFloat(termInput.value) || 0;
// 3. Validation
if (price <= 0 || months <= 0) {
errorMsg.style.display = "block";
errorMsg.innerText = "Please enter a valid Vehicle Price and Loan Term.";
resultArea.style.display = "none";
return;
}
errorMsg.style.display = "none";
// 4. Calculations
// Sales Tax is usually calculated on (Price – TradeIn) in many regions,
// though some tax the full price. We will assume (Price – TradeIn) for this standard calculator.
// If Taxable Amount is negative, tax is 0.
var taxableAmount = Math.max(0, price – trade);
var totalTax = taxableAmount * (taxRate / 100);
// Amount Financed = (Price + Tax) – Down Payment – Trade In
var amountFinanced = (price + totalTax) – down – trade;
if (amountFinanced <= 0) {
// No loan needed
resultArea.style.display = "block";
paymentDisplay.innerText = "$0.00";
financedDisplay.innerText = "$0.00";
taxDisplay.innerText = "$" + totalTax.toFixed(2).replace(/\d(?=(\d{3})+\.)/g, '$&,');
interestDisplay.innerText = "$0.00";
costDisplay.innerText = "$" + (price + totalTax).toFixed(2).replace(/\d(?=(\d{3})+\.)/g, '$&,');
return;
}
var monthlyPayment = 0;
var totalInterest = 0;
var totalLoanCost = 0;
if (interestRate === 0) {
monthlyPayment = amountFinanced / months;
totalInterest = 0;
} else {
var monthlyRate = (interestRate / 100) / 12;
// Formula: M = P [ i(1 + i)^n ] / [ (1 + i)^n – 1 ]
var x = Math.pow(1 + monthlyRate, months);
monthlyPayment = (amountFinanced * x * monthlyRate) / (x – 1);
totalInterest = (monthlyPayment * months) – amountFinanced;
}
var totalCostOfCar = price + totalTax + totalInterest;
// 5. Output Formatting
function formatCurrency(num) {
return "$" + num.toFixed(2).replace(/\d(?=(\d{3})+\.)/g, '$&,');
}
paymentDisplay.innerText = formatCurrency(monthlyPayment);
financedDisplay.innerText = formatCurrency(amountFinanced);
taxDisplay.innerText = formatCurrency(totalTax);
interestDisplay.innerText = formatCurrency(totalInterest);
costDisplay.innerText = formatCurrency(totalCostOfCar);
resultArea.style.display = "block";
}
Understanding Your Car Loan Options
Buying a new or used vehicle is one of the most significant financial decisions many households make. While the sticker price of the car is the most visible number, the details of your financing arrangement can have a massive impact on the total amount you eventually pay. Our Car Loan Calculator helps you look beyond the monthly payment to understand the true cost of borrowing.
How Your Monthly Payment is Calculated
Your auto loan payments are determined by three primary factors: the loan amount (principal), the interest rate (APR), and the loan term (duration). Understanding how these interact is crucial for getting the best deal.
- Principal: This is the cost of the car plus taxes and fees, minus any down payment or trade-in value. The lower your principal, the less interest you pay.
- Interest Rate (APR): This is the cost of borrowing money. Rates are influenced by your credit score and the current economic environment. A difference of just 1-2% can save you hundreds or even thousands of dollars over the life of the loan.
- Loan Term: Common terms range from 36 to 72 months. While a longer term lowers your monthly bill, it significantly increases the total interest paid.
The Impact of Down Payments and Trade-Ins
One of the most effective ways to lower your monthly payment and avoid "being underwater" on your loan (owing more than the car is worth) is to maximize your upfront contribution. This calculator allows you to input both cash down payments and trade-in values to see exactly how they reduce your financed amount.
For example, on a $30,000 vehicle, a $5,000 down payment doesn't just reduce the loan to $25,000—it also saves you the interest that would have accrued on that $5,000 over 4 or 5 years.
Sales Tax Considerations
Many buyers forget to factor in sales tax, which can add significant cost to the transaction. In many jurisdictions, sales tax is calculated on the difference between the new car price and your trade-in value, providing a tax benefit to trading in a vehicle. Our tool estimates this tax to give you a more accurate "out-the-door" price estimate.
Tips for Securing a Better Rate
Before heading to the dealership, check your credit score and consider getting pre-approved by a bank or credit union. Dealership financing can be convenient, but having a pre-approval letter in hand gives you leverage to negotiate a better interest rate.