Financing a vehicle is one of the most significant financial commitments most people make. Using an Auto Loan Calculator helps you look beyond the sticker price to understand the true cost of ownership. The monthly payment is determined by the vehicle price, your down payment, the trade-in value of your old car, the interest rate (APR), and the loan term.
Key Factors That Affect Your Car Payment
Interest Rate (APR): Your credit score significantly impacts the Annual Percentage Rate. A lower APR means less money goes toward interest and more toward the principal balance.
Loan Term: Stretching a loan to 72 or 84 months lowers your monthly bill but drastically increases the total interest paid over the life of the loan. A standard term is usually 60 months.
Down Payment & Trade-in: Putting money down or trading in a vehicle reduces the principal loan amount immediately. This lowers both your monthly payment and total interest costs.
Sales Tax: Don't forget state and local taxes. These are often rolled into the loan, meaning you pay interest on the tax as well.
How to Use This Calculator
Simply enter the negotiated price of the car, the sales tax rate for your area, and your financing details. If you have a vehicle to exchange, enter its estimated value in the "Trade-in Value" field. Adjust the loan term to see how extending the years changes your monthly budget versus the total cost.
function calculateAutoLoan() {
// 1. Get Input Values
var price = parseFloat(document.getElementById('vehiclePrice').value);
var taxRate = parseFloat(document.getElementById('salesTax').value);
var down = parseFloat(document.getElementById('downPayment').value);
var tradeIn = parseFloat(document.getElementById('tradeInValue').value);
var interestRate = parseFloat(document.getElementById('interestRate').value);
var term = parseInt(document.getElementById('loanTerm').value);
// 2. Handle Inputs (Default to 0 if empty/NaN)
if (isNaN(price)) price = 0;
if (isNaN(taxRate)) taxRate = 0;
if (isNaN(down)) down = 0;
if (isNaN(tradeIn)) tradeIn = 0;
if (isNaN(interestRate)) interestRate = 0;
if (isNaN(term)) term = 60;
// 3. Logic & Calculation
// Calculate Tax Amount
var taxAmount = price * (taxRate / 100);
// Calculate Net Loan Amount
// Loan Amount = (Price + Tax) – Down Payment – Trade In
var loanAmount = (price + taxAmount) – down – tradeIn;
var monthlyPayment = 0;
var totalInterest = 0;
var totalCost = 0;
if (loanAmount > 0) {
if (interestRate === 0) {
// Simple division if 0% APR
monthlyPayment = loanAmount / term;
totalInterest = 0;
} else {
// Amortization Formula: M = P [ i(1 + i)^n ] / [ (1 + i)^n – 1 ]
var monthlyRate = (interestRate / 100) / 12;
var mathPower = Math.pow(1 + monthlyRate, term);
monthlyPayment = loanAmount * ( (monthlyRate * mathPower) / (mathPower – 1) );
totalInterest = (monthlyPayment * term) – loanAmount;
}
totalCost = price + taxAmount + totalInterest;
} else {
// If down payment + trade in covers the cost
loanAmount = 0;
monthlyPayment = 0;
totalInterest = 0;
totalCost = price + taxAmount;
}
// 4. Formatting Results
var formatter = new Intl.NumberFormat('en-US', {
style: 'currency',
currency: 'USD',
minimumFractionDigits: 2
});
// 5. Update DOM
document.getElementById('monthlyPaymentResult').innerText = formatter.format(monthlyPayment);
document.getElementById('loanAmountResult').innerText = formatter.format(loanAmount);
document.getElementById('totalTaxResult').innerText = formatter.format(taxAmount);
document.getElementById('totalInterestResult').innerText = formatter.format(totalInterest);
document.getElementById('totalCostResult').innerText = formatter.format(totalCost);
// Show results area
document.getElementById('results-area').style.display = 'block';
}