Calculator for Buying a Car
function calculateCarCost() {
var carPrice = parseFloat(document.getElementById('carPrice').value);
var tradeInValue = parseFloat(document.getElementById('tradeInValue').value);
var upfrontPayment = parseFloat(document.getElementById('upfrontPayment').value);
var salesTaxRate = parseFloat(document.getElementById('salesTaxRate').value);
var regFees = parseFloat(document.getElementById('regFees').value);
var loanTermMonths = parseFloat(document.getElementById('loanTermMonths').value);
var annualFinancingRate = parseFloat(document.getElementById('annualFinancingRate').value);
var monthlyInsurance = parseFloat(document.getElementById('monthlyInsurance').value);
var monthlyFuel = parseFloat(document.getElementById('monthlyFuel').value);
var monthlyMaintenance = parseFloat(document.getElementById('monthlyMaintenance').value);
// Validate inputs
if (isNaN(carPrice) || carPrice < 0) carPrice = 0;
if (isNaN(tradeInValue) || tradeInValue < 0) tradeInValue = 0;
if (isNaN(upfrontPayment) || upfrontPayment < 0) upfrontPayment = 0;
if (isNaN(salesTaxRate) || salesTaxRate < 0) salesTaxRate = 0;
if (isNaN(regFees) || regFees < 0) regFees = 0;
if (isNaN(loanTermMonths) || loanTermMonths <= 0) loanTermMonths = 1; // Prevent division by zero or invalid term
if (isNaN(annualFinancingRate) || annualFinancingRate < 0) annualFinancingRate = 0;
if (isNaN(monthlyInsurance) || monthlyInsurance < 0) monthlyInsurance = 0;
if (isNaN(monthlyFuel) || monthlyFuel < 0) monthlyFuel = 0;
if (isNaN(monthlyMaintenance) || monthlyMaintenance < 0) monthlyMaintenance = 0;
// Step 1: Calculate Net Car Price
var netCarPrice = carPrice – tradeInValue;
if (netCarPrice < 0) netCarPrice = 0; // Cannot have negative car price after trade-in
// Step 2: Calculate Sales Tax
var totalSalesTax = netCarPrice * (salesTaxRate / 100);
// Step 3: Calculate Total Amount to Finance
var totalCostBeforeFinancing = netCarPrice + totalSalesTax + regFees;
var amountToFinance = totalCostBeforeFinancing – upfrontPayment;
if (amountToFinance 0 && loanTermMonths > 0) {
if (monthlyInterestRate > 0) {
var factor = Math.pow(1 + monthlyInterestRate, loanTermMonths);
monthlyLoanPayment = amountToFinance * (monthlyInterestRate * factor) / (factor – 1);
} else {
monthlyLoanPayment = amountToFinance / loanTermMonths; // No interest
}
}
// Step 5: Calculate Total Interest Paid
var totalInterestPaid = (monthlyLoanPayment * loanTermMonths) – amountToFinance;
if (totalInterestPaid < 0) totalInterestPaid = 0; // Should not be negative
// Step 6: Calculate Total Monthly Car Expenses
var totalMonthlyExpenses = monthlyLoanPayment + monthlyInsurance + monthlyFuel + monthlyMaintenance;
// Step 7: Calculate Total Cost of Ownership (over the loan term)
var totalOwnershipCost = upfrontPayment + (monthlyLoanPayment * loanTermMonths) + (monthlyInsurance * loanTermMonths) + (monthlyFuel * loanTermMonths) + (monthlyMaintenance * loanTermMonths);
// Display Results
document.getElementById('resultNetCarPrice').innerHTML = 'Net Car Price (after trade-in): $' + netCarPrice.toFixed(2);
document.getElementById('resultTotalSalesTax').innerHTML = 'Total Sales Tax: $' + totalSalesTax.toFixed(2);
document.getElementById('resultAmountToFinance').innerHTML = 'Total Amount to Finance: $' + amountToFinance.toFixed(2);
document.getElementById('resultMonthlyLoanPayment').innerHTML = 'Estimated Monthly Loan Payment: $' + monthlyLoanPayment.toFixed(2);
document.getElementById('resultTotalInterestPaid').innerHTML = 'Total Interest Paid (over term): $' + totalInterestPaid.toFixed(2);
document.getElementById('resultTotalMonthlyExpenses').innerHTML = 'Total Estimated Monthly Car Expenses: $' + totalMonthlyExpenses.toFixed(2);
document.getElementById('resultTotalOwnershipCost').innerHTML = 'Total Estimated Cost of Ownership (over ' + loanTermMonths + ' months): $' + totalOwnershipCost.toFixed(2);
}
// Run calculation on page load with default values
window.onload = calculateCarCost;