*This calculation assumes your monthly payment limit covers the loan principal and interest only. Insurance and maintenance should be budgeted separately.
function calculateAffordability() {
// 1. Get input values
var income = parseFloat(document.getElementById('cac_income').value);
var downPayment = parseFloat(document.getElementById('cac_down').value) || 0;
var tradeIn = parseFloat(document.getElementById('cac_trade').value) || 0;
var termMonths = parseInt(document.getElementById('cac_term').value);
var interestRate = parseFloat(document.getElementById('cac_rate').value);
var taxRate = parseFloat(document.getElementById('cac_tax').value) || 0;
var budgetPercent = parseFloat(document.getElementById('cac_budget_percent').value);
// 2. Validation
if (isNaN(income) || income <= 0) {
alert("Please enter a valid monthly net income.");
return;
}
if (isNaN(interestRate) || interestRate < 0) {
alert("Please enter a valid interest rate.");
return;
}
// 3. Logic: Calculate Max Monthly Payment
// Max Payment = Income * Budget Percent
var maxMonthlyPayment = income * budgetPercent;
// 4. Logic: Calculate Loan Amount (Present Value of an Annuity)
// PV = Pmt * (1 – (1 + r)^-n) / r
var monthlyRate = (interestRate / 100) / 12;
var maxLoanAmount = 0;
if (interestRate === 0) {
maxLoanAmount = maxMonthlyPayment * termMonths;
} else {
maxLoanAmount = maxMonthlyPayment * (1 – Math.pow(1 + monthlyRate, -termMonths)) / monthlyRate;
}
// 5. Logic: Calculate Total Purchasing Power
// Purchasing Power = Loan Amount + Down Payment + Trade In
var totalCashAvailable = maxLoanAmount + downPayment + tradeIn;
// 6. Logic: Back out Sales Tax
// Price * (1 + TaxRate) = TotalCashAvailable
// Price = TotalCashAvailable / (1 + TaxRate)
var maxCarPrice = totalCashAvailable / (1 + (taxRate / 100));
// 7. Calculate Totals for display
var totalInterest = (maxMonthlyPayment * termMonths) – maxLoanAmount;
if (totalInterest < 0) totalInterest = 0; // Edge case for 0 interest
// 8. Formatting currency
var formatter = new Intl.NumberFormat('en-US', {
style: 'currency',
currency: 'USD',
maximumFractionDigits: 0
});
// 9. Display Results
document.getElementById('cac_max_price').innerHTML = formatter.format(maxCarPrice);
document.getElementById('cac_monthly_payment').innerHTML = formatter.format(maxMonthlyPayment) + "/mo";
document.getElementById('cac_loan_amount').innerHTML = formatter.format(maxLoanAmount);
document.getElementById('cac_total_interest').innerHTML = formatter.format(totalInterest);
// Show result box
var resultBox = document.getElementById('cac_result');
resultBox.classList.add('active');
}
Understanding Your Car Buying Budget
Determining how much car you can afford is a critical step in maintaining financial health. Buying a vehicle is often the second largest purchase an individual makes, right after buying a home. Our Car Affordability Calculator helps you reverse-engineer the purchase price based on your monthly income and financial comfort level, ensuring you don't overextend yourself.
The Golden Rules of Car Affordability
Financial experts often cite specific rules of thumb to keep your car budget in check. The most common metric is the 10% to 15% rule. This suggests that your total monthly car payment (including principal and interest) should not exceed 10% to 15% of your monthly net (take-home) income.
Conservative (10%): Best for those with high rent/mortgage costs or aggressive savings goals.
Standard (15%): The typical balance for most budgets.
Aggressive (20%): Only recommended if you have very few other debts and low living expenses.
How the Calculator Works
Unlike a standard loan calculator that tells you the payment for a specific price, this tool works backward:
Input Income: We start with your net monthly income to establish a payment ceiling.
Determine Loan Capacity: Using the interest rate and loan term, we calculate how much money that monthly payment can borrow.
Add Equity: We add your down payment and trade-in value to the loan amount.
Adjust for Taxes: Finally, we divide by the tax rate to find the "Sticker Price" you should look for on the lot.
The 20/4/10 Rule Explained
Another popular framework for car buying is the 20/4/10 rule. It suggests:
20% Down Payment: To avoid being "underwater" on the loan immediately.
4 Year Term: Limit financing to 48 months to minimize interest paid.
10% of Income: Keep total transportation costs (payment + insurance + gas) under 10% of gross income.
While strict, adhering to the 20/4/10 rule is the safest way to build wealth while driving a reliable vehicle. You can simulate this in our calculator by selecting "48 Months" and a "10%" budget cap.
Hidden Costs to Consider
Remember that the "Sticker Price" isn't the only cost. When budgeting, ensure you leave room for:
Insurance Premiums: Sportier and more expensive cars cost significantly more to insure.
Fuel or Electricity: Calculate your expected mileage and local fuel costs.
Maintenance & Repairs: Set aside $50-$100 monthly for tires, oil changes, and unexpected repairs.