How the Car Affordability Calculator works
This tool flips the usual car-loan question around. Instead of starting from a sticker price and asking what the payment would be, it starts from what you can comfortably pay each month and works backward to the loan — and vehicle price — that payment can support.
The formula
First, the calculator caps your monthly car payment at a percentage of your take-home pay you choose: PMT = Monthly income × payment cap%. It then applies the standard loan-payment formula in reverse to find the largest loan that payment can carry over your chosen term:
Loan = PMT × (1 − (1 + i)−n) / i
where i is the monthly interest rate (APR ÷ 12) and n is the number of monthly payments (loan term in months). If the APR is 0%, this reduces to Loan = PMT × n — the payment simply multiplied by the number of months. Your down payment and trade-in value are then added on top of the loan amount, dollar-for-dollar, to give the maximum vehicle price: Max price = Loan + down payment + trade-in.
Worked example
Take a $5,000 monthly take-home pay with a 15% payment cap, a $3,000 down payment, no trade-in, a 60-month loan, and a 6.5% APR. The affordable payment is $5,000 × 15% = $750/month. The monthly rate is 6.5% ÷ 12 ≈ 0.005417, and with n = 60 payments the formula supports a loan of roughly $38,300. Adding the $3,000 down payment gives a maximum vehicle price of about $41,300, with total interest over the loan term of around $6,700 (60 payments of $750 minus the $38,300 loan).
What moves the affordable price most
- Payment cap: raising the percentage of income allotted to the car payment increases the affordable payment directly, which raises both the loan amount and the maximum price by the same proportion.
- Down payment and trade-in: every dollar added here raises the maximum vehicle price by exactly one dollar, without changing the monthly payment or loan term at all — this is the most direct way to afford a pricier vehicle on the same budget.
- Loan term: a longer term lets a fixed monthly payment support a larger loan, but it also means paying interest for longer, so total interest paid rises even though the monthly payment itself stays flat.
- Interest rate: a higher APR shrinks the loan amount a given payment can support, because more of each payment goes to interest instead of principal.
What this calculator does not include
This is a loan-affordability estimate, not a full cost-of-ownership picture. It does not add sales tax, title and registration fees, dealer add-ons, or ongoing costs like insurance, fuel, and maintenance — all of which affect what you can actually afford to drive. Use the result as a starting cap on the loan and price, then budget separately for those additional costs before committing to a purchase.