How the car affordability calculator works
Instead of asking "what will this car cost per month?", this tool answers the opposite question. It takes your monthly budget, the APR and the loan term and computes the largest loan that payment can support (the present value of the payments). It then adds your down payment and trade-in, and backs out sales tax and fees, to find the maximum vehicle price.
The formula
Maximum loan P = M × (1 − (1 + r)−n) / r. Because tax is owed on the price after the trade-in credit, the price that fits is (loan − fees + down payment + trade-in × (1 + tax rate)) ÷ (1 + tax rate).
Example
A $500 monthly budget at 6% APR over 60 months supports a loan of about $25,863. With $4,000 down, a $2,000 trade-in, $400 of fees and 7% sales tax, you could shop for a car priced up to roughly $29,500 — and the auto loan calculator confirms that price produces a $500 payment.
The 20/4/10 rule
A popular guideline says to put at least 20% down, finance for no more than four years, and keep total car costs (payment plus insurance and fuel) under 10% of gross income. If you enter your monthly income, the calculator shows what share your payment alone would take. It is a guideline, not a law, but it keeps transportation from crowding out savings.
Frequently asked questions
How much of my income should go to a car payment?
Many planners suggest keeping the payment around 10% of gross monthly income and all car costs below about 15–20%.
Does a longer loan let me afford more car?
Yes, but you pay more interest and depreciation can leave you owing more than the car is worth. A longer term mainly makes an expensive car feel cheaper.
Should I include insurance in my budget?
Budget for it separately. Insurance can vary by hundreds of dollars a month depending on the vehicle, so get a quote before you buy.
What if my trade-in is worth more than the car?
Then no sales tax is due and the calculator simply adds the full trade-in value to your buying power.
Results are estimates for educational purposes only and are not financial, tax, legal or medical advice. See our disclaimer. Last reviewed 2026-10-22.