Car Affordability Calculator

How much car can you actually afford?

Ask a dealer and the answer is a monthly payment you can be talked into. Ask a lender and the answer is the largest loan your credit file will support. Neither party is asked to care about your insurance premium, your commute, or the transmission that fails in year five. That is why buyers use a rule of their own, decided at the kitchen table rather than in the finance office, and why this calculator shows three of them side by side instead of one.

The three rules, and what each one assumes

The 20/4/10 rule is the conservative standard: put 20% down, finance for no more than four years, and keep total car costs — payment, insurance, fuel, maintenance — under 10% of gross income. The 35% rule is a blunter shortcut used widely in the US: your car budget is 35% of annual gross income, full stop. The 10% payment cap keeps the income limit but stretches the loan to five years, which buys more car for the same monthly outlay and slows down how quickly you build equity.

Worked example: 70,000 income, 5,000 down, 7% APR

Monthly gross is 5,833. Ten percent of that is a 583 monthly car budget. Financed at 7% over 48 months, 583 a month supports a loan of about 24,360, so with the 5,000 down the 20/4/10 budget lands at roughly 29,360. The 35% rule ignores the loan entirely: 0.35 x 70,000 = 24,500, which with 5,000 down means borrowing 19,500 and paying about 386 a month over five years. The 10% cap over 60 months supports a 29,459 loan, for a 34,459 car at the same 583 payment.

Same income, same deposit, a 10,000 spread in what you are "allowed" to spend. The calculator also flags that 20/4/10 wants 5,872 down on a 29,360 car — slightly more than the 5,000 in this example — because the deposit requirement scales with the price the rule just produced.

The cost iceberg under the payment

The payment is the part above the waterline. In the US, average annual insurance runs well over 2,000 for a financed car carrying full coverage, fuel adds 1,500-2,500 depending on mileage, and maintenance plus tyres averages 900-1,200 once a car is out of warranty. Registration, inspection and property tax vary by state but rarely round to zero. Together these commonly add 40-60% on top of the loan payment: budget 583 for the loan and the real monthly drain is closer to 850.

That single fact is the whole argument for 20/4/10. Its 10% limit is deliberately applied to all car costs, not just the payment, which is why the price it produces looks stingy next to what a lender will approve. If your insurance is cheap and you drive 5,000 miles a year, you have room to lean toward the 35% figure. If you are 24 with a sports coupe quote, you do not.

Buy in the right order: price, then financing, then trade-in

Dealers make margin on all three, and mixing them lets a concession in one reappear in another. Settle the out-the-door price of the car first, in writing. Only then discuss financing, ideally against a pre-approval you already hold from a credit union or bank — a dealer can beat it, and letting them try is free. Bring up the trade-in last, once the other two numbers are locked, so its value cannot be quietly used to absorb a price you thought you had negotiated down.

The 84-month trap

Long loans have gone mainstream: 72- and 84-month terms are routine, and they always look affordable because the monthly figure is what gets quoted. Take the same 583 payment at 7% over 84 months and it supports a 43,650 car — 14,000 more than the four-year rule allows. The cost is 10,350 in interest instead of 3,640, and roughly four years spent owing more than the car is worth. Insurers total a car at market value, not loan balance, so an accident in year three of an 84-month loan leaves you paying for a vehicle you no longer have unless you bought gap coverage.

Limitations of any affordability rule

These rules use gross income because it is the number everyone knows, but you spend net. In a high-tax jurisdiction the same 10% of gross can be 14% of take-home, which is why the conservative rule earns its keep. They also assume car costs sit alongside a normal housing burden; if rent already eats 45% of your income, cut the result rather than trusting it. And they say nothing about whether the specific car is reliable — a 30,000 budget spent on a model with a known transmission fault is worse than 20,000 spent well. Use the number as a ceiling, not a target, and check the actual insurance quote for the exact car before you sign anything.