How Much Car Can You Afford?

The 28/36 rule is the household-debt framework lenders have used since the early 1980s — originally codified by the FHA for mortgage underwriting, then adopted by auto, credit card, and personal-loan lenders. It says no more than 28% of your gross monthly income should go to housing (mortgage, rent, taxes, insurance) and no more than 36% should go to ALL household debt combined (housing + auto + student loans + credit cards + personal loans).

For an auto-loan applicant earning $75,000/year ($6,250/mo gross), the 36% ceiling caps total household debt at $2,250/mo. If your rent or mortgage already runs $1,500/mo and student loans add $400/mo, you have $350/mo of headroom for the car payment — and that's before insurance, fuel, or maintenance. The calculator uses your annual income and a conservative 25% transportation-budget slice of gross to model how the 36% ceiling translates into a buyable car price.

Many lenders will technically approve loans that push household debt to 40–45% of gross — but at those ratios you're one missed paycheck or insurance claim from default. The 36% ceiling is a buyer-protection guardrail, not a regulatory one.

The 10% rule is the editorial complement to 28/36. It says your total transportation cost — loan payment, insurance, fuel, maintenance, parking, and tolls — should not exceed 10% of your gross monthly income. For the $75,000-earner above, that's $625/mo for everything related to owning the car, not just the loan.

This is the rule most buyers violate, because dealer financing conversations focus exclusively on the loan payment. A $400/mo payment looks affordable in isolation — but layered with $150/mo insurance, $200/mo fuel, $50/mo maintenance reserve, and $50/mo parking, the all-in cost crests $850/mo. That's 13.6% of the budget on a $75k income — outside the 10% guardrail and a major contributing factor to the 30%+ delinquency rate Experian reports on subprime auto loans.

The calculator's "all-in monthly budget" tile shows the 10% cap explicitly. The split assumes ~35% of the transportation budget goes to insurance, fuel, and maintenance — leaving the remaining 65% for the loan payment.

Debt-to-income is the formal version of the 28/36 rule that auto lenders run on every application. They compute DTI as total monthly debt obligations divided by gross monthly income. Most national lenders quote tier-1 APRs only at DTI under 35%, prime APRs at DTI 35–43%, and require specialized underwriting above 43%. Above 50% DTI, the loan is functionally a deep-subprime product and the APR penalty often exceeds 8 points.

DTI is also why the calculator asks for your gross income — not net. Lenders price against gross because that's what they can verify on a paystub or tax return. Net income varies with state tax, 401(k) contributions, and HSA elections, none of which lenders adjudicate. If your DTI is borderline, every additional $100 of monthly debt (a new credit card, a personal loan, a co-signed lease) can bump you into the next APR tier.

You can lower DTI before applying by paying down credit cards (immediate effect on the next statement), refinancing existing high-rate debt, or removing a co-signed account you're no longer responsible for. See our DTI glossary entry for the full calculation and lender-by-lender thresholds.

The calculator above has four inputs. Each maps to a specific lever in the affordability math + a specific decision a lender will make about your application.

Annual gross incomeHighest weight (35%)

Pre-tax annual earnings — the same figure you'd put on a W-2 or a loan application. Lenders care about the gross figure because that's what they compare against the 28/36 rule. Include W-2 wages, 1099 self-employment, recurring bonuses, and verifiable rental income — but NOT one-time windfalls or anything that requires a tax return more than 24 months old.

Down paymentCuts loan principal directly

Cash you bring to the deal (or trade-in equity applied to the new loan). Every dollar down reduces principal 1:1 and lowers the loan-to-value ratio that lenders use to price APR tiers. The industry default is 10–20% — but the math here is real even at 5%: a $2,500 down payment on a $30,000 car shaves roughly $50/mo at typical APRs.

Loan APRSingle biggest cost lever

The Annual Percentage Rate quoted by the lender. APR is the all-in number — it includes interest and any origination fees, expressed as an annualized rate. APRs vary 10+ points between credit tiers (5% top-tier, 17%+ subprime) and 0.5–1.5 points between lender types (credit unions cheaper than banks, banks cheaper than buy-here-pay-here).

Loan term (months)Trade-off lever — payment vs. total interest

How long you'll repay the loan. 48–60 months is the conservative band; 72–84 lowers the payment but increases total interest by 30–60% AND increases the time you're underwater on the loan. Calculator defaults to 60 months because that's the editorial-best balance for the average buyer.

1. 01

   Budgeting only the loan payment — not the all-in cost

   The single most common mistake. A car that's 'affordable' at $400/mo loan payment can easily run $700–900/mo all-in once insurance, fuel, maintenance, parking, and tolls are included. The 10% rule corrects this by capping ALL transportation cost at 10% of gross income — not just the payment.

2. 02

   Stretching the term to make a more expensive car 'fit'

   A $35,000 car at 72 months looks like a $550/mo payment. The same car at 60 months runs $650/mo. The 72-month option is rarely the right choice — you pay ~$2,500 more in total interest, stay underwater 18+ months longer, and face higher insurance gap-coverage costs.

3. 03

   Ignoring the trade-in's negative equity

   If you owe $18,000 on your trade and the dealer values it at $14,000, you're $4,000 underwater. Rolling that into the new loan inflates principal by $4k and pushes your loan-to-value over 100% — which usually triggers a 1–2 APR-point penalty.

4. 04

   Treating dealer financing as 'just the rate they offered'

   Dealer financing typically marks up the lender's buy-rate by 1–3 APR points. A pre-approval from a credit union or direct lender eliminates this markup. The single fastest way to save $1,000+ on the loan is to walk in pre-approved.