Car dealers want to sell you a monthly payment. You should be buying a total cost. The 20/4/10 rule is the simplest guardrail that keeps a car from eating your budget.

The rule

  • 20% down — at least one-fifth of the price upfront
  • 4-year loan max — never stretch to 6, 7 or 8 years to "afford" the payment
  • 10% of gross income — total transportation costs under a tenth of your pay

That last 10% includes everything: payment, insurance, gas, maintenance. Not just the loan.

What it means at real salaries

10% of gross monthly income is your all-in ceiling:

| Salary | Max all-in car cost/month | |---|---| | $60,000 | $500 | | $70,000 | $583 | | $80,000 | $667 | | $100,000 | $833 |

On a $70,000 salary, your ceiling is $583/month for the payment plus insurance plus gas. Now watch what happens with a typical purchase: a $35,000 car with 20% down ($7,000) leaves a $28,000 loan. At 7% over 4 years, that loan alone is $670/month — already over budget before insurance and fuel. The rule is telling you the truth the dealer's payment quote hides: on $70k, that car is too much car.

Why the 4-year cap matters

Stretching to 72 or 84 months lowers the payment but keeps you underwater for years — owing more than the car is worth. Cars depreciate ~20% in year one. A 7-year loan on a depreciating asset is how people end up rolling $8,000 of negative equity into the next loan, forever.

The costs buyers forget

The payment is roughly two-thirds of ownership. Budget the rest:

  • Insurance: $150–$250/month for full coverage, more if you are young or the car is expensive
  • Fuel: 12,000 miles/year at 25 MPG and $3.50/gallon ≈ $140/month
  • Maintenance: ~$75/month averaged over the car's life

A "$500/month" car is really a $750–$850/month commitment. The 10% rule exists precisely because of this gap.

Run your numbers

Use our auto loan calculator to compare terms and see total interest, or the loan payment calculator for any loan type. And check what your paycheck actually leaves you each month with the salary paycheck calculator before you sign.

Estimates for planning only, not financial advice. Insurance and fuel costs vary widely by location and vehicle.