Car Affordability Calculator (20/4/10 Rule)

The 20/4/10 rule unpacked

Each number blocks a different mistake. The 20% down payment keeps you from driving off the lot underwater, owing more than the car is worth the moment it becomes used. The 4-year cap on the loan keeps the interest bill honest and gets you back to payment-free ownership quickly. And the 10% ceiling on total car costs protects the rest of your budget: cars are the classic category where lifestyle creep hides, because the upgrade always arrives one payment at a time.

Depreciation: the steepest cost nobody budgets

A new car typically loses around 20% of its value in the first year and close to half by year five. On a $40,000 purchase that's roughly $8,000 gone before the first oil change, more than most people's annual savings. No bill ever arrives for it, which is exactly why it goes unbudgeted; it simply shows up years later as a small trade-in number. Buying lightly used, putting real money down, and saving toward the purchase in advance (a savings goal with a date on it) are the three moves that blunt it.

Why longer loans feel cheaper and cost more

Stretching a loan from 48 to 72 months can cut the payment by a quarter while raising the total interest by half or more, and it keeps you underwater on the car for most of the term. Dealers quote monthly payments instead of prices for precisely this reason. Run the same car through the auto loan calculator at different terms and the trade becomes visible in dollars. The deeper discipline is knowing what a car payment displaces in your month, which is easier when your whole budget lives in one place; that's the job Stoia does once your accounts are connected.