Car affordability calculator (20/4/10 rule)

How much car can you actually afford?

Cars are the second-largest household expense for most Americans, and the single most common way people derail their financial plans. The combination of long loan terms, dealer financing add-ons, and the emotional pull of a new vehicle makes it easy to commit to payments that feel manageable in the showroom but create chronic cash flow stress for years afterward. The 20/4/10 rule gives you a clear, non-negotiable framework to evaluate any car purchase.

The 20/4/10 rule

20% down — ensures you're not immediately underwater on a depreciating asset. New cars lose 15–20% of value in the first year. A 20% down payment roughly matches that first-year depreciation, keeping your loan balance close to the car's value.

4-year maximum loan term — the average new car depreciates approximately 50% in 5 years. A 4-year loan keeps your payoff schedule ahead of depreciation, so you're never deeply underwater. 7-year loans are now common but financially dangerous — you're still paying for a 6-year-old car while the manufacturer is marketing you the next model.

10% of gross income — total monthly car costs (loan payment + insurance) capped at 10% of gross monthly income. Some sources extend this to 15%, which is more realistic in high-cost cities, but 10% is the conservative standard that leaves room for other financial goals.

Monthly payment formula

The standard loan payment formula calculates the fixed monthly installment on a fully amortizing loan:

P = L × r(1+r)^n / [(1+r)^n - 1]

Where L is the loan amount (price minus down payment), r is the monthly interest rate (annual rate ÷ 12), and n is the number of monthly payments. On a $25,000 loan at 7% APR for 48 months, the payment is $598/month.

Five-year total cost of ownership

The sticker price is just the beginning. TCO over 5 years includes loan interest, insurance, fuel, maintenance, and depreciation. A $35,000 car might cost $55,000–$65,000 to own for 5 years when all costs are tallied. Reliable, moderately-priced used vehicles typically have significantly lower TCO because you're not absorbing the steepest portion of the depreciation curve.

New vs. used considerations

Certified pre-owned vehicles that are 2–3 years old offer the best value proposition: someone else absorbed the worst of the first-year depreciation, the car likely still has some factory warranty, and you can often finance at rates close to new-car rates. The "sweet spot" is often a 2–4 year old vehicle with under 40,000 miles — significantly below new-car price with many years of reliable service remaining.