The 15% Rule for Car Affordability
Your total transportation costs—loan payment, insurance, fuel, and maintenance—should stay at or below 15% of your gross monthly income. On a $5,000/month salary, that's a $750 budget. Most people only look at the loan payment and forget that insurance, gas, and repairs add $200–$400/month on top. This calculator accounts for all four costs so you see the real number, not a fantasy one.
The 20/4/10 Rule: A Stricter Test
Financial planners recommend the 20/4/10 rule: put at least 20% down, finance for no more than 4 years (48 months), and keep total transportation costs under 10% of gross income. On $60K/year income, that's $500/month max. This rule is aggressive but keeps you from being underwater on a depreciating asset. If you can't hit all three numbers, the car is too expensive.
New vs Used: Where the Real Savings Are
A new car loses 20–25% of its value in the first year. A 3-year-old certified pre-owned (CPO) vehicle has already absorbed that hit. On a $35,000 new car, that's $7,000–$8,750 in depreciation you never recover. Used cars carry slightly higher interest rates (7–9% vs 5–7%), but the lower purchase price more than compensates. A $20,000 used car at 8% for 48 months costs $488/month—$197/month less than the same car bought new at $35,000 and 6%.
Total Cost of Ownership: The Number That Matters
The sticker price is a fraction of what you actually pay. Over 5 years, a $30,000 car costs roughly $45,000–$55,000 when you add interest ($3,000–$5,000), insurance ($9,000–$12,000), fuel ($7,200–$10,000), maintenance ($3,600–$5,000), and depreciation ($10,000–$15,000). That's $750–$917/month in real cost. Always calculate the full ownership cost, not just the monthly loan payment.
5 Mistakes That Make Cars Unaffordable
- Stretching to 72–84 month loans. Longer terms lower your payment but pile on interest. A 72-month loan at 7% on $30,000 adds $6,800 in interest vs $3,200 on a 48-month term. You'll also be underwater for most of the loan.
- Ignoring insurance costs before buying. A $40,000 SUV can cost $200–$300/month to insure. Get insurance quotes before you sign the loan paperwork—not after.
- Skipping the down payment. Zero-down financing means instant negative equity. Put at least 10–20% down to stay ahead of depreciation and lower your monthly payment.
- Focusing on monthly payment, not total cost. Dealers love negotiating on payment because it hides the total price. Always negotiate on the out-the-door price first, then figure out the monthly number.
- Buying more car than you need. A reliable $15,000–$20,000 car gets you from A to B just as well as a $40,000 one. The extra $20,000 invested at 8% for 10 years becomes $43,000. That's the real price of the upgrade.
To estimate your monthly loan payment on a specific vehicle, use our car payment calculator. Want to see how much a car actually costs per mile driven? Try the cost per mile calculator. For general loan amortization on any type of loan, see the loan payment calculator.