What is the 20/4/10 rule?
The 20/4/10 rule is a simple guideline for buying a car without straining your budget:
| Number | Rule | Why it helps |
|---|---|---|
| 20 | Put at least 20% down | New cars lose value quickly. A big down payment keeps you from owing more than the car is worth. |
| 4 | Finance for 4 years (48 months) or less | Shorter loans cost less interest and you build equity faster. |
| 10 | Keep car payments plus insurance under 10% of gross income | Leaves room for housing, savings and everything else. |
It's stricter than what most buyers actually do. The average new-car loan now runs close to 70 months, with payments in the mid-$700s. That's exactly why the rule is useful as a reality check.
How the calculator works
Your 10% budget is your gross monthly income × 10%. Subtract insurance (and gas and upkeep, if you include them) to get the most you should spend on a loan payment. The loan amount that payment supports over 48 months comes from the standard loan formula:
Your maximum price is the lower of two limits: what your down payment plus that loan can buy, and the price at which your down payment is still 20%. Sales tax and fees are added to the price.
Example
You earn $75,000 a year, have $8,000 to put down, pay $180 a month for insurance, and can get a 6.35% APR. Ten percent of your monthly income is $625; after insurance, $445 is left for the payment. Over 48 months that supports a loan of about $18,800, so with your down payment and 7% tax and fees your maximum price is about $25,000.
A $35,000 car ($37,450 with tax and fees) would cost $696 a month over 48 months: 14% of your income. Stretching to 84 months brings the payment down to $435, but you'd pay $7,105 in interest instead of $3,976, and the loan would far outlast the rule's 4-year limit.
The new car loan interest deduction
For cars bought from 2025 through 2028, you may deduct up to $10,000 a year of auto loan interest, even if you don't itemize. The car must be new, for personal use, under 14,000 pounds, with final assembly in the United States. The deduction phases out by $200 for every $1,000 of income above $100,000 (single) or $200,000 (married filing jointly). Final assembly location is on the window sticker and can be confirmed with the VIN.
A deduction lowers your taxable income, not your tax bill dollar-for-dollar: in the 22% bracket, $2,000 of deductible interest saves about $440.
Frequently asked questions
What is the 20/4/10 rule for buying a car?
Put at least 20% down, finance for no more than 4 years, and keep your car payment plus insurance under 10% of your gross monthly income.
How much car can I afford on a $75,000 salary?
Under the 20/4/10 rule, 10% of $75,000 is $625 a month for payment and insurance. With $180 for insurance, that leaves $445 for the payment, which supports a 48-month loan of about $18,800 at 6.35% APR. Add $8,000 down and allow 7% for sales tax and fees, and the maximum price is about $25,000. Enter your own numbers to see your limit.
Is a 72- or 84-month car loan a bad idea?
Longer loans lower the monthly payment but cost much more interest, and because cars lose value quickly, you're more likely to owe more than the car is worth for years.
Can I deduct car loan interest?
For new, U.S.-assembled cars bought from 2025 through 2028 for personal use, you may deduct up to $10,000 a year of loan interest without itemizing. The deduction phases out above $100,000 of income for single filers and $200,000 for joint filers.
Does the 10% include insurance?
In the most common version, yes: car payment plus insurance should stay under 10% of gross income. Stricter versions also include gas and maintenance.
Estimates only. Your actual rate, taxes and fees vary by lender, state and credit. The interest deduction estimate is simplified and uses the first year's interest; it does not account for every eligibility rule. Average loan figures from Experian's State of the Automotive Finance Market, Q2 2026. Not financial or tax advice. Last updated October 1, 2026.