Free Car Affordability Calculator (20/4/10 Rule)
The monthly payment is only part of what a car costs. Enter the price, your down payment, the loan and what you'll spend on insurance and gas to see the full monthly cost and how it compares to the 20/4/10 rule.
It runs in your browser. Nothing you type is saved or sent anywhere. The numbers filled in below are a made-up example; change them to yours.
How the math works
- Loan = price − down payment. Monthly payment uses the standard loan formula: loan × (APR ÷ 12) ÷ (1 − (1 + APR ÷ 12)^−months).
- Total interest = monthly payment × months − loan.
- 20/4/10 is a common rule of thumb: put at least 20% down, borrow for no more than 4 years, and keep total car costs under 10% of your pay before taxes. It's a guide, not a law.
Example (made up): an $18,000 car with $3,600 (20%) down is a $14,400 loan. At 7% APR for 48 months, the payment is about $344.83 and total interest about $2,151.64. Add $250 a month for insurance, gas and upkeep and the car costs about $594.83 a month, or 11.9% of $5,000 pay, so it's a little over the 10% mark.
Your real rate depends on your credit and the lender. This is an estimate for planning, not financial advice.
Save up the down payment first
The Sinking Funds Tracker ($5) shows how much to save per month and per paycheck for each goal, like a down payment, car repairs or insurance, with progress bars. Works in Google Sheets and Excel.
Get the tracker on Gumroad – $5
This template is also in the Complete Household Money Bundle (what's inside): all 13 home money spreadsheets for $29 (about $69 if bought one by one).
We make this template, so take that into account. The calculator above is free to use either way.
Related free guides
- How to Save for a Car or Down Payment (Example Numbers)
- How to Budget for Car Repairs and Maintenance
- How to Pay Off a Personal Loan Early (Example Numbers)
- Debt Snowball vs Debt Avalanche: How to Pick One
More free tools: all calculators.
See all Cedar & Ledger templates