How to use this affordability calculator
- Enter your gross monthly income, before taxes. If you only know your annual salary, divide it by 12.
- Add your expected monthly insurance premium and fuel spending for the kind of vehicle you're considering.
- Enter the APR you expect to qualify for.
- Choose the strict 20/4/10 mode or the flexible mode.
- Read your maximum monthly payment, maximum loan and maximum vehicle price.
How it's calculated
The 20/4/10 guideline sets three limits: at least 20% down, a loan of no more than 48 months, and total monthly car costs (payment plus insurance plus fuel) of no more than 10% of gross monthly income. The flexible mode raises that cap to 15% and allows terms up to 60 months.
The calculator starts with your maximum payment: gross monthly income × the percentage, minus insurance, minus fuel. It then finds the largest loan that payment can repay at your APR over the term, known as the present value: max loan = payment × (1 − (1 + r)−n) ÷ r, where r is the APR divided by 12 and then by 100, and n is the number of months. Because the loan only covers the part of the price you don't put down, max price = max loan ÷ (1 − down payment share). With 20% down, that means dividing by 0.8.
Example: you earn $6,500 a month before tax ($78,000 a year) and expect $160 for insurance and $140 for fuel. Ten percent of income is $650, which leaves $350 for the payment. At 6.5% APR for 48 months, $350 a month repays a loan of about $14,759. Dividing by 0.8 gives a maximum price of about $18,448, with roughly $3,690 down. In flexible mode, 15% of income leaves $675 a month, which over 60 months supports a loan of about $34,498 and, keeping 20% down, a price near $43,123.
Making your car budget go further
- Get insurance quotes for the specific models you're considering. Premiums can differ by hundreds of dollars a year between similar cars.
- Sales tax, registration and dealer fees usually come out of the same budget, so shop a little below your maximum price.
- Set money aside for maintenance, repairs and parking, which the 10% guideline doesn't count.
- A shorter loan builds equity faster, which matters if you need to sell or trade in early.
- If the result is lower than you hoped, a reliable used car often fits the guideline far more easily than a new one.
Good to know: Lenders may approve you for much more than this calculator suggests. Approval only means you qualify for the loan, not that the payment fits comfortably in your budget.
Frequently asked questions
What is the 20/4/10 rule for buying a car?
It's a budgeting guideline: put at least 20% down, finance for no more than four years (48 months), and keep total monthly car costs, including the payment, insurance and fuel, at or below 10% of your gross monthly income.
Is the 20/4/10 rule realistic with today's car prices?
It's strict, and many buyers can't meet it on a new car. That's why the calculator also offers a flexible mode at 15% of income and up to 60 months. Treat the strict result as a comfortable target and the flexible one as an upper limit.
Should I use gross or net income?
The guideline is based on gross income, before taxes and deductions. If you carry other large debts or have high living costs, aim below the result, because your take-home pay is what actually covers the payment.