20/4/10 Rule Calculator


What is 20/4/10 Rule Calculator?

The 20/4/10 rule is a guideline used by lenders to determine how much car you can afford based on your income and expenses. The rule states that you should:

  • Put down at least 20% of the car’s purchase price as a down payment.
  • Finance the car for no more than 4 years.
  • Keep your monthly car payments (including principal, interest, and insurance) to no more than 10% of your gross income.

Using a 20/4/10 rule calculator can help you determine how much car you can afford based on these guidelines.


The formula used by the 20/4/10 rule calculator is:

maximum affordable car price = (monthly gross income * 0.1) * (1 – (1 + monthly interest rate)^(-number of months)) / monthly interest rate + down payment


  • monthly gross income is your total income before taxes and deductions, divided by 12 to get a monthly amount.
  • monthly interest rate is your annual interest rate divided by 12 to get a monthly rate.
  • number of months is the length of your car loan in months.
  • down payment is the amount of money you plan to put down on the car, expressed as a dollar amount.


Let’s say you make $5,000 a month and plan to finance a car for 48 months with a 5% annual interest rate. You have $10,000 saved up for a down payment. Using the 20/4/10 rule calculator, you can determine the maximum car price you can afford by following these steps:

  1. Calculate your monthly interest rate: 0.05 / 12 = 0.00417.
  2. Calculate the number of months: 48.
  3. Calculate your monthly gross income: $5,000.
  4. Calculate the maximum monthly car payment: $5,000 * 0.1 = $500.
  5. Calculate the maximum car loan amount: $500 * (1 - (1 + 0.00417)^(-48)) / 0.00417 = $21,449.
  6. Add your down payment: $21,449 + $10,000 = $31,449.

Based on this calculation, you can afford a car with a maximum purchase price of $31,449 if you follow the 20/4/10 rule.

How to Calculate

To use a 20/4/10 rule calculator, you will need to input the following information:

  • Your monthly gross income.
  • The length of your car loan in months.
  • Your annual interest rate.
  • The amount of money you plan to put down on the car.

The calculator will then use the formula described above to determine the maximum car price you can afford based on the 20/4/10 rule.


1. Why is the 20/4/10 rule important?

The 20/4/10 rule is important because it helps ensure that you don’t overextend yourself when buying a car. By sticking to this guideline, you can avoid taking on more debt than you can handle and reduce the risk of financial hardship.

2. What if I can’t afford to put down a 20% down payment?

While a 20% down payment is ideal, it’s not always possible for everyone. In this case, you may want to consider a less expensive car or a longer loan term to reduce your monthly payments.


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