Transparent method
The formula
Worked example
Example inputs
How to interpret the result
The 20/4/10 rule is a conservative screen: 20 percent down, a loan of at most four years and total transportation cost under 10 percent of gross income. On a $40,000 car and $100,000 of income, the four year loan at 7 percent after $8,000 down is about $766 a month. Adding $350 of insurance and fuel gives $1,116, which is about $283 over the $833 target. The rule says the car is a stretch, and it does not forbid the purchase.
Before you rely on it
What to check
Try a cheaper car or a larger down payment and see which one brings the total under the target. A smaller change in price is often enough.
The common error
Where people go wrong with 20/4/10 car affordability calculator
Treating the rule as a lender's approval test. Lenders use their own ratios, and a loan you can qualify for may still be more than you should take.
Sensitivity evidence
How auto-loan rate changes the monthly cost over 10% target
| Auto-loan rate | Monthly cost over 10% target | Four-year loan payment with 20% down | 10% monthly transportation target |
|---|---|---|---|
| 5.0% | $254 | $737 | $833 |
| 6.0% | $268 | $752 | $833 |
| 7.0%worked example | $283 | $766 | $833 |
| 8.0% | $298 | $781 | $833 |
| 9.0% | $313 | $796 | $833 |
Every input, tested
Which input moves the monthly cost over 10% target most
| Input | Tested from | To | Monthly cost over 10% target at each end | Swing |
|---|---|---|---|---|
| Gross annual income | $90,000 | $110,000 | $366 to $200 | $167 (59%) |
| Vehicle price | $36,000 | $44,000 | $206 to $360 | $153 (54%) |
| Monthly insurance and fuel | $315 | $385 | $248 to $318 | $70 (25%) |
| Auto-loan rate | 6.0% | 8.0% | $268 to $298 | $30 (10%) |
Two variables at once
Monthly cost over 10% target by auto-loan rate and vehicle price
| Auto-loan rate \ Vehicle price | $32,000 | $40,000 | $48,000 |
|---|---|---|---|
| 5.0% | $106 | $254 | $401 |
| 6.0% | $118 | $268 | $418 |
| 7.0% | $130 | $283 | $436 |
| 8.0% | $142 | $298 | $454 |
| 9.0% | $154 | $313 | $472 |
Step by step
The worked example, input by input
| Input | Value used |
|---|---|
| Vehicle price | $40,000 |
| Gross annual income | $100,000 |
| Auto-loan rate | 7.0% |
| Monthly insurance and fuel | $350 |
| Monthly cost over 10% target | $283 |
| Four-year loan payment with 20% down | $766 |
| 10% monthly transportation target | $833 |
Where the inputs come from
Vehicle price is what you plan to pay including taxes and fees, gross annual income is your pay before taxes, the auto loan rate is a preapproved quote, and monthly insurance and fuel are your estimated running costs. The 20/4/10 guide says to put at least 20% down, finance for no more than four years and keep total transportation costs under 10% of gross monthly income. Include the down payment from cash you actually have.
Using the monthly cost over 10% target in a real decision
The rule is conservative on purpose, so treat it as a guardrail rather than a law.
When the result stops applying
The 10% target uses gross income, so the burden on take-home pay is heavier. High-cost areas, or people with low other expenses, may exceed the rule sensibly, and people with high debts should stay under it. Maintenance and depreciation are not in the 10% figure. Financing for four years raises the payment compared with longer terms, which is the point.
What to compare it with
See the payment on the auto loan calculator and the maximum loan on the auto affordability page. The vehicle total cost of ownership calculator gives a fuller picture, and the lease versus buy page compares payment types. Save for a larger down payment before shopping.
People also ask
Frequently asked questions
What is the 20/4/10 rule?
Put 20% down, finance for no more than four years, and keep total car costs, including insurance and fuel, under 10% of your gross income.
Is 20/4/10 realistic today?
For newer, expensive cars it can be hard, which is why many people buy used or a less expensive model to stay within it.
What if I cannot meet all three parts?
Prioritize the total cost limit and a short loan term. A smaller down payment is less harmful than a long loan on a car that depreciates.