How much of your income a car payment should take

Four incomes, four percentages, sixteen payments. Each bar below is gross monthly income multiplied by a share — nothing more exotic than that. The bars are all drawn against the same scale, so the longest one ($1,600) runs the full width and everything else is proportional to it. The incomes are illustrative round numbers, not typical or average anything.

Two things jump out once the bars are stacked. The first is that the same percentage produces wildly different dollar amounts, which is obvious but easy to forget when a rule is repeated as a single number. The second is less obvious: the $4,500 household at 20% and the $6,000 household at 15% land on exactly the same payment, $900. A percentage rule treats those two cases as different decisions. Your bank account will not.

What the percentages leave out

Every bar is computed on gross income, because that is how the rules of thumb are usually phrased. Nobody pays a car loan with gross income. Taxes, retirement contributions and health premiums come out first, and how much they take differs a great deal from one paycheck to the next. A payment that is 12% of gross might be 16% or 17% of what actually lands in checking — work it out from your own pay stub rather than trusting our guess.

The bars also stop at the payment. They do not include insurance, fuel, parking, tolls, registration or the repair that arrives the week after the warranty ends. The well-known 20/4/10 rule tries to handle this by capping all car costs, not just the loan, at 10% of gross — which is a much tighter limit than it first sounds, and the reason many households fail it on paper. If you want your real running number rather than a rule, the worksheet in your real cost per mile will get you there.

And the bars ignore what is already spoken for. Two households at $6,000 a month are not in the same position if one pays $2,400 in rent and a student loan while the other owns a paid-off house. A percentage of income is blind to every other line in the budget, which is precisely the information that decides whether a payment is comfortable.

Two cases the ladder handles especially badly. If your pay swings from month to month — commission, tips, seasonal work — a percentage of "monthly income" is a percentage of a number that doesn't hold still. Size the payment against a lean month, not an average one; the good months can go to savings. And if two incomes cover the household, ask what happens to the payment if one of them pauses for a while. A car loan runs for years, and a lot of life fits inside that many years.

What a payment actually buys

It helps to translate a few of those bars into a loan amount. At an illustrative 7% APR over 60 months — a made-up rate for arithmetic, not a quote — the payments above would support roughly these amounts financed:

Those are amounts financed, not sticker prices. Taxes, fees, a down payment and a trade-in all move the relationship between the two, and a longer term would make each payment "buy" more car — while making the car cost more in total and leaving you owing more than it is worth for longer.

The argument

Here is our case, and it is not a percentage. Pick the payment from the bottom up. Start with take-home pay. Subtract the fixed bills that exist whether or not you own this car. Subtract what you want to be saving. Subtract the car's running costs — insurance quote in hand, a fuel estimate from your actual commute, a monthly amount for maintenance. What remains is the most a payment could be. Then take some off that, because a budget that balances to the dollar breaks the first month anything goes wrong.

Once you have that number, look back up the ladder and see where it sits. If it lands on one of the short navy bars, good — you have room. If it lands on a blue bar, a rule of thumb has not been violated so much as a warning light has come on: the payment is large relative to income, and everything else in the budget has to be unusually quiet for it to work. Some households genuinely are that quiet. Most find out otherwise in month seven.

The one thing we would steer anyone away from is working the ladder in reverse — starting from the car, asking what payment it needs, and then hunting for a term long enough to make the payment fit. That is how a $540 budget becomes a $675 commitment stretched over seven years. The three numbers to set before you shop exist to stop exactly that, and a maximum payment is one of them.

So how much of your income should a car payment take? Less than whatever share leaves the rest of your life running normally — and you can only find that share by writing the rest of your life down first.