Ask how much car you can afford and you'll be handed a rule. Ask twice and you'll be handed a different one. The 20/4/10 rule, the 35% rule, half your salary, one year's salary, 10% of gross, 15% of take-home — all quoted with equal confidence, as though they're saying the same thing in different words.
They aren't. On a $60,000 salary they put your ceiling anywhere from $7,800 to $60,000.
Quick answer: The rules disagree because they measure different things — some cap the monthly payment, some cap the sticker price, some include insurance and fuel while others ignore them, and the salary-multiple rules assume you're paying cash. If you're financing, use 20/4/10. If you're paying cash, half your annual salary is the sensible ceiling. Everything else is either looser or measuring something you didn't ask about.
Here's what each rule permits on a $60,000 gross salary, assuming a 7.2% APR and $350 a month in insurance, fuel and maintenance:
| Rule | What it caps | Max car price |
|---|---|---|
| 20/4/10 | All car costs, 10% of gross | $7,800 |
| 15% of take-home | All car costs | $11,867 |
| 35% of annual income | Sticker price | $21,000 |
| 10% of gross income | Payment only | $27,923 |
| Half your annual salary | Sticker price (cash) | $30,000 |
| One year's salary | Sticker price (cash) | $60,000 |
A 7.7× spread. Someone following the first rule buys a used compact; someone following the last buys a new luxury sedan. Both believe they're being responsible, and both can cite a rule.
"10% of gross monthly" limits what leaves your account each month. "35% of annual income" limits the sticker. Those aren't the same constraint, and the gap between them widens with your loan term. Stretch to 72 months and a payment cap permits a much more expensive car; a price cap doesn't move at all.
This is the single biggest driver of the spread. Running a car costs most people $300–450 a month before any loan payment. A rule capping total car costs at 10% of gross is dramatically stricter than one capping just the payment at the same 10%.
On our $60,000 example, 10% of gross is $500 a month. Subtract $350 of running costs and the payment budget is $150 — which is why 20/4/10 lands so much lower than the payment-only rules. It isn't being arbitrary; it's counting what the others leave out.
"Half your salary" and "one year's salary" come from a cash-purchase tradition. They say nothing about interest, term or down payment, which makes them close to meaningless applied to a financed car. If you're borrowing, a rule that ignores the loan isn't giving you affordability advice.
The one-year's-salary rule was reasonable when cars cost far less relative to income. With average new car transaction prices around $50,000, it now permits a purchase most people at that income genuinely shouldn't make. It survives because it's memorable, not because it's sound.
Enter your income and running costs to see where each rule lands — and which to follow.
Compare the Rules →Each of its three parts fixes a specific failure mode:
It is deliberately strict. Most buyers fail it — and on lower incomes, running costs alone can exceed the 10% cap, meaning the rule says you shouldn't be financing at all. That's uncomfortable, but it's information rather than a flaw in the arithmetic.
The payment rules don't apply, and the right ceiling is a salary multiple. Half your annual salary works for most people. One year's salary is defensible only if you have no other debt, a fully funded emergency fund, and you plan to keep the car a long time.
Worth saying plainly: paying cash removes interest entirely, which is worth more than any of these rules will tell you. A $25,000 car bought outright and a $25,000 car financed over five years at 7.2% are not the same purchase — the second costs roughly $4,800 more.
They're all built on income alone. None of them asks about:
Use whichever rule fits your situation as an upper bound, then subtract for whatever above applies to you. A number that clears every rule can still be wrong for you.