The 20/4/10 rule, tested on real Ontario numbers
The short version
The 20/4/10 rule says put 20 percent down, finance for four years at most, and keep everything the car costs within 10 percent of your income. It is a good stress test and a bad gate: on real Ontario numbers, a $30,000 used SUV needs about $160,000 a year to pass it. We test the rule instead of preaching it, and we answer the question its tellings dodge: that 10 percent is measured on gross pay, which is money you never actually see.
The 20/4/10 rule circulates wherever car advice is given: put at least 20 percent down, borrow for no more than 4 years, and keep your total transportation costs, payment, insurance, fuel, everything, at or under 10 percent of your income.
It travels with rivals. The 20/3/8 variant tightens the term to three years and the share to 8 percent. The 10 percent rule keeps only the income cap. And the mildest telling caps just the loan payment at 15 percent of monthly income, and says nothing about the down payment, the term, or the insurance bill at all.
Most articles pick one and preach it. We would rather run all four against one real car and show you what each one actually demands, because a rule you cannot check is just a person on the internet with a confident voice.
Gross or net? The question every telling dodges
People search this question in exactly these words, and the honest answer is: the rule means gross, your before-tax income, because that is the number budgeting rules are historically written on. But the objection to that is real and correct: you cannot spend gross pay. Someone earning $60,000 takes home a lot less, and a rule quoted on money you never see always sounds more affordable than it is.
Our position, used everywhere on this site: measure the share on gross, because that is what every rule and every lender ratio is defined on, and never show the share without the dollars beside it, because the dollars come out of your take-home. If a rule quotes you a percentage and no dollar figure, do the multiplication yourself before you nod along.
The test: one car, four rules
Here is a $30,000 used SUV in Toronto, bought the way 20/4/10 tells you to buy it: $6,000 down and 48 months of financing. The engine prices the whole cost of having it, loan, insurance, fuel, maintenance and licensing, at about $1,333 a month. Then each rule sets its own bar for the income that cost must fit inside.
One $30,000 used SUV in Toronto, and the income each rule demands
- Loan payment on the 20/4/10 purchase, 20% down, 48 months$703
9.64% APR48 months7.29% to 11.99% for your credit
- Income 20/4/10 needs, everything within 10% of gross$160,000 a year
- Income 20/3/8 needs, everything within 8% over 3 years$229,000 a year
- Income the payment-only 15% rule needs$57,000 a year
- Income our manageable ceiling needs, everything within 15%$107,000 a year
Same car every row. The spread between the strictest rule and the mildest is the whole debate: the strict rules are safe and nearly nobody passes them, the mild one is passable and counts only the loan.
Read the spread before you pick a side. The strict rules are strict enough that a plain used SUV requires an income most Canadians do not earn: about $160,000 for 20/4/10 and about $229,000 for 20/3/8. The payment-only rule blesses the same car at about $57,000, and it manages that by ignoring insurance, fuel and maintenance, which in Ontario can rival the payment itself.
The community has noticed. The standing rebuttal runs: if only the top few percent of earners can pass the rule, where are all the used cars coming from? It is a fair question, and the honest answer is that almost nobody passes, almost everybody buys anyway, and a rule that fails everyone teaches no one.
What the rules get right
Direction, not thresholds. Every part of 20/4/10 points the right way: a bigger down payment means less borrowed and less time upside down, a shorter term means less interest and an earlier payoff, and counting every cost instead of just the payment is the single most protective habit a buyer can have. The arithmetic behind all three is real and we compute it on every result this site shows.
What the rules get wrong is pretending the threshold is the wisdom. Whether the right cap is 8, 10 or 15 percent depends on your rent, your debts and your city, and no three-number slogan knows any of that.
How to actually use them
- Use 20/4/10 as a stress light, not a gate. Pass it and the car is comfortable by any standard anyone publishes. Miss it and you have company: our own bands call up to 10% of gross easy, up to 15% manageable, and up to 20% tight, all measured all-in.
- Never test a rule on the payment alone. The payment-only rule passes cars that fail every other test for a reason. Test on the all-in number or you are testing a fraction of the bill.
- Assume financing is normal, and rate the cost. Most Canadians finance. The useful question is never whether borrowing is virtuous, it is what this loan, on this car, in this city, does to this budget.
That last question is exactly what the calculator answers: your income, your city, your credit, every cost counted, and a verdict in plain words instead of a slogan.
Sources
Every claim in this guide either comes from one of these, or is worked out in front of you by the same engine that runs the calculator. Figures we hold in our own dated assumptions file are listed with their sources on the assumptions page.