How Much Car You Can Afford, and Why the Answer Feels Low
The 20/4/10 guidance is three constraints applied together: twenty percent down, four years maximum, and total vehicle costs under ten percent of gross income. Applied properly it produces a number well below what most people spend and well below what any lender would approve. That gap is the point of running it. This calculator applies all three honestly, counts running costs inside the ten percent where they belong, and shows what relaxing each constraint actually buys.
Table of Contents
What 20/4/10 Actually Says
Twenty percent down. Enough to get above water quickly, so an early write off or sale does not leave a shortfall.
Four years maximum. Long enough to be manageable and short enough that the loan clears well before the car becomes unreliable.
Ten percent of gross income for the car in total. Not the payment. The payment plus insurance, fuel, tax and servicing.
All three together, not whichever is convenient. Meeting the ten percent by stretching to seven years defeats the purpose of the four.
How to Use This Calculator
Be honest about running costs. Insurance and fuel for the car you are actually considering, not a hopeful figure. A more expensive car usually costs more to insure and run.
Start with the defaults and see the number. For most incomes it is lower than expected, and that reaction is the useful part.
Then relax one constraint at a time. The panel tells you when you have left the guidance, and the table shows what the extra years cost.
Use gross income. The rule is defined on it, and using take home pay makes the result stricter than intended.
Running Costs Belong Inside the Ten
This is the part most often misread. Applying ten percent to the loan payment alone and treating insurance and fuel as separate makes the rule far more permissive than it is.
They are not small. Insurance, fuel, tax and servicing frequently come to as much as a modest payment, and they rise with the value of the car.
They scale with the purchase. A more expensive car costs more to insure, usually more to service, and often more to fuel, so the two constraints tighten together.
Depreciation is still not counted. Even this stricter reading leaves out the largest cost of all, which is why the rule is a floor rather than a full budget.
Why Twenty Percent Down
It gets you above water quickly. A car loses value fastest in the first year, and a substantial deposit is what keeps the balance below the value through that period.
Which matters if anything happens. A write off pays the value, not the balance. Being above water means there is no gap to find.
It also reduces the interest. Less financed for less time, which compounds with the shorter term the rule also asks for.
It is a test of readiness. Not having twenty percent available is itself information about whether the purchase is affordable.
Why Four Years and Not Seven
Long terms let any car fit any budget. Which turns the question from what you can afford into what payment you will tolerate.
Interest rises sharply. The table above shows the difference on the same monthly budget, and over seven years it is usually thousands more.
You are underwater for most of it. On a seven year loan the balance can exceed the value for four or five years, which is a long exposure.
The car ages into the loan. Paying for a vehicle that has started needing repairs is a common and unpleasant position, and short terms avoid it.
When Breaking the Rule Is Reasonable
A subsidised manufacturer rate. At zero or near zero percent, a longer term costs very little extra and the calculation changes genuinely.
A car needed for work. If it generates income, it is closer to an investment than a consumption decision.
A very high income. Ten percent of a large income is a lot of car, and the constraint stops being the binding one.
Deliberately, not by drift. The difference between choosing to exceed the guidance and discovering you have is the whole point of checking.
What a Lender Will Approve Instead
Considerably more. Lenders assess whether you will keep paying, not whether the purchase is wise, and those are different questions.
Approval is not endorsement. Being offered a seven year loan on a car at twenty percent of your income says nothing about whether it is a good idea.
The dealer optimises for the payment. Because that is the number you react to, and it can be lowered without improving anything.
Knowing your own number first is the defence. Deciding what you will spend before the conversation removes the leverage entirely.
The Cost the Rule Still Misses
Depreciation. Usually the single largest cost of ownership, and absent from the guidance entirely because it never arrives as a payment.
Repairs after the warranty. Predictable in aggregate and unpredictable in timing, which is exactly what an emergency fund is for.
The opportunity cost of the deposit. Twenty percent of a car price is a meaningful sum doing nothing else.
Which is why the rule is a minimum standard. Passing it is not the same as the purchase being a good use of money, only that it is unlikely to be a problem.
Common Mistakes to Avoid
Applying the ten percent to the payment alone. The most common misreading, and it makes the rule far weaker than intended.
Meeting the ten percent by extending the term. It satisfies one constraint by breaking another.
Using take home pay. The rule is defined on gross, so net makes it stricter than it should be.
Forgetting insurance rises with the car. Budgeting current insurance costs for a considerably more expensive vehicle understates the running total.
Frequently Asked Questions
Financial Disclaimer: 20/4/10 is widely repeated guidance rather than a lending standard or advice. It omits depreciation, which is usually the largest cost of ownership. Treat the result as a conservative benchmark and consider your own circumstances before deciding.