How it works
The calculation applies three illustrative limits together: at least a 20% down payment, a 48-month loan, and total car expenses within 10% of gross monthly income. It also caps combined other debt and car expenses at 36% of gross income. That extra constraint is a disclosed planning assumption, not a lender’s underwriting standard.
Monthly running costs are subtracted before calculating how much payment is available for financing. The lower of the cash-down limit and the payment-supported limit determines the purchase budget. If no payment capacity remains, the result may still show your available cash as a cash-purchase budget; it does not mean ongoing expenses are affordable.
The formula behind the estimate
Every result starts with a defined model and visible assumptions.
Payment budget = max(0, min(0.10 × gross income − running costs, 0.36 × gross income − other debts − running costs)) Finance capacity = payment budget ÷ payment on $1 financed for 48 months Purchase budget = min(down payment ÷ 0.20, down payment + finance capacity)
A worked example
With $6,000 gross monthly income, $700 of other debt payments, $5,000 down, and $250 of monthly running costs, the 10% guideline leaves $350 for a loan payment. At a 7% annual rate over 48 months, financing capacity is approximately $14,618. The resulting total purchase budget is approximately $19,618 before separating taxes and fees from the vehicle price.
A rule of thumb is a starting point
Gross income is convenient for a consistent comparison, but taxes and deductions determine what reaches your bank account. Two households with the same salary can have very different room for transportation. Childcare, medical expenses, housing, support obligations, and variable income are not captured in a simple percentage. Use the result to narrow a search, then build a full cash-flow budget.
The 20/4/10 framework is not a legal rule, a guarantee of good financial outcomes, or a promise of loan approval. Some buyers can comfortably spend less; others cannot cover necessary transport within the percentage. A shortfall is useful information: consider a less expensive vehicle, a larger independently affordable down payment, or delaying the transaction.
Work backward from the complete purchase
Treat the output as a total acquisition budget rather than a listing-price target. If the result is $20,000 and verified taxes and fees total $1,500, a $20,000 sticker price does not fit that same budget. Leave room for inspection, immediate service, and other costs you need to pay without borrowing.
Higher interest reduces the amount financed by a fixed payment. Extending the term can hide that pressure in a low monthly bill while increasing interest and the period of negative-equity risk. This tool deliberately keeps the loan at 48 months; use the separate loan calculator to see the tradeoff explicitly rather than silently changing the guideline.
Questions worth asking
Is 20/4/10 an official lending rule?
No. It is a budgeting guideline implemented here with clearly stated assumptions. Lenders use their own criteria and may approve amounts that are uncomfortable for your household.
Why is the budget zero with no down payment?
This version enforces at least 20% down. Zero down cannot satisfy that constraint for a positive purchase price, even if income could support payments.
Should I enter take-home or gross income?
The model uses gross monthly income. Check the proposed payment against take-home pay separately before relying on the budget.
Does the result include insurance?
Only through the monthly running-cost amount you provide. Get an actual quote and update that number; the tool does not estimate insurance rates.
What is the 36% limit?
It is an additional illustrative cap on other debt payments plus modeled transport spending. It is not a standard debt-to-income calculation or an approval threshold.
This is general information, not personalized financial advice. Read the limitations.
