Car Affordability Calculator

First, it estimates disposable income using gross monthly income minus monthly debt payments. Next, it computes a conservative payment cap using: min(15% of gross income, 25% of (gross income − monthly debt payments)). Then it uses the standard amortization formula (fixed APR, fully amortizing loan) to estimate the maximum loan principal supported by that payment for your chosen term.

Because the calculator uses a rule of thumb based on gross income (not take-home pay), your actual comfort level may differ—especially if you have high taxes, insurance, or other non-debt obligations. Longer terms (72 months and up) can reduce the payment but increase total interest, which the calculator flags. APR also matters: even if the payment fits the cap, a higher APR can materially raise total interest cost.

The estimate is for principal + interest only; it does not include car insurance, fuel, maintenance, registration, taxes, or dealer fees. It also doesn’t model trade-in value, rebates, or price adjustments unless you reflect them via your down payment input. If monthly debt payments are close to your gross income, the “safe” payment may drop sharply—so review results carefully before committing.

If APR is 0%, total interest should come out to $0 (the loan cost should be driven by the financed amount and term). If monthly debt payments equal or exceed gross monthly income, the payment cap can become very low or zero, limiting affordability. If your down payment is large enough to cover the estimated purchase price, the calculator effectively treats the financed portion as $0 and shows that the purchase could be paid in cash.