Car Affordability Calculator

What this computes

A dealer answers "how much can I afford?" with the largest loan you'll be approved for. That's the wrong number — it's built to maximize what you borrow, not to leave you a livable budget. This calculator answers the question the right way round: it starts from your income and works back to a price.

It takes a sane share of take-home pay as the ceiling for all car costs, subtracts your estimated insurance, fuel, and maintenance, and applies a second debt-to-income ceiling. The lower of the two becomes your affordable monthly payment, which it reverse-amortizes into a loan principal and grosses up — through down payment, trade-in, and tax — into the price you can actually carry.

The math

Two ceilings; the lower one binds:

Transport budget = take-home × car-cost %
Payment ceiling  = transport budget − insurance − fuel − upkeep
DTI ceiling      = take-home × DTI cap − existing debt
Max payment      = min(payment ceiling, DTI ceiling)

That payment is reverse-amortized into a principal:

P = M × [ (1+r)ⁿ − 1 ] / [ r(1+r)ⁿ ]

Where M is the affordable payment, r is the monthly rate (APR ÷ 12), and n is the term in months. The principal is then grossed back up through sales tax, down payment, and trade-in to the vehicle price — the inverse of how a loan is normally built.

A worked example

$6,000 take-home, $400 existing debt, 20% car-cost ceiling, $400/mo insurance + fuel + upkeep, 60-month loan at 6.9%, $4,000 down.

• Transport budget: $6,000 × 20% = $1,200
• Left for payment: $1,200 − $400 = $800
• Reverse-amortized principal: ≈ $40,500
• Affordable price with $4,000 down: ≈ $41,600

The dealer optimizes the loan you'll accept. This optimizes the budget you'll keep. Those are not the same number.

How to use this

1. Use real take-home pay. Net, not gross, and only income you can count on. Variable bonuses don't belong in a fixed monthly payment.
2. Quote insurance before you buy. Premiums vary wildly by model — a sportier or pricier car can add hundreds a month and shrink the payment you can afford.
3. Run a 48-month term first. If the car only fits on a 72- or 84-month loan, the calculator is telling you it doesn't fit — the long term just hides it.
4. Walk in with the number. Decide the price ceiling here, then negotiate to it. The dealer's approval amount is irrelevant to what your budget can hold.

Approved ≠ affordable

The single most expensive mistake car buyers make is treating loan approval as a budget. The two answer different questions:

• Approval asks: will this person probably repay the loan? It ignores your other goals, your savings rate, and every non-loan cost of the car.
• Affordability asks: after this car, do you still have a functioning budget? That's the question that keeps you out of a payment you resent in year three.

A car that's "approved" but unaffordable doesn't announce itself for a year or two — until the payment is crowding out saving, the car needs its first big repair, and the loan is still underwater. Starting from the budget avoids that entirely.

What this calculator doesn't model

• Depreciation and resale. Affordability is cash-flow; it doesn't tell you whether the car holds value. See the Car Depreciation calculator.
• Irregular income. The model assumes a steady monthly take-home. Self-employed or commission income should be budgeted conservatively against a low month.
• Total cost of ownership. Running-cost inputs here are estimates you supply. For a rigorous figure, use the True Cost of Ownership calculator and feed the result back in.
• Emergency buffer. Affordable on paper still assumes you have savings for a surprise repair. A car that consumes your entire margin isn't truly affordable.