Car Payment vs Income + Negative Equity
Car Payment vs Income + Negative Equity Calculator
What this calculator does
This calculator exposes the two numbers that trap car buyers: the car payment as a share of take-home income, and negative equity, which is what you owe minus what the car is worth.
The math, with a worked example
Payment percentage = monthly payment ÷ monthly take-home income × 100. Negative equity = loan balance − car value. Break-even months are estimated using the portion of the payment that goes to principal after interest. A $450 payment on $3,000 take-home is 15%. A $22,000 loan on an $18,000 car is $4,000 underwater. The calculator flags both and estimates how long until the loan drops to the car value.
Why this matters
A car payment is measured against income or it isn't measured: $450 on $3,000 take-home is 15% — the common guardrail's ceiling — before gas and insurance. Add being $4,000 underwater and the car owns you until the equity math flips.
Methodology and transparency
Payment share = payment ÷ take-home × 100; negative equity = loan balance − car value; the break-even estimate tracks the principal portion of each payment until the balance crosses the car's value. The car-value input decides the equity math and it's the one people flatter — use a real trade-in quote, not the retail listing price. Everything runs in your browser; no data is sent or stored.
Last reviewed: 2026-08-08
Common questions
What is the 15% red line?
It is a common rule of thumb: a car payment above 15% of take-home squeezes the rest of the budget and leaves little room for repair or savings.
What is negative equity?
You owe more on the car than it is worth. If you sell or trade it, you still owe the difference, which often gets rolled into the next loan.
Should I trade in a car with negative equity?
Almost never. Rolling the unpaid balance into the next loan just makes the next car more expensive from day one.