Negative Equity Calculator

What this computes

When you trade in a car you still owe money on, the dealer pays off your loan with the trade-in credit. If the car is worth more than you owe, the surplus helps buy the next car. If you owe more than it's worth — you're underwater — that shortfall doesn't disappear. It either comes out of your pocket in cash or gets bolted onto the new loan.

Enter your current payoff, the trade-in offer, and the new car's price and financing. The calculator shows your equity position, how much negative equity there is, the amount you'd finance, the new monthly payment, and — crucially — how much extra interest and payment rolling the shortfall forward adds versus paying it in cash.

The math

Your equity position comes first:

Equity          = trade-in − current payoff
Negative equity = max(0, current payoff − trade-in)

A positive equity number means the trade-in covers your loan with room to spare. A negative one means you're underwater by that much. The shortfall is either rolled into the new loan or paid in cash:

Sales tax       = new price × tax rate
Rolled in       = negative equity   (only if you roll it in)
Amount financed = new price + tax − down + rolled-in

Then the standard amortization formula gives the payment:

M = P × [ r(1+r)ⁿ ] / [ (1+r)ⁿ − 1 ]

Where P is the amount financed, r is the monthly rate (APR ÷ 12), and n is the term in months. To isolate the cost of rolling the shortfall, the same loan is priced again without the rolled-in balance, and the two are differenced — that's the extra payment and extra interest the negative equity is responsible for.

A worked example

You owe $22,000; the best trade-in offer is $18,000. New car: $32,000, $2,000 down, 7% sales tax, 72 months at 8% APR.

• Negative equity: $22,000 − $18,000 = $4,000
• Sales tax: $32,000 × 0.07 = $2,240
• Amount financed (rolled in): $32,000 + $2,240 − $2,000 + $4,000 = $36,240
• New payment: ≈ $635/month
• Extra from the rolled-in $4,000: ≈ $70/month and ≈ $1,050 in extra interest over the loan

You financed $36,240 to drive off in a $32,000 car — already $4,000-plus underwater before the first payment clears.

Rolling negative equity forward doesn't erase the hole. It finances it — and digs the next one before you've climbed out.

How to use this

1. Get your exact payoff, not your balance estimate. Call your lender for the 10-day payoff. It includes accrued interest and is the number the dealer actually pays off.
2. Get the trade-in offer in writing. Use a real quote (dealer, CarMax, Carvana), not an optimistic guess. Your equity position swings on this number.
3. Toggle "roll the negative equity into the loan." Compare the two paths side by side: financed vs paid in cash. The difference is the true price of rolling it forward.
4. Watch the amount financed against the car's price. If you're financing thousands more than the car costs, you're starting the new loan underwater — and the cycle repeats.

Why rolling negative equity is dangerous

Dealers will happily roll your shortfall into the new loan because it makes the deal close. The math works against you in three ways:

• You pay interest on a car you don't own. The rolled-in balance is part of the new principal, so you pay APR on it for the full term — money spent on a car already gone.
• You start underwater again — deeper. The new car depreciates the moment you drive it off the lot, and you've financed more than it's worth. The next time you want to trade, the hole is bigger.
• The cycle compounds. Roll negative equity into loan after loan and the financed balance keeps outrunning the car's value. This is how people end up owing $40,000 on a $25,000 car.

The clean escapes: keep the current car and pay it below its value before switching, or pay the shortfall in cash so it never gets financed. If neither is possible right now, the most honest answer is usually to wait.

What this calculator doesn't model

• Trade-in tax credit on the new car. Many states tax (new price − trade-in). This keeps it simple and taxes the full new price; if your state credits the trade-in, your tax — and financed amount — will be a bit lower. See the Auto Loan calculator for that toggle.
• Dealer add-ons. Extended warranties, gap insurance, and fees are often financed too, raising the amount financed beyond what's modeled here.
• Depreciation of the new car. How fast you climb back to positive equity depends on the new car's depreciation — see the Car Depreciation calculator.
• Refinancing or early payoff. A lower rate or extra principal builds equity faster. Model those in the refinance calculator.