Auto Loan Calculator With Trade-In Negative Equity

This auto loan calculator estimates a monthly payment and total interest when you roll negative trade-in equity into a new loan. Enter the amount financed after adding the shortfall, the APR, and the term; it returns an estimate, not a lender quote.

Formula

The calculator uses the standard fixed-payment loan formula. First determine the amount financed, including any negative equity rolled into the replacement vehicle loan:

Amount financed = vehicle price + taxes and fees financed + negative equity − cash down payment − trade-in allowance.

For the payment calculation, the monthly payment is M = P × r / (1 − (1 + r)⁻ⁿ), rounded to the nearest cent. Total interest is I = M × n − P. In these formulas, P is the total amount financed in dollars, r is the monthly interest rate as a decimal (APR ÷ 1200), n is the number of monthly payments (loan term in years × 12), M is the monthly payment in dollars, and I is total interest in dollars. The calculator’s input called amount financed must already include the trade-in shortfall and any other amount included in the loan.

Worked Example

Using the calculator’s default values—$20,000 financed, 8% APR, and 5 years—the monthly rate is 8 ÷ 1200 = 0.0066667, and the term is 5 × 12 = 60 payments.

1. Payment before rounding = 20,000 × 0.0066667 ÷ [1 − (1 + 0.0066667)⁻⁶⁰].
2. Monthly payment, rounded to cents = $405.53.
3. Total of rounded payments = $405.53 × 60 = $24,331.80.
4. Total interest = $24,331.80 − $20,000 = $4,331.80.

Assumptions

• The APR and payment amount remain fixed for the full term.
• Payments are monthly and made at the end of each payment period.
• Interest compounds monthly at APR ÷ 12.
• The estimate does not separately calculate taxes, title or registration charges, dealer fees, optional products, insurance, or other costs; include an amount only if it is part of the amount financed you enter.

How Negative Equity Changes the Loan

Negative equity means the amount still owed on a vehicle is greater than the amount credited for it as a trade-in. The difference is a shortfall. For example, if a payoff is $18,000 and a dealer allows $15,000 for the vehicle, the negative equity is $3,000. If that $3,000 is added to the new loan, it raises the amount financed by $3,000 before considering any down payment or other financed costs.

To prepare the calculator input, add the shortfall to the replacement vehicle’s price and any other costs being financed, then subtract cash down and the trade-in allowance if you are working from the full purchase figures. Avoid counting the trade-in allowance twice: when starting with a vehicle price already reduced by the trade-in credit, add only the amount needed to cover the old loan payoff beyond that credit.

Using the default APR and term as an illustration, financing an additional $3,000 would raise the payment by about $60.83 per month and add about $650 in interest over 60 payments. The precise result depends on the APR, term, and amount actually financed. This illustrates why a relatively small monthly increase can represent a larger total repayment over time.

Using and Reading the Estimate

Enter the amount that will be financed, not simply the advertised vehicle price. Use the APR and term shown in a lender’s written quote or disclosure for a more relevant estimate; do not treat the example defaults as a rate offer. The calculator returns a level monthly payment and the interest implied by making that rounded payment for every scheduled month.

Compare the payment with the total repayment and the principal. A longer term generally spreads repayment across more months, which can lower the scheduled monthly amount while increasing the amount of interest paid when the rate is above zero. The calculator does not decide whether a payment fits a household budget or whether a lender will approve an application.

Common Errors and Calculator Limits

• Using the trade-in value instead of the payoff: Negative equity is the gap between the payoff and the trade-in credit, not the full payoff.
• Entering the monthly rate as APR: Enter annual APR as a percent, such as 8, rather than 0.6667. The formula converts APR to a monthly rate.
• Leaving out financed amounts: If taxes, fees, or other items are included in the actual loan, leaving them out understates the payment. Get their amounts from the transaction paperwork.
• Rounding too early: The calculator rounds the scheduled payment to cents and uses that rounded payment to estimate total interest. A lender’s final schedule may differ because of payment dates, rounding conventions, or final-payment adjustments.

This is a simplified amortizing-loan estimate. It does not calculate a separate payoff quote for the old vehicle, verify its value, or establish whether a lender will permit a shortfall to be included. It also does not model variable rates, irregular payments, deferred-payment terms, add-on products, or extra payments. If any of those apply, use the specific figures and schedule supplied by the lender.

Frequently Asked Questions

Can I put negative equity into a new auto loan?
The calculator can estimate payments when a shortfall is included in the amount financed. Whether it can be included in a particular transaction depends on the lender and deal terms; confirm the figures with the lender.

How do I calculate negative equity on a trade-in?
Subtract the trade-in allowance from the current loan payoff. For example, an $18,000 payoff minus a $15,000 allowance leaves $3,000 in negative equity, before any other costs or credits.

Does rolling negative equity increase the monthly payment?
It increases the amount financed, so it increases the payment when APR and term stay the same. It also generally increases total interest on a positive-rate loan because interest is calculated on a larger principal.

Why is my lender’s payment different from the calculator?
The quoted amount may include different financed costs or use a different APR, term, payment date, or rounding method. Compare the amount financed and repayment schedule line by line with the estimate.

Does this calculator include my old loan payoff?
Only if you account for the payoff-versus-trade-in difference in the amount financed you enter. Do not enter the entire old payoff as negative equity; the trade-in credit offsets part of it.

The result is an estimate, and the lender’s disclosure gives the exact figures.

Related

• Mortgage Loan Amount Calculator: The Loan a Monthly Payment Repays
• Home Equity Loan Calculator: Amount Up to a CLTV Limit and Payment
• Mortgage Calculator With Escrow: Payment With Tax and Insurance

in p | Amount financed after trade-in | $ | 20000 | 1 | 100000000
in apr | Interest rate (APR) | % | 8 | 0.01 | 100
in yrs | Loan term | years | 5 | 0.0833 | 50
out pmt | Monthly payment | $ | round(p * (apr / 1200) / (1 - (1 + apr / 1200) ^ -(yrs * 12)), 2) | 2
out int | Total interest | $ | pmt * yrs * 12 - p | 2