Auto Loan Calculator With an Upside-Down Trade

An auto loan calculator with an upside-down trade estimates the monthly payment and total interest when you add negative equity from your current car to a new loan. Enter the amount financed—including the trade-in shortfall—along with APR and term; the calculator shows the payment and estimated interest.

Formula

This calculator treats the entered loan amount as the full amount financed, including any negative equity rolled into the next loan. It applies the standard fixed-rate, level-payment loan formula:

• Monthly payment: M = P × r ÷ [1 − (1 + r)⁻ⁿ]
• Total interest: I = M × n − P

P is the total amount financed in dollars; APR is the annual percentage rate entered as a percentage; r is the monthly rate, APR ÷ 1,200; n is the number of monthly payments, years × 12; M is the monthly payment in dollars; and I is estimated interest in dollars. The calculator rounds the payment to cents and uses that rounded payment for total interest.

Worked Example

Using the default inputs—$20,000 financed, 8% APR, and 5 years:

1. Monthly rate: 8 ÷ 1,200 = 0.0066667.
2. Number of payments: 5 × 12 = 60.
3. Payment: $20,000 × 0.0066667 ÷ [1 − (1.0066667)⁻⁶⁰] = $405.53 per month, rounded to cents.
4. Total interest: $405.53 × 60 − $20,000 = $4,331.80.

Assumptions

• The APR is fixed for the full term.
• Payments are equal, made monthly, with the first payment one month after borrowing.
• Interest compounds monthly at APR ÷ 12.
• The estimate excludes fees, taxes, insurance, optional products, and other charges unless they are included in the loan amount you enter.

How to Include an Upside-Down Trade

You are upside down, or have negative equity, when the amount needed to pay off your current auto loan is greater than the vehicle’s trade-in value. The difference is the shortfall. If that shortfall is included in financing for another vehicle, it increases the new loan amount and the payment is calculated on the larger balance.

Work out the amount to enter by adding the vehicle’s negotiated price, applicable charges and any negative equity being financed, then subtracting money down, trade-in credit, rebates or other credits that actually reduce the amount borrowed. Use the final amount financed if the lender has provided one. Do not enter only the new car’s price if the old loan shortfall will also be borrowed.

For example, suppose the payoff on a trade-in is $18,000 and its agreed trade value is $15,000. The negative equity is $3,000. If the new transaction otherwise requires $22,000 of financing, adding the shortfall makes the amount financed $25,000, before any other adjustments. Enter $25,000—not $22,000—to estimate the payment. This illustration is arithmetic only; actual figures depend on the transaction documents.

What the Estimate Shows—and What It Leaves Out

The payment result represents principal and interest under the inputs. Total interest is the sum of the rounded monthly payments minus the amount financed; it is not the total cash cost of owning the vehicle. A longer term generally spreads repayment over more months and changes both the payment and cumulative interest. Compare the same amount financed and APR when examining different terms.

Taxes, title or registration charges, dealer or lender fees, service contracts, insurance, and other products may affect the transaction or monthly budget. They are not calculated separately here. If a charge is financed, include it in the loan amount; if it is paid separately, the calculator does not add it to the payment. Ask for an itemized out-the-door price and a written amount-financed figure, and use the lender’s quoted APR and term rather than an assumed rate.

Common Problems and Alternatives

• Using the wrong payoff figure: A loan’s current balance may differ from the payoff amount for a particular date. Get an up-to-date payoff figure from the current loan servicer.
• Assuming trade value equals payoff: Obtain a written trade offer and compare it with the payoff. The difference can change as either figure changes.
• Forgetting cash down or credits: These may reduce the amount financed, but only include amounts confirmed for the transaction.
• Expecting an exact lender quote: Rounding, payment dates, fees, and contract terms can make the actual schedule differ from this estimate.

Rolling negative equity forward does not erase it: it adds the shortfall to the new debt. A reader evaluating the arithmetic can also calculate the new purchase separately, examine the trade and payoff figures independently, or consider whether a private sale or keeping the current vehicle changes the numbers. Those alternatives have their own costs and practical constraints; this calculator does not determine which choice to make.

Frequently Asked Questions

Can I roll negative equity into a new car loan?
It may be possible in a transaction, but whether it can be included depends on the lender and deal details. This calculator only estimates payments after you enter a total financed amount that includes the shortfall.

How do I calculate negative equity on a trade-in?
Subtract the trade-in value from the current loan payoff. If payoff is $18,000 and the trade value is $15,000, the shortfall is $3,000.

Does rolling over negative equity increase my payment?
It increases the amount financed, all else equal, so the standard payment formula produces a higher payment. The exact change depends on APR and term.

Does this calculator include taxes and fees?
No. Add any charges that will be financed to the loan amount you enter; amounts paid separately are not reflected in the payment result.

Why is my lender’s payment different?
The contract may use a different APR, financed amount, term, payment schedule, fees, or rounding method. Compare the itemized figures and use the lender’s written disclosure for the actual terms.

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

Related

• Amortization Schedule XLS: Formula and Example
• Annual Amortization Schedule Calculator and Loan Breakdown
• Amortization Schedule With Start Date Explained

in p | Loan amount | $ | 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 | $ | p * (apr / 1200) / (1 - (1 + apr / 1200) ^ -(yrs * 12)) | 2
out int | Total interest | $ | pmt * yrs * 12 - p | 2