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Auto Loan Early Payoff Calculator — Lump Sum + Extra Monthly Payments

A borrower on Reddit asked: ‘I can’t find a tool/calculator that’ll allow me to add both.’ They had paid $4,500 toward principal and wanted to add $200 each month (r/personalfinance, March 19, 2024). This calculator combines those two moves in one comparison: time left, interest avoided and savings after any prepayment fee. No account or original loan paperwork is required.

How this calculator works

Use the current unpaid principal, the annual contract interest rate and your required monthly payment. Let B be that balance, L the principal-only lump sum paid now, M the required payment, E the monthly extra and r = annual rate / 1200. The accelerated plan starts at B − L. Each month, interest I = remaining balance × r; payment A = min(M + E, remaining balance + I); next balance = remaining balance + I − A. Repeat until paid off. The standard plan uses L = 0 and E = 0. Interest saved = standard interest − accelerated interest; net savings = interest saved − your fee. Total accelerated cash paid = B + accelerated interest + fee, including the lump sum once. The final payment is smaller when necessary. At 0% interest, months = ceiling((B − L) / (M + E)). This is a monthly simple-interest estimate, not a daily payoff quote or a precomputed-interest rebate calculation.

Scenario examples

Extra principal each month ($)Accelerated payoffInterest saved
03 yr 8 mo$1,300.17
503 yr 2 mo$1,518.73
1002 yr 10 mo$1,688.11
2002 yr 4 mo$1,933.96
3002 yr 0 mo$2,103.66

Assumptions: $20,000 current principal, 6% annual interest, $400 required monthly payment, $4,500 lump sum paid now and $0 prepayment fee. These are illustrative inputs, not market rates. All results use the in-browser engine.

FAQ

Can I combine a lump sum and extra monthly payments?

Yes. Enter the one-time principal payment in the lump-sum field and the recurring increase in extra monthly payment. They are applied together, not as separate competing options. The lump sum reduces principal before the first modeled month; the monthly extra then continues until payoff. Leave either field at zero to isolate its effect.

Should I enter my original loan amount or current balance?

Use the current unpaid principal balance from your latest statement, after payments already posted. Do not enter the original car price, total scheduled payments or a payoff quote containing accrued interest. The required monthly payment stays the amount in your existing contract. This models the remaining loan, not interest already paid.

What interest rate belongs in the rate field?

Use the contract annual interest rate used to accrue interest on the balance. If the disclosed APR includes financed or upfront fees, it may differ from that rate. Using APR as a substitute is an approximation. Do not add already financed fees again: they are already part of the principal balance.

How do I account for a prepayment penalty?

Ask the lender for the fee associated with your proposed plan and enter that dollar amount. It is subtracted from interest savings and added to accelerated total cash paid, not financed into the balance. The fee is counted whenever entered. A negative net-savings result means that entered fee exceeds the modeled interest benefit.

Why does my lender show a different payoff amount?

Many auto loans accrue simple interest daily. Month lengths, posting dates, unpaid fees and interest since the last statement change a real settlement amount. This tool assumes equal monthly intervals and principal applied immediately. Request a dated payoff quote before settling the loan, even when the calculator says the remaining principal can be paid now.

Does the calculator work for precomputed-interest loans?

No. In a precomputed or add-on-interest contract, the finance charge is set using a different method and early-settlement rebates depend on the agreement and applicable law. The CFPB distinguishes these contracts from simple-interest loans. Ask the lender which method applies; do not treat this calculator’s savings as a promised refund.

Sources and borrower question

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