Extra Payment Loan Payoff Calculator
Add extra principal to any loan and see your new payoff date, interest saved, and a complete amortization schedule.
An extra-payment loan calculator shows how paying more than your scheduled payment changes any fixed-rate amortizing loan — an auto loan, personal loan, student loan, or mortgage. On these loans, each payment is split between interest on the current balance and principal that reduces it. Anything you pay above the scheduled amount goes entirely to principal, so it permanently lowers the balance that interest is charged on. The result is always the same pair of outcomes: the loan finishes early, and the total interest falls.
One point worth being precise about: on most fixed loans, extra principal shortens the term while your scheduled monthly payment stays the same. You reach zero sooner; the contractual payment does not drop. The exception is recasting (re-amortization), which some mortgage servicers offer for a fee after a large lump sum. This tool models the standard case — same payment, shorter term, less interest — and builds the complete month-by-month schedule so you can see exactly how the balance falls.
How it's calculated
The scheduled payment on a fixed-rate loan comes from the standard annuity formula:
Payment = P · r / (1 − (1 + r)−n)
where P is the balance, r is the monthly rate
(annual rate ÷ 12), and n is the number of payments remaining.
The calculator then simulates the loan month by month: interest of
balance × r accrues, the scheduled payment plus your extra is
applied, and the reduced balance carries forward. The final payment is
trimmed so the balance lands exactly on zero. Full formulas are on our
methodology page.
A note on auto loans specifically: most U.S. auto loans are simple-interest loans where interest accrues daily on the outstanding balance rather than monthly. The monthly model here is a close approximation — and the daily-accrual structure means an extra payment starts saving interest the day it posts. The CFPB's explainer on amortization and auto loans covers how the interest-vs-principal split shifts over the life of the loan.
Worked example: $25,000 auto loan at 9% over 60 months
Take the calculator's default scenario: a $25,000 balance at 9% APR with 60 months remaining. The scheduled payment is $518.96, and riding out the full schedule costs about $6,138 in interest. In the first month alone, interest is $25,000 × (0.09 ÷ 12) = $187.50, so only $331 of that first payment reduces the balance. Here is what recurring extras do, computed with the same engine as the tool above:
| Extra per month | Payoff time | Total interest | Interest saved | Time saved |
|---|---|---|---|---|
| $0 (baseline) | 60 months | $6,138 | — | — |
| $50 | 54 months | $5,444 | $694 | 6 months |
| $150 | 45 months | $4,446 | $1,691 | 15 months |
| $300 | 35 months | $3,498 | $2,640 | 25 months |
The $150 row is a useful benchmark: it pays the loan off 15 months early and saves $1,691 — about 28% of the loan's total interest cost — for an extra commitment of roughly 29% on top of the scheduled payment. Notice the pattern across rows: savings grow with the extra amount, but not linearly, because each additional dollar attacks a balance that earlier dollars have already shrunk. The higher your loan's rate and the earlier in the term you start, the more each extra dollar is worth. Enter your own balance, rate, and term above for exact figures and the full amortization schedule.
When should you use this calculator?
Use it before committing extra money to any fixed-payment loan, to see the exact payoff date and interest saving a given extra amount buys. Typical cases: sizing a monthly extra to hit a target payoff date — say, finishing a 60-month auto loan before a planned trade-in; comparing where an extra $100 does the most mechanical good across several loans by running each one through the tool; checking how much of your current payment is going to interest versus principal at this point in the term; or verifying a lender's payoff quote against an independent schedule.
The calculator answers the cost question — what a prepayment strategy does to this loan's timeline and interest. It does not answer whether that money would serve you better elsewhere, such as higher-rate debt, an emergency fund, or investments. Prepaying a loan earns a guaranteed return equal to the loan's rate; how that compares to your alternatives is a judgment this tool leaves to you.
Assumptions and limitations
- The rate is fixed and interest compounds monthly. Daily simple-interest loans (most auto loans) will differ slightly; the direction of the savings is the same.
- Extra amounts are applied to principal with each scheduled payment. Confirm your lender does not treat extras as an advance on future installments.
- No prepayment penalty is modeled. Check your agreement — the CFPB explains how prepayment penalties work.
- Fees, late charges, deferred interest, and variable rates are not modeled.
- Recasting to a lower payment is not modeled; the payment is assumed constant.
- Results are estimates for planning; lender day-count conventions can shift figures slightly.
This page is educational content about how amortizing loans respond to extra payments. It is not financial advice and does not consider your personal circumstances. For decisions that matter, consult a qualified professional.
Last reviewed: 2026-07-09