Loan Overpayment Calculator, See Exactly What You Save
This loan overpayment calculator shows exactly how much interest and time you save by paying extra toward your mortgage, auto loan, or personal loan each month. Compare your original payoff schedule against an overpayment scenario side by side, with a visual balance chart and a full amortization table.
The one time extra payment is applied in the month you choose below, on top of any recurring extra monthly payment.
How the Loan Overpayment Calculator Works
This loan overpayment calculator runs a full month by month amortization schedule twice, once with your original payment and once with your extra payments added, then compares the two side by side. It works for mortgages, auto loans, personal loans, or any fixed rate installment loan with monthly compounding.
Every extra dollar you pay goes straight toward the principal balance rather than future interest, which is what makes overpaying so effective early in a loan. Because interest is calculated on the remaining balance each month, a lower balance today means less interest charged for every month that follows, compounding your savings the earlier you start.
The calculation runs entirely in your browser using standard amortization formulas. Your loan amount, rate, and payment details are never sent to a server or stored anywhere.
What This Tool Does
How to Use It
Enter loan details
Add your loan amount, annual interest rate, and term in years, plus the date your loan started or will start.
Set overpayment
Enter an extra monthly amount, a one time lump sum, or both, and choose which month the lump sum applies.
Calculate
Click Calculate My Savings to see the two schedules compared, along with the balance chart.
Export
Download the complete month by month schedule as a CSV file for your records or to share with a partner.
The Amortization Formula
This is the same standard formula banks and mortgage calculators use to compute your fixed monthly payment.
M = P × [ r(1+r)^n ] / [ (1+r)^n − 1 ]
P = loan principal
r = monthly interest rate (annual rate ÷ 12)
n = total number of monthly payments
Every month, the interest portion of your payment equals the remaining balance times the monthly rate. Whatever is left of the payment reduces principal. Extra payments reduce principal directly, without changing the required minimum payment.
Why Overpaying Early Saves the Most
Interest compounds on balance
Interest is recalculated on your current balance every month, so a smaller balance today reduces every future interest charge, not just one payment.
Early payments carry more weight
Because loans front load interest, extra principal paid in year one saves more total interest than the same amount paid in year twenty.
Consistency compounds too
A modest but consistent extra monthly payment often outperforms a single large lump sum paid later, since it starts reducing interest sooner.
FAQ: Loan Overpayment Calculator
It depends on your rate, balance, and how early you start, but the effect is often larger than people expect. On a $300,000 mortgage at 6.5% over 30 years, an extra $200 a month typically saves tens of thousands of dollars in interest and cuts several years off the loan. Enter your own numbers into the calculator above to see your exact figures, since even small differences in rate or balance change the outcome meaningfully.
Not always, and this is the single most common mistake with loan overpayments. Some lenders apply extra funds to next month’s payment instead of the principal balance, which defeats the purpose entirely. Always contact your loan servicer or check your account settings to confirm extra payments are marked “apply to principal” and not “advance payment due date.” This calculator assumes every extra dollar reduces principal immediately, which is the scenario that produces real interest savings.
Mathematically, paying extra sooner always saves more interest than paying the same amount later, because interest compounds on a lower balance for longer. A steady monthly overpayment tends to build savings gradually and predictably, while a lump sum (a bonus or tax refund, for example) delivers a bigger one time drop in balance. Try both scenarios in the calculator above, entering the lump sum in the “One-Time Extra Payment” field, to compare the exact interest and time saved for your situation.
Some loans, particularly certain mortgages, auto loans, and personal loans from specific lenders, include a prepayment penalty clause that charges a fee for paying off the balance faster than scheduled. Most standard conforming mortgages in the United States do not carry this penalty today, but it is worth checking your loan agreement or asking your lender directly before making large overpayments, since a penalty could offset some of the interest savings this calculator projects.
This depends on your loan’s interest rate compared to the return you could reasonably expect from investing. If your loan rate is higher than what you would likely earn after tax in the market, paying down the loan is the mathematically safer, guaranteed return. If your rate is low, investing may produce a higher long term return, though with more risk and less certainty. Many people split the difference, overpaying a modest amount while also investing, which this calculator can help you model by testing a few different extra payment amounts.
Enter the amount in the “One-Time Extra Payment” field and the month number it applies to, where month 1 is your first payment. The calculator adds that amount directly to principal in that specific month, on top of any recurring monthly overpayment you have set, then recalculates the entire schedule from that point forward with the reduced balance.
Yes. This calculator works for any fixed rate, fixed term loan with monthly compounding, including mortgages, auto loans, personal loans, and student loans with a fixed rate. Simply enter the loan amount, interest rate, and term for whichever loan you are analysing. It is not designed for variable rate loans or loans with irregular payment schedules, where the actual interest calculation may differ from the standard amortization formula used here.
No. Every calculation, including the full amortization schedule and CSV export, is generated locally in your browser using JavaScript. Your loan amount, interest rate, and payment details are never transmitted to a server, logged, or stored anywhere.