Mortgage Prepayment Calculator
Created by: Liam Turner
Last updated:
Enter your outstanding mortgage balance, interest rate, and remaining term to see exactly how much sooner you could pay off your home — and how much interest you would save — by adding extra monthly payments or a one-time lump sum.
Mortgage Prepayment Calculator
FinanceSee how extra monthly payments or a one-time lump sum reduce your mortgage payoff date and total interest — with a sensitivity table for common extra-payment amounts.
The Power of Mortgage Prepayments
On a typical 30-year mortgage at today's rates, you will pay nearly as much in total interest as the original loan amount by the time you make your final payment.
A mortgage prepayment calculator shows you exactly how much of that interest you can eliminate by making additional principal payments — and how many years you can cut from your loan term.
Mortgage prepayment is one of the highest-certainty returns available to homeowners.
Unlike investing in the stock market, where returns fluctuate, every dollar of extra principal you pay reduces your outstanding balance by exactly one dollar and eliminates the interest that dollar would have generated over every remaining month of the loan.
The return is exactly equal to your mortgage rate — guaranteed.
How the Prepayment Calculation Works
The calculator first computes your regular monthly payment using the standard amortization formula based on your current balance, rate, and remaining term.
It then runs two parallel month-by-month simulations: one with only the regular payment (the baseline), and one with your extra monthly payment and any lump sum applied from day one.
Each simulation tracks the outstanding balance month by month until it reaches zero.
The difference in the number of months between the two scenarios is your "months saved," and the difference in total interest paid is your "interest saved." The sensitivity table runs this comparison for four standard extra-payment levels so you can pick the amount that fits your budget.
Mortgage Prepayment Formulas
Regular payment = P × r(1+r)^n / ((1+r)^n − 1)
where P = outstanding balance, r = monthly rate (annual rate ÷ 12), n = remaining months
Each month: Interest = Balance × r; Principal = Payment − Interest; Balance = Balance − Principal
With extra payment: Balance = Balance − (Payment + Extra)
Interest saved = Total interest (no extra) − Total interest (with extra)
Example Scenarios
$280,000 Balance / 6.75% / 25 Years Remaining
Outstanding balance: $280,000. Rate: 6.75%. Remaining term: 25 years (300 months). Regular monthly payment: $1,923. Baseline total interest: $296,953. Adding $250/month ($2,173 total payment): payoff in 21 years 4 months, saves 44 months and $67,400 in interest. Adding $500/month: payoff in 18 years 7 months, saves 76 months and $108,300 in interest.
Lump Sum: $15,000 One-Time Payment
Same loan as above ($280,000, 6.75%, 25 years). Applying a $15,000 lump sum today reduces the starting balance to $265,000. Result: payoff 16 months earlier, saving $34,200 in interest — significantly more than the same $15,000 spread over 25 months as a monthly extra, because the full amount starts compounding against the balance immediately.
How People Use This Calculator
- Homeowners who have received a bonus, inheritance, or tax refund and want to see the impact of a one-time principal payment.
- Buyers comparing 15-year versus 30-year mortgages who want to model how extra payments on the 30-year can replicate the 15-year payoff timeline at a lower required payment.
- Anyone approaching retirement who wants to be mortgage-free by a specific date and needs to know how much extra to pay each month to hit that target.
- Refinancing evaluators who want to compare the long-term cost of a refi (closing costs + new interest) against an aggressive prepayment plan on the existing loan.
- Couples building a shared financial plan who want to see what happens to their mortgage if one partner's income temporarily increases.
Maximizing the Impact of Prepayments
Always specify to your servicer that extra payments are to be applied to principal, not to prepay future installments.
Some servicers — particularly older ones — will apply surplus payments to advance your next due date rather than reduce your balance, which delays the compounding benefit of principal reduction.
If you are early in a 30-year mortgage, prepayments have the highest leverage.
In the first several years, nearly all of each payment goes to interest.
An extra $200 in year one eliminates all the future interest that would have accrued on that $200 over the remaining term — potentially $400+ in interest savings from a single extra payment.
The further into the loan you are, the smaller the leverage because the remaining balance (and thus the interest base) is already declining.
Frequently Asked Questions
How does an extra mortgage payment save so much interest?
Mortgage interest is calculated on your outstanding balance each month. When you reduce the principal early, every future month accrues less interest. On a 30-year mortgage, an extra $200/month in the early years can save tens of thousands of dollars because it eliminates years of compound interest charges from the tail end of the loan.
Should I apply extra payments as lump sums or monthly additions?
A lump sum applied today saves more than the same total amount spread over many months because the principal reduction starts compounding sooner. That said, consistent monthly extra payments are more manageable and still dramatically effective over time.
Do I need to tell my lender to apply extra payments to principal?
Yes — this is critical. Contact your servicer or include a note with your payment specifying that any amount above the required payment should be applied to principal. Without instruction, some servicers apply extra amounts to the next month's payment, which does not reduce principal early.
What if I have a prepayment penalty?
Some mortgages — especially older ones or certain ARM products — include prepayment penalties for paying off the loan early. Check your loan documents or call your servicer before making large prepayments to avoid unexpected charges.
Is it better to prepay my mortgage or invest the extra money?
This depends on your mortgage rate versus expected investment returns. If your mortgage rate is 7% and you can earn 10% in the market, investing is mathematically better — but prepayment provides a guaranteed, risk-free return equal to your mortgage rate. Your risk tolerance and peace-of-mind factor matter.
Can I pay biweekly instead of monthly?
Biweekly payments result in 26 half-payments per year, equivalent to 13 full monthly payments — one extra per year. This alone can reduce a 30-year mortgage by several years. Many servicers offer biweekly programs; check for any associated fees.
Sources and References
- Freddie Mac. "Understanding Your Mortgage Amortization." FreddieMac.com, 2024.
- Consumer Financial Protection Bureau. "What is Amortization?" CFPB, consumerfinance.gov, 2023.
- Board of Governors of the Federal Reserve System. "Consumer's Guide to Mortgage Refinancings." federalreserve.gov, 2022.