Calculator inputs
Use the original loan details, then add any extra payment strategy you want to test.
Example data table
| Example field | Sample value |
|---|---|
| Original loan amount | $350,000.00 |
| Annual interest rate | 6.50% |
| Original term | 30 years |
| First payment date | Jan 15, 2023 |
| Payments already made | 36 |
| Extra monthly payment | $250.00 |
| Annual lump-sum extra | $2,000.00 in December |
| One-time extra payment | $5,000.00 at payment 6 |
| Estimated current balance | $337,460.35 |
| Standard payoff month | Dec 2052 |
| Accelerated payoff month | Dec 2043 |
| Months saved | 108 |
| Estimated interest saved | $147,081.82 |
Formula used
1) Standard monthly payment
Payment = P × r ÷ (1 - (1 + r)^(-n))
Where P is the original principal, r is the monthly interest rate, and n is the total number of monthly payments.
2) Monthly interest
Interest = Current Balance × r
3) Principal repaid each month
Principal = Total Payment - Interest
4) Extra principal strategy
Adjusted Principal = Principal + Monthly Extra + Annual Extra + One-Time Extra
5) New balance
New Balance = Current Balance - Adjusted Principal
6) Time saved
Months Saved = Standard Remaining Months - Accelerated Remaining Months
7) Interest saved
Interest Saved = Standard Remaining Interest - Accelerated Remaining Interest
If the rate is 0%, the payment becomes principal divided by total months. All extra payments are treated as direct principal reduction.
How to use this calculator
- Enter the original mortgage amount, rate, term, and first payment date.
- Add how many payments you have already completed.
- Choose your extra payoff plan, such as monthly, annual, or one-time principal payments.
- Click Calculate payoff to see the estimated current balance, payoff dates, months saved, and interest savings.
- Review the chart to compare standard and accelerated balances over time.
- Download the full accelerated amortization plan in CSV or PDF for planning, sharing, or recordkeeping.
Frequently asked questions
1) What counts as an extra payment?
Any amount paid above the scheduled monthly payment can count. This calculator treats extra amounts as direct principal reduction, which helps shorten the loan and reduce future interest charges.
2) Does this include taxes, insurance, or HOA dues?
No. It focuses on principal and interest only. Escrow items like property taxes, homeowners insurance, mortgage insurance, and HOA fees are not included in payoff savings.
3) Why is my current balance estimated?
The estimate is based on your original loan details and payment count. Your servicer balance may differ slightly because of payment timing, escrow adjustments, rounding, or prior extra payments.
4) Can one-time lump sums make a big difference?
Yes. Large one-time principal payments early in the remaining schedule often reduce interest meaningfully because future interest is calculated on a smaller balance.
5) What happens if my interest rate is 0%?
The calculator divides the balance evenly across the remaining months. Extra payments still shorten the schedule, but there is no interest-saving component because the loan has no finance charge.
6) Should I check for prepayment penalties?
Yes. Some loans include prepayment penalties or servicing rules for applying extra funds. Review your note or contact your lender before relying on any accelerated payoff strategy.
7) Why might my lender’s payoff date differ slightly?
Lenders may apply payments on different dates, use exact daily interest, or post partial payments differently. This tool gives a strong estimate, not a legal payoff quote.
8) Is monthly extra better than annual extra?
Monthly extras usually reduce interest sooner because principal falls earlier. Annual lump sums can still work very well, especially when they align with bonuses, refunds, or irregular cash flow.