How the extra-payment comparison works
This calculator compares a normal fixed-rate amortization schedule with a second schedule that applies the same extra principal amount every month.
Two schedules are calculated from the same starting balance, rate and original term. The baseline schedule uses only the required principal-and-interest payment. The accelerated schedule adds the entered extra principal after interest is calculated each month, capped so the model never pays more principal than remains. The calculator then compares payoff month and total interest between the two schedules.
Standard fully amortizing monthly paymentProvides the reference schedule.
Baseline payment + entered extra principalThe extra amount is treated as principal in every modeled month until payoff.
Baseline total interest − accelerated total interestShows the modeled reduction in interest from retiring principal earlier.
Use your remaining balance, note rate and remaining term.
Enter only the extra amount you realistically expect to apply to principal each month.
Compare months saved and interest saved—not just the larger monthly outflow.
Confirm with the servicer how extra funds must be labeled and whether any contractual restrictions apply.
Best used for
- Planning recurring extra-principal payments
- Estimating a target payoff date
- Comparing prepayment with keeping cash available for other goals
What this tool does not answer
- The model assumes the same extra amount every month.
- It does not value alternative investment returns or emergency-fund needs.
- Actual servicer posting dates, curtailment rules and prepayment terms are outside the model.
How to read a sample result
Illustrative example: on $300,000 at 6% with 360 months remaining, adding $200 of principal each month raises the modeled payment from about $1,798.65 to $1,998.65, shortens payoff by about 81 months and reduces modeled interest by about $91,173.43.
The methodology below explains how this calculator treats the inputs and assumptions. Replace the sample values with your own planning figures. Results are estimates, not quotes, approvals, disclosures or recommendations.
Inputs and units
Principal, rate, term and extra principal per month. All currency entries are U.S. dollars. Percentage fields take a number such as 6 for six percent, not 0.06. Repayment terms marked in months use whole months; horizons marked in years use whole years.
Calculation workspace
Enter your own figures in the calculator above. The starting values are examples only. No personal identifiers are needed to run a scenario.
How to interpret the result
Read every result together with the assumptions you entered. Changing an input changes the scenario, not product availability. Displayed amounts are rounded for readability.
Assumptions and limitations
Check the loan’s prepayment terms and principal-application instructions. The calculator does not send a payment.
Review more than one scenario
Run a base case and then change one uncertain input at a time. Compare a shorter holding period, a higher cost or a different repayment term where those inputs are supported. Record which costs have not been entered. A precise-looking number can still rest on uncertain assumptions.
Invalid inputs and error recovery
If an input is missing or outside the calculator’s supported range, the page will ask you to correct it. Use negative values only in fields that explicitly allow them. A calculation error is not a credit decision.
Model documentation
The calculator methodology explains the inputs, assumptions and supported models. Use the result for planning and compare it with the provider’s written terms and applicable disclosures before making a decision.
Choose a product for your calculation
Use these comparisons for planning; final eligibility, pricing and terms are set by the provider.
Extra principal
The model applies the chosen additional amount to principal each month. Confirm the real servicer’s application rules and any relevant prepayment terms.
Program references and comparison sources
CFPB: Loan Estimate explainer — Comparison of covered mortgage disclosures, cash to close, points, lender credits and monthly costs.
CFPB: Understand the different kinds of loans available — Loan structures, terms and repayment risks; not provider-specific eligibility.
Check the linked source for its effective date and applicable scope. A source link is not a lender partnership or an individual offer.