Focused calculator
Mortgage Extra Payment Calculator
Compare a baseline mortgage payoff path with a strategy that adds a fixed amount to every ordinary scheduled payment, a repeated annual lump sum, or both. The calculator estimates the change in payoff time and interest under the entered assumptions; it does not determine what a lender permits or calculate a prepayment penalty.
What this calculator compares
The baseline follows the regular principal-and-interest payment for the entered mortgage balance, nominal annual rate, remaining amortization period, and ordinary payment frequency. The strategy follows the same mortgage assumptions and adds the extra payments entered.
An extra payment here means either a fixed dollar amount added after every scheduled payment, the same lump sum applied after each complete model year, or both. The primary result is the estimated payoff time saved. The comparison also shows how total interest and the total amount paid change.
Extra-payment strategy and contract limits
Enter the current mortgage balance, the nominal annual mortgage rate, a remaining amortization period that can be expressed as a whole number of months, and an ordinary payment frequency. The entered rate remains constant throughout both payoff paths.
For an extra amount on every scheduled payment, enter a fixed dollar amount. It begins with the first payment, repeats at every payment, and is applied after the scheduled principal-and-interest payment. Leave it blank or enter zero if this strategy is not included.
For an annual lump sum, enter the fixed amount to apply after every complete model year. A model year begins with the first payment and contains 12 monthly, 24 twice-monthly, 26 biweekly, or 52 weekly payment periods. The same amount repeats after each complete model year. Leave it blank or enter zero if this strategy is not included.
Optional dollar limits may be entered for the maximum extra amount permitted with each scheduled payment and the maximum annual lump sum. A blank limit means no contractual limit was supplied and the calculator does not check one; zero means the entered contractual limit is zero. If a strategy exceeds a supplied limit, the strategy cannot be calculated until the conflicting amount or limit is corrected. The calculator does not reduce the strategy silently.
Mortgage contracts express prepayment privileges in different ways. Because a percentage may be based on the original mortgage amount or another contractual amount, this calculator does not infer a dollar limit from a percentage. Limit checking therefore uses the dollar amounts shown in the mortgage contract or provided by the lender.
The payment-frequency choices are ordinary, non-accelerated arrangements. Ordinary biweekly and weekly payments are different from accelerated biweekly and accelerated weekly payments, which use higher payment amounts and are not modelled here.
How the comparison is calculated
- Calculate the canonical monthly principal-and-interest payment from the entered nominal annual rate with semi-annual compounding and the remaining amortization in whole months. Use the unrounded monthly payment as the basis for ordinary frequencies: monthly uses it as-is; twice monthly divides it by 2; biweekly multiplies it by 12 and divides by 26; weekly multiplies it by 12 and divides by 52.
- For the baseline, convert the entered rate to the periodic rate for the selected frequency. At the end of each period, calculate interest on the opening balance, apply the scheduled payment to interest and principal, and continue until the mortgage is paid off.
- For the strategy, follow the same schedule and then apply the fixed extra amount after each scheduled payment. The amount actually applied is limited only by the remaining balance.
- After every complete model year, apply the entered annual lump sum after that period's scheduled and regular extra payments. The same lump sum repeats annually. If the mortgage is paid off before an annual event, no lump sum is applied. If the remaining amount due is lower than the entered lump sum, apply only the amount needed to pay it off.
- The calculation does not carry any unused portion of a supplied annual limit into a later model year. This is an OpenBook modelling assumption and does not establish how a lender applies a contractual privilege.
- Continue both paths until actual payoff, allowing a smaller final payment. Compare their actual payment counts, elapsed payoff time, total interest, and total amount paid. A strategy with no extra payments is a valid no-change comparison.
- Retain unrounded precision during the calculation and display public currency values to cents. Separately rounded displayed amounts may not add exactly to a displayed total.
The timing and repetition of the annual lump sum are modelling assumptions. A lender may allow prepayments only on certain dates, apply a different annual period, impose minimum or maximum amounts, or use other contractual rules.
Example
Suppose the current mortgage balance is $500,000, the nominal annual rate is 5%, the remaining amortization period is 25 years, and ordinary biweekly payments are selected. The scheduled payment is about $1,342.17. Without extra payments, the illustration reaches payoff in 649 payments, with a final payment of about $779.23 and total interest of about $370,502.38.
Now add $200 after every biweekly payment and a $30,000 lump sum after every complete model year. The annual amount is assumed to repeat each year. Under these assumptions, the strategy reaches payoff in 234 payment periods, or 9 years, and total interest is about $125,086.01. The estimated payoff time saved is about 15 years and 11.5 months, and estimated interest saved is about $245,416.37.
Across the strategy, $46,800 of regular extra payments and about $264,219.31 of annual lump sums are actually applied. Only about $24,219.31 of the final annual lump sum is needed. Including the scheduled payment and the $200 extra amount, the final payoff-period total is about $25,761.48. The strategy's total amount paid is about $625,086.01. No contract limits are entered in this example, so it does not determine whether a lender would permit the strategy.
How to interpret the result and limits
The result compares two calculated payoff paths; it is not a lender quote, mortgage statement, contract interpretation, or recommendation to make extra payments. The time and interest differences depend on the entered assumptions.
Prepayment privileges, permitted payment increases, lump-sum amounts, calculation bases, eligible dates, minimums, maximums, and unused-privilege rules vary by mortgage contract and lender. An amount above a permitted privilege may result in a prepayment penalty. This calculator neither determines nor calculates that penalty.
The illustration assumes the same extra dollar amount is added to every scheduled payment and the same annual lump sum repeats after each complete model year. It does not model a one-time dated lump sum, changing extra amounts, skipped payments, carryforward of unused privilege, or lender-specific transaction dates.
The entered interest rate remains constant for the entire illustration. A mortgage commonly spans several contractual terms, and its rate, payment, prepayment terms, and other conditions may change at renewal.
The calculator does not model accelerated weekly or biweekly payment frequencies, property taxes, insurance, condominium fees, other ownership costs, mortgage qualification or affordability, or lender-specific day-count or rounding practices.
The final scheduled payment, regular extra payment, or annual lump sum may be smaller than the amount entered because the calculation applies only what remains due. Displayed amounts are rounded to cents while internal calculations retain greater precision.
This calculator provides educational information only. It does not provide mortgage, borrowing, legal, tax, investment, or other professional advice. Applicable payment and prepayment terms are determined by the mortgage contract and lender records.