Mortgage Extra Payment Calculator
See how extra monthly or one-time mortgage principal payments can change your payoff time and estimated interest.
Last updated: August 2026
Baseline versus extra principal
Monthly extra-payment examples
Each example includes the currently entered lump sum.
View yearly balance comparison
Balances come from the same full-precision monthly simulations used above.
Planning estimate only. Confirm that your lender applies extra payments to principal and check for servicing instructions or prepayment restrictions. Taxes, insurance, escrow, HOA, mortgage insurance, recasting, refinancing, adjustable rates, and lender-specific rounding are not modeled.
What changes when you pay extra principal?
Extra principal reduces the outstanding mortgage balance sooner. Future monthly interest is then calculated on a smaller balance, which can shorten the estimated payoff period and reduce total interest. The calculator keeps the scheduled principal-and-interest payment unchanged; it does not model a mortgage recast.
How does an extra mortgage payment save interest?
Each month, estimated interest equals the remaining principal multiplied by the monthly interest rate. Principal reduction equals the payment minus that month's interest. The calculator applies the entered extra monthly amount to principal and carries full precision into the next month.
How is the normal mortgage payment estimated?
For a fixed-rate amortizing mortgage, M = P × r(1+r)n ÷ ((1+r)n − 1), where P is remaining principal, r is the monthly interest rate, and n is remaining monthly payments. At 0% interest, principal is divided by the number of months.
The result is scheduled principal and interest only—not necessarily your full housing payment. Property tax, homeowners insurance, HOA fees, mortgage insurance, and escrow are excluded.
What does a lump-sum principal payment do?
The calculator subtracts the one-time amount from today's remaining principal before the first future scheduled payment, then keeps the recurring P&I payment unchanged. A lender-approved recast can change the required payment and is not modeled here.
Assumptions and lender differences
This is a planning estimate using one constant APR, monthly interest, no new fees, and no payment changes. Actual results may differ because of servicing dates, daily-interest details, rounding, payment-application rules, loan modifications, or adjustable rates. Confirm that extra payments are applied to principal as intended and whether prepayment restrictions apply.
Frequently asked questions
How much does an extra $100 a month save on a mortgage?
It depends on the remaining balance, APR, and term. Enter those values with $100 extra principal to compare the normal and accelerated schedules.
Does paying extra on my mortgage reduce interest?
Under this fixed-rate monthly model, applying extra money to principal lowers the balance sooner and therefore reduces future interest.
Should I pay monthly or make a lump sum?
The calculator can model either or both, but it does not recommend a financial strategy.
Does extra principal lower my monthly payment?
Not in this model. The recurring scheduled P&I payment stays the same and the mortgage pays off earlier. Recasting is separate.
Does this include taxes and insurance?
No. It models principal and interest only.
Can I enter a one-time extra payment?
Yes. It is applied immediately to principal before the future monthly simulation.
Why might my lender's result differ?
Payment timing, daily-interest practices, escrow, rounding, payment instructions, prepayment terms, and loan modifications can affect actual results.
What if my mortgage has an adjustable rate?
This calculator assumes the entered APR stays constant. Future rate adjustments are not modeled.