Mortgage Amortization Calculator with Extra Payments
This calculator models mortgage amortization and shows how recurring extra payments or a single lump-sum principal payment change payoff timing and total interest paid. It produces conservative numeric estimates, highlights assumptions, and surfaces how results were calculated.
Use the recurring extra payments option to see the effect of adding a fixed amount to each scheduled payment. Use the lump sum option to estimate the immediate effect of applying a one-time principal payment.
Estimates the effect of recurring extra payments or a single lump-sum payment on payoff timing and interest paid. Provides conservative numeric estimates and identifies assumptions and rounding limits.
Inputs
Advanced inputs
Recurring extra payment
Lump sum payment
Results
Payment including extra
$97.57
Estimated years to payoff
—
Estimated interest with extra payments
—
Estimated interest saved
—
| Output | Value | Unit |
|---|---|---|
| Payment including extra | $97.57 | currency |
| Estimated years to payoff | — | years |
| Estimated interest with extra payments | — | currency |
| Estimated interest saved | — | currency |
Visualization
Methodology
Calculations use standard amortization formulas: the periodic rate is APR divided by the number of payments per year. The scheduled payment is computed from the loan principal, periodic rate, and number of payments using the standard annuity formula.
For recurring extras, we model an increased periodic payment (scheduled payment plus the extra). The estimated payoff number of payments is computed by solving the annuity equation for the remaining term; natural logarithms are used to invert the geometric series. For a lump sum applied immediately, we subtract the lump from principal and recompute the remaining term using the same approach.
Numerical results are approximate. Rounding, payment timing conventions, escrow, fees, variable-rate resets, prepayment penalties, and lender-specific posting rules are not modeled. Always verify results with your lender and account statements.
Key takeaways
This advanced calculator supports three scenarios: scheduled payments only, recurring extra payments, and a one-time lump-sum payment. It reports estimated payoff duration and estimated interest saved.
Results are estimates. Confirm with your lender before making financial decisions.
Further resources
External guidance
Expert Q&A
Are results exact for every lender?
No. This tool provides mathematical estimates. Actual payoff timing and interest depend on lender posting rules, daily interest accrual conventions, potential prepayment penalties, escrow adjustments, and whether payments are applied to principal immediately. Use results for planning and verify with your lender.
Does the calculator model adjustable-rate mortgages or interest-only periods?
Not in this version. This tool assumes a fixed APR for the modeled term and that payments are applied according to the standard amortization schedule. For adjustable-rate loans or interest-only periods, consult a lender-specific schedule or an advisor.
How accurate are the logarithm/inversion formulas?
Formulas use standard algebraic inversion of the annuity equation. Results are subject to floating-point rounding; see the accuracy note. For very small extra payments close to zero, the estimated payoff may be numerically unstable.
Is this financial advice?
No. This is an educational calculator. It does not replace professional financial, tax, or legal advice.
Sources & citations
- IEEE floating-point standard (information) — https://standards.ieee.org
- National Institute of Standards and Technology (NIST) — https://www.nist.gov
- International Organization for Standardization (ISO) — https://www.iso.org
- Occupational Safety and Health Administration (OSHA) — https://www.osha.gov