Home Loan Amortization Calculator with Extra Payments
This calculator models home loan amortization under multiple scenarios: standard monthly amortization, recurring extra monthly payments, a one-time extra principal payment, and an approximate biweekly payment schedule. Use it to estimate monthly amounts, payoff timing, and interest saved.
Outputs are estimates based on the inputs and assumptions shown below. This tool is for planning and educational purposes and does not replace loan documents, lender payoff quotes, or professional financial advice.
Add a fixed extra amount to each monthly payment and estimate accelerated payoff and interest savings.
Inputs
Results
Adjusted monthly payment
$1,347.13
Months to payoff
360
Total interest (with extra)
$184,968.26
Interest saved
-$0.00
| Output | Value | Unit |
|---|---|---|
| Adjusted monthly payment | $1,347.13 | currency |
| Months to payoff | 360 | months |
| Total interest (with extra) | $184,968.26 | currency |
| Interest saved | -$0.00 | currency |
Visualization
Methodology
Monthly compounding and standard amortization formulas are used unless a biweekly model is selected. Recurring extras are assumed to be applied to principal at each scheduled payment; a closed-form logarithmic solution is used to estimate the reduced payoff period. One-time extras reduce principal at the selected payment date and remaining months are recomputed.
Biweekly simulations convert the loan to the biweekly frequency (26 payments per year) and compute payments with the equivalent periodic rate. This produces an approximation of acceleration; exact schedules depend on lender application of partial payments and posting rules.
Worked examples
If you add $200/month extra on a $300,000, 30-year, 3.5% loan, the calculator shows the new months to payoff and the approximate interest saved versus the standard schedule.
A one-time extra of $5,000 applied at month 12 reduces the principal; the tool estimates the new remaining months and interest paid after that payment.
Key takeaways
Use the recurring extra mode to understand steady extra contributions. Use the one-time mode to evaluate tactical lump-sum payments. Use the biweekly mode as an approximation of accelerated payoff from more frequent payments.
Results are estimates. Lender posting conventions, escrow, fees, or prepayment penalties are not modeled.
Further resources
Expert Q&A
Are results exact?
Results are estimates produced by standard amortization formulas. Actual payoff dates and interest may differ due to lender posting rules, rounding, escrow, fees, or prepayment penalties. Always confirm payoff figures with your lender.
Does the calculator include taxes and insurance?
No. This tool models principal and interest only. Taxes, insurance, HOA fees, and other escrow items are excluded.
What if my lender applies extra payments differently?
Some lenders apply extra payments to future payments or hold them in escrow rather than to principal immediately. The calculator assumes extras are applied directly to principal at each payment; contact your lender to confirm how extras are handled.
Is biweekly exact?
The biweekly mode is an approximation converting to 26 payment periods per year. Exact results depend on lender-specific implementation of biweekly plans.
Sources & citations
- National Institute of Standards and Technology (NIST) — Guidance for software assurance and testing — https://www.nist.gov/
- International Organization for Standardization (ISO) — Information security and software quality standards — https://www.iso.org/
- IEEE — Recommendations for numerical accuracy and floating-point practices — https://www.ieee.org/
- Occupational Safety and Health Administration (OSHA) — General guidance on testing environments (operational safety context) — https://www.osha.gov/