Student Loan Amortization Calculator with Extra Payments
This tool estimates how extra payments — either recurring contributions or a one-time lump sum — affect the amortization of a fixed-rate student loan. Use it to gauge payoff time, interest savings, and how different payment strategies change your total cost.
Results are estimates based on fixed-rate math and typical amortization formulas. The tool does not model loan-specific rules such as capitalization timing, variable interest rates, borrower benefits, institutional fees, or income-driven repayment enrollments.
Models the effect of adding a fixed extra payment each period on payoff time and interest. Assumes extra payment is applied to principal in each period and borrower keeps scheduled payment amount plus extra.
Inputs
Advanced inputs
Recurring extra payment
One-time lump sum
Results
Scheduled payment
$325.58
Payment with extra
$375.58
Months to payoff (estimated)
99.6895
Total interest (estimated)
$7,441.28
Interest saved (estimated)
$1,628.18
| Output | Value | Unit |
|---|---|---|
| Scheduled payment | $325.58 | — |
| Payment with extra | $375.58 | — |
| Months to payoff (estimated) | 99.6895 | months |
| Total interest (estimated) | $7,441.28 | — |
| Interest saved (estimated) | $1,628.18 | — |
Visualization
Methodology
Scheduled payment is computed using the standard fixed-rate annuity formula: payment = r * principal / (1 - (1 + r)^-n), where r is the periodic rate and n is the scheduled number of payments.
Recurring extra payments are applied each period in addition to the scheduled payment and reduce principal faster. Estimated payoff months with extra payments are calculated by solving the geometric progression for remaining balance using logarithms.
A one-time lump-sum reduces principal at the chosen payment index. Remaining payoff is estimated by computing the balance immediately before the lump payment, subtracting the lump amount, and re-solving the amortization equation for remaining periods under the assumption the scheduled payment amount is maintained.
Key takeaways
This advanced amortization calculator supports three scenarios: standard amortization, recurring extra payments, and a single lump-sum payment. It provides estimated payoff timing, total interest, and interest savings using closed-form amortization formulas.
Outputs are estimates. For exact payoff figures and instructions on applying extra payments, contact your loan servicer. Use this tool to compare strategies and approximate savings before making payment decisions.
Further resources
Expert Q&A
How accurate are these estimates?
Estimates use standard amortization math and continuous formulas for payoff time. They are accurate for fixed-rate loans with payments applied each period and no fees or balance capitalization events. Edge cases such as payment totals that are less than or equal to the interest due per period can produce invalid estimates; the tool warns when inputs make payoff impossible without increasing payment.
Does this model income-driven repayment, loan forgiveness, or variable rates?
No. This calculator assumes a fixed interest rate and scheduled payments. Income-driven plans, forgiveness programs, variable rates, deferment, forbearance, payment exemptions, or capitalization rules are not modeled. Consult your loan servicer or plan documents for those scenarios.
Are rounding and payment timing included?
Results are shown as decimal estimates for payoff months and currency values rounded for display. The model assumes payments occur each period on schedule. Real-world rounding, payment timing differences, and servicer policies can change exact amounts; treat outputs as close estimates, not billing statements.
What if an extra payment would pay off the loan early?
If an extra payment would fully repay the balance before the next scheduled payment, the model may compute a fractional number of periods. Many lenders will accept a final smaller payment; check with your servicer for exact payoff handling.
Sources & citations
- NIST (National Institute of Standards and Technology) — https://www.nist.gov/
- ISO (International Organization for Standardization) — https://www.iso.org/
- IEEE (Institute of Electrical and Electronics Engineers) — https://www.ieee.org/
- OSHA (Occupational Safety and Health Administration) — https://www.osha.gov/