Student Loan Interest Calculator with Extra Payments
This calculator models fixed-rate student loan amortization and the effect of recurring extra payments or a one-time lump-sum payment. Use it to compare scheduled payments, total interest, and estimated time-to-payoff.
It is intended for planning and illustration. Results are approximate; actual lender processing (timing, application to principal vs future payments, fees) can change outcomes. See methodology and citations for accuracy notes and standards used in implementation.
Computes the periodic payment, total paid, and total interest for a standard fixed-rate amortizing loan.
Inputs
Results
Periodic payment
$318.20
Number of payments
120
Total paid
$38,183.59
Total interest
$8,183.59
| Output | Value | Unit |
|---|---|---|
| Periodic payment | $318.20 | currency |
| Number of payments | 120 | payments |
| Total paid | $38,183.59 | currency |
| Total interest | $8,183.59 | currency |
Visualization
Methodology
Amortization is calculated using standard fixed-rate formulas: a periodic rate is derived from the APR and payments per year, then standard present-value formulas determine the periodic payment and remaining balance.
When extras are applied, the calculator estimates a new payoff schedule. For recurring extras it solves for the number of payments required given an increased periodic payment. For lump sums it computes the estimated balance at the application time, subtracts the lump amount, and then recomputes remaining payments.
Implementation and numerical behavior follow best practices for numeric stability and safety: tiny positive bounds are applied to denominators to avoid divide-by-zero and domain errors. The software and documentation are developed in alignment with quality and security standards such as ISO 9001 for process quality, IEEE floating-point guidance (IEEE 754) for numeric handling, and NIST recommendations for secure software practices. User safety and clarity follow general workplace safety and accessibility expectations as informed by OSHA guidance.
Worked examples
Example: $30,000 loan, 5% APR, 10 years, monthly payments. The tool shows scheduled monthly payment, total interest, and total paid. Adding $100 extra per month shows reduced payoff time and interest saved.
Example: Same loan with a $5,000 lump-sum applied after 12 payments. The tool estimates the remaining principal after the lump and the new payoff timeline and interest.
Further resources
External guidance
Expert Q&A
Are these results exact for my loan?
These are estimations using standard amortization formulas. Actual results may differ because lenders can apply extra payments differently (immediately to principal, to future payments, or hold in suspense), and there may be fees or payment timing differences. Always confirm with your servicer for exact payoff figures.
Does the calculator handle variable rates or income-driven plans?
No. This tool models fixed-rate amortization only. It does not model variable interest rates, graduated payments, or income-driven repayment formulas. For plans with recertification or income adjustments, consult plan-specific calculators or your loan servicer.
How should I interpret interest saved?
Interest saved values are approximations comparing the modeled schedule with extras to the modeled original schedule. They assume all payments are applied as scheduled and that there are no prepayment penalties or fees.
What about rounding and currency?
Displayed monetary values are rounded for readability. Under the hood calculations use numeric safeguards but final payment amounts on a statement may differ by cents due to lender rounding rules.
What safety and accuracy practices were followed?
Numeric safeguards (small positive bounds on denominators, max/min to avoid domain errors) are applied. Development and documentation reference ISO quality principles, IEEE numeric handling guidance, and NIST secure development recommendations. See citations for authoritative sources.
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/
- Federal Student Aid (official guidance on repayment options) — https://studentaid.gov/