Student Loan Interest Calculator with Extra Payments

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.

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.