Personal Loan Payment Calculator with Extra Payments

Calculations use standard amortization formulas: periodic rate r = annual_rate divided by payment_frequency, number of periods n = term_years × payment_frequency, and the annuity formula for level payments when r is above zero. For a zero rate a simple division is used.

Number of payments to payoff with a higher periodic payment uses the logarithmic inversion of the annuity formula. Remaining balance after k payments uses the future value formula for amortizing loans.

Design and testing practices follow general engineering and security guidelines drawn from standards such as NIST for secure handling of input/output, ISO quality principles for validation, and IEEE best practices for numerical robustness. This tool documents assumptions and rounding behavior to support reproducibility.

Use the recurring extra mode to model disciplined ongoing additional principal contributions. Use the one-time extra mode to model lump-sum contributions such as bonuses or tax refunds.

All outputs are estimates. Differences may occur because of rounding, payment posting timing, loan fees, variable rates, or lender-specific rules. See accuracy and regulatory notes below.

Example 1: $25,000 loan, 7.5% APR, 5 years, monthly payments. Adding $50 to each monthly payment reduces total interest and shortens payoff; see interest saved and payments saved outputs.

Example 2: Same loan, one-time $2,000 payment at payment 12 reduces the remaining balance immediately and shortens the remaining amortization; the calculator estimates the new number of payments and interest saved.