Extra Payment Impact Calculator

This calculator runs the same month-by-month payoff simulation twice — once with only Minimum Monthly Payment, once adding Extra Monthly Payment and any Extra Annual Payment (lines 3-28) — and reports the difference in months and interest between the two runs. Before either simulation starts, the engine checks whether Minimum Monthly Payment even covers a month's interest on Current Balance (line 90): if it doesn't exceed Current Balance times the monthly rate, both Original Payoff Time and Accelerated Payoff Time return the literal sentinel -1, and Months Saved is explicitly set to 0 rather than computed from the difference. Doing the arithmetic on the $20,000-at-18%-APR defaults gives a floor of exactly $300 a month — the displayed $400 default clears it, but a minimum payment at or below $300 flips both payoff times to -1.

Minimum Monthly Payment dominates Months Saved: a 10% nudge moves it by roughly 78%, ahead of Current Balance (about 63%) and Interest Rate (about 44%) — because a larger minimum payment shrinks Original Payoff Time on its own before any extra payment is even added, which mechanically compresses the gap the extra payment can still close. Extra Annual Payment barely moves Months Saved at these defaults — a one-time yearly lump sum changes total interest paid by only a couple of dollars, not enough to shift the calculator's whole-number month count at all, even though it is subtracted the same way as Extra Monthly Payment inside the simulation loop (lines 19-21). This calculator does not model a variable interest rate, a change in your minimum payment requirement, or new debt added during the payoff period.