Loan

Every figure is computed in whole cents. Each period’s interest is the balance times the period rate, rounded half-up to the cent, exactly once. The payment comes from the standard annuity formula, rounded to the cent; the final payment simply pays whatever is left, so the schedule sums exactly — principal portions add up to the loan to the penny, every row.

Day counts matter and are named. Under 30/360 every month is a twelfth of a year (the U.S. convention, with the standard end-of-month adjustments). Under Actual/360 — how most commercial loans accrue — interest is charged on the real days elapsed over a 360-day year, which quietly raises the effective rate; the payment stays the annuity amount, so the split shifts and the last payment differs. Actual/365 counts real days over 365.

A zero rate does not divide by zero. At 0% the payment is the loan over the term, to the cent — a case a surprising number of calculators get wrong.

Extra money goes to principal. The monthly extra and the one-time amount reduce the balance the moment they are paid, and the savings tile is the difference between this schedule and the no-extra schedule, computed, not estimated.

It runs entirely on your machine — the policy at the top of this file makes the browser refuse every network connection, and nothing you type is stored anywhere. You are reading the whole program: this one file is the tool and its own source.