How an amortization schedule works
An amortizing loan is repaid with a fixed payment that stays the same for the whole term, but the split inside that payment changes every month. Interest is charged on whatever you still owe, so early payments are mostly interest and barely touch the balance; as the balance falls, less of each payment goes to interest and more to principal. The schedule above shows that shift payment by payment.
The payment itself comes from the standard formula M = P × r ÷ (1 − (1 + r)−n), where P is the amount borrowed, r the monthly rate (annual rate ÷ 12) and n the number of payments. For a $336,000 mortgage at 6.5% over 30 years that gives $2,123.75 a month — and $428,549 of interest over the life of the loan, more than the amount borrowed. Each row then applies the same three steps: interest = balance × r, principal = payment − interest, new balance = old balance − principal.
Extra payments go entirely to principal, which is why they are so effective. Adding $200 a month to that same mortgage cuts the term by 6 years 4 months and saves about $106,900 of interest. The calculator recomputes the whole schedule with the extra amount and reports both the interest saved and the new payoff date, so you can compare "what if I round up to $2,500?" in one edit.
Reading the yearly view: each row totals the twelve payments of that year and shows the balance at year end. Switch to monthly for the payment-level detail you would need to reconcile against a lender statement or to check a specific payoff figure.