How the Amortization Calculator works
Amortization is why year one of a 30-year mortgage barely dents the balance. Nearly the whole payment is interest at the start, and the split flips slowly. This shows the balance and cumulative interest at the year you choose.
Balance after k payments = P × ((1 + r)^n − (1 + r)^k) ÷ ((1 + r)^n − 1).