Amortization

Amortization is the mathematical structure that makes a fixed-rate mortgage have a constant monthly payment. The payment stays the same dollar amount each month, but its composition shifts: in the early years, most of each payment is interest because the outstanding balance is large; in the later years, most of each payment is principal because the balance has been paid down. The line where principal first exceeds interest is called the crossover point, and on a 30-year loan at typical rates it lands roughly halfway through the term.

How it works: the monthly payment is computed by the standard amortization formula, which solves for the constant payment that fully amortizes the loan over the term at the agreed rate. Each month's interest is calculated against the current outstanding balance; the rest of the payment goes to principal, which reduces the balance for next month's interest calculation. Over 360 months on a 30-year loan, the schedule produces a smooth handoff from interest-heavy to principal-heavy payments.

Why it matters: amortization explains why early-mortgage years build equity slowly and late-mortgage years build it quickly. A borrower five years into a 30-year fixed has paid down only about 9% of the original balance, despite making 60 monthly payments. The amortization visualizer at /tools/amortization shows this curve for any loan and rate.

Common gotcha: refinancing resets the amortization clock. A borrower five years into a 30-year mortgage who refinances into a new 30-year loan is now paying interest on a fresh 30-year amortization curve, meaning the early years of the new loan are mostly interest again. The monthly payment may go down, but the total interest paid over the life of the loan can go up. The break-even calculation in /tools/amortization captures this trade.