Understanding Mortgage Amortization

Amortization decides how much of every payment becomes yours (equity) and how much becomes the bank's (interest). Once you see the curve, you will never look at a mortgage the same way.

The core mechanic

Each month, interest = remaining balance × monthly rate. The payment is fixed, so whatever is left after interest reduces principal. At the start the balance is huge → interest eats most of the payment. As years pass the balance shrinks → interest shrinks → principal's share grows. The loan is designed so the final payment zeroes the balance exactly at term's end. Watch it happen in the amortization calculator.

A $340,000 loan at 6.5%, in snapshots

• Payment 1: $2,149 → $1,842 interest, $307 principal. Balance: $339,693.
• After 5 years: you have paid $128,949 — but only ~$23,200 was principal. Balance ≈ $316,800.
• After 10 years: balance ≈ $285,600. Equity from payments: ~16% of the loan.
• Crossover: around year 19–20, principal finally exceeds interest in each payment.
• After 30 years: total interest paid ≈ $433,689.

Why this matters for real decisions

• Selling early: after 5 years of a 30-year mortgage you have barely dented the principal — most 'equity' in a quick sale comes from appreciation and your down payment, not payments. This drives the rent vs buy math.
• Refinancing restarts the clock: a fresh 30-year schedule re-front-loads interest — the trap the refinance calculator exposes with lifetime totals.
• Extra payments punch above their weight early: principal removed in year 2 avoids interest for 28 years. See the payoff calculator.
• Negative amortization (payment < interest) grows the balance — a feature of some exotic loans and a serious warning sign in any offer.

Amortization across loan types

Fixed loans amortize predictably. ARMs re-amortize at every adjustment over the remaining term. Interest-only loans do not amortize at all during the IO period — then amortize brutally fast over what remains. Biweekly plans amortize a touch faster via the 13th yearly payment. And shorter terms amortize dramatically faster: a 15-year loan's first payment is already majority-principal. Compare term curves in the 15 vs 30 calculator, and total interest across any scenario in the interest calculator.