What Is Amortization? How Your Loan Payments Actually Work

Each monthly payment on an amortizing loan covers two things: (1) interest on the remaining balance, and (2) principal reduction. Interest is calculated as: Monthly Interest = Outstanding Balance × (Annual Rate ÷ 12). In month 1 on a $300,000 mortgage at 7%, monthly interest = $300,000 × (0.07 ÷ 12) = $1,750. If the total payment is $1,996, only $246 reduces the balance.

The next month you owe $299,754. Interest = $1,748.60. Principal paid = $247.40. Each month the balance drops slightly, interest drops slightly, and more goes to principal. This slow ramp-up in principal repayment is why payoff feels impossibly slow in early years — because it is, mathematically.

An amortization schedule is a table showing every payment: number, amount, interest paid, principal paid, remaining balance. In year 1 of a 30-year $300,000 mortgage at 7%, you pay $23,952 in payments but reduce your balance by only $2,916. $21,036 went to interest. By year 10, each payment applies $500–$600 to principal. By year 25, most of the payment is principal.

• Year 1–5: ~85% of each payment is interest
• Year 10: ~75% interest
• Year 20: ~55% interest
• Year 25: ~40% interest
• Year 30 (last 12 months): ~3% interest

The inflection point where you pay more in principal than interest occurs at month 253 (year 21) on a 30-year at 7%. For the first two-thirds of the loan, you're primarily paying interest. This is why refinancing early and restarting the amortization clock is costly: you restart the high-interest phase.

On the same $300,000 at 7% 30-year loan, paying an extra $200/month from day one reduces the loan term by about 6 years and saves roughly $86,000 in total interest. Extra principal reduces the balance on which future interest is calculated, creating a compounding effect that accelerates over time.

A one-time extra payment has disproportionate early impact. Making a $5,000 lump sum payment in year 1 saves more in interest than the same amount in year 20, because it reduces a higher base balance for more remaining years. The earlier the extra payment, the greater the leverage.

A 15-year mortgage on $300,000 at 6.5% (rates are typically 0.5–0.75% lower than 30-year): monthly payment = $2,613. Total interest over 15 years = $170,340. Compare to 30-year at 7%: payment = $1,996, total interest = $418,560. Difference: $248,220 less in total interest with the 15-year, and you own the home in half the time.

The counter-argument: the $617/month payment difference, invested at 7% for 30 years, grows to $746,000. Theoretically, 30-year plus investing the difference wins. In practice, most people don't consistently invest that difference. Your answer depends on whether you'll actually invest $617/month — honestly.

Negative amortization occurs when your monthly payment is less than the interest charged. The unpaid interest gets added to the principal balance — your loan grows even as you make payments. This happened with "option ARM" mortgages in the 2000s: borrowers paid minimums that didn't cover interest, and balances grew until they owed more than the home was worth.

Today, negatively amortizing mortgages are largely prohibited for consumers under Dodd-Frank. But income-driven repayment plans for federal student loans can have negative amortization — your balance can grow while in repayment. Check your specific plan's interest subsidy provisions to understand whether your balance is shrinking or growing.