How Mortgage Amortization Works

What amortization means

Amortization comes from the Latin "amortire" — to extinguish a debt. An amortizing loan is one where each payment covers the interest owed for that period plus some principal, so the loan balance steadily decreases to zero by the end of the term. The key feature of an amortizing mortgage is that the monthly payment stays constant throughout the loan, but the split between interest and principal shifts dramatically over time.

The math behind the payment

The monthly payment formula is:

M = P × [r(1+r)^n] / [(1+r)^n - 1]

Where:
  P = principal (loan amount)
  r = monthly interest rate (annual rate ÷ 12)
  n = total number of payments (years × 12)

For a $300,000 loan at 7% over 30 years: r = 7%/12 = 0.5833%, n = 360 payments. The result is a fixed monthly P&I payment of $1,996. Over 360 payments that's $718,560 total — more than twice the original loan. The difference ($418,560) is the total interest paid over the life of the loan.

Why early payments are mostly interest

In the first month on that $300,000 loan at 7%, interest accrues at 0.5833% per month:

Month 1:
  Interest:  $300,000 × 0.5833% = $1,750
  Principal: $1,996 - $1,750 = $246
  Balance:   $300,000 - $246 = $299,754

Month 180 (Year 15):
  Interest:  ~$1,050
  Principal: ~$946
  Balance:   ~$180,000

Month 360 (Final):
  Interest:  ~$12
  Principal: ~$1,984
  Balance:   $0

In month 1, 87.6% of your payment is interest. By year 15, the split is roughly equal. In the final years, nearly all of each payment goes to principal. This is why the total interest paid is so large — the lender earns interest on the full balance for the first years, when it's still near $300,000.

How extra payments change everything

Every extra dollar paid toward principal immediately reduces the balance — and since future interest is calculated on the remaining balance, that extra payment eliminates future interest charges too. The compounding works in your favor. On the $300,000 / 7% / 30-year loan:

• An extra $100/month saves ~$38,000 in interest and cuts 4+ years off the term
• An extra $500/month saves ~$135,000 in interest and cuts nearly 12 years off the term
• One extra payment per year (bi-weekly plan) saves ~$55,000 and cuts ~5 years

Apply extra payments specifically to principal (confirm with your lender), not to future payments. Some lenders apply extra cash to the next month's scheduled payment — check your loan servicer's policy.

30-year vs 15-year: the real numbers

On a $300,000 loan at comparable market rates (7% for 30-year, 6.5% for 15-year), the 15-year monthly payment is ~$2,614 versus $1,996 for the 30-year — about $618 more per month. But total interest paid: the 30-year accrues ~$418,000 in interest while the 15-year accrues only ~$170,000. The 15-year saves $248,000 in interest despite a lower rate — purely from the shorter compounding period. If you can afford the higher payment, the 15-year is almost always better financially. A middle path: take the 30-year for cash flow flexibility and make extra principal payments to accelerate payoff when your budget allows.