How Loan Amortization Works

What is amortization?

Amortization is the process of paying off a loan through regular, equal payments over a fixed period. Each payment covers the interest owed for that period plus a portion of the principal. By the final payment, the balance is exactly zero.

The word comes from the Old French amortir, meaning "to kill off" — as in, gradually killing off the debt. Amortized loans include mortgages, auto loans, student loans, and most personal loans.

Not all loans amortize. Interest-only loans, balloon loans, and revolving credit lines (like credit cards) work differently — your payments may not reduce the principal at all until a specific point.

Why early payments are mostly interest

With an amortized loan, your monthly payment stays the same throughout the term. What changes is the split between interest and principal each month.

Here is why: interest for each period is calculated on the remaining balance. Early in the loan, the balance is high, so interest is high and leaves little room for principal reduction. As you pay down the balance, interest charges shrink — meaning more of each fixed payment goes to principal. This compounds in your favor over time, but slowly.

Payment #1  (month 1):
  Interest  →  $1,458   (87.7%)
  Principal →    $205   (12.3%)
  Balance   → $249,795

Payment #60 (year 5):
  Interest  →  $1,415   (85.1%)
  Principal →    $248   (14.9%)
  Balance   → $238,619

Payment #180 (year 15):
  Interest  →  $1,134   (68.2%)
  Principal →    $529   (31.8%)
  Balance   → $193,500

Payment #360 (final):
  Interest  →      $10
  Principal →  $1,653
  Balance   →      $0

After 5 years of payments, you have paid roughly $99,780 but reduced the balance by only $11,381. The remaining $88,399 went to interest.

The amortization formula

The fixed monthly payment M for a loan is:

M = P × [r(1+r)^n] / [(1+r)^n - 1]

• P — the loan principal (amount borrowed)
• r — the monthly interest rate (annual rate ÷ 12)
• n — the total number of payments (years × 12 for monthly)

For each period, the interest portion is balance × r, and the principal portion is M − interest. The new balance is the old balance minus the principal portion.

How to read an amortization schedule

An amortization schedule lists every payment for the life of the loan. Each row typically shows:

The most eye-opening column is cumulative interest. On a typical 30-year mortgage, total interest paid often exceeds the original loan amount.

When extra payments make a big difference

Because early payments are dominated by interest, any extra principal payment you make early in the loan is extremely effective. Every dollar of extra principal directly reduces the balance on which future interest is calculated.

                 Standard     +$200/mo extra
Payoff time:     30 years      23 years 4 months
Total interest:  $348,772      $253,606

Interest saved:  $95,166
Payoff faster:   6 years 8 months

An extra $200/month — less than $7/day — saves nearly $100,000 in interest and eliminates nearly 7 years of payments. The earlier you start making extra payments, the larger the impact, because you interrupt the compounding of interest sooner.

Frequently asked questions

Is it better to make one extra payment per year or add extra each month?

Both strategies save roughly the same amount if the total extra money is the same — what matters is getting the money to the lender sooner. Monthly extra payments are slightly more effective because you reduce the balance a bit earlier each month, lowering the interest accrual faster. One lump-sum payment per year produces nearly the same result but requires more discipline to set aside the money.

What happens if I refinance a loan partway through?

Refinancing starts a new amortization clock. If you're 10 years into a 30-year mortgage and refinance into a new 30-year loan, you've extended your payoff by a decade — even if the new rate is lower. Always calculate the break-even point: how many months of lower payments it takes to recoup the closing costs. And consider refinancing into a shorter term (e.g. 15 or 20 years) to avoid resetting the amortization entirely.

What is negative amortization?

Negative amortization occurs when your payment is less than the interest owed for that period. The unpaid interest is added to the principal, so your balance grows instead of shrinking. This happened frequently with certain adjustable-rate mortgages before 2008. Today, most qualified mortgages are required to be fully amortizing — the payment must at least cover interest — but negative amortization products still exist in some markets.