Amortization is a repayment pattern
A fully amortizing loan is designed so scheduled payments reduce the balance to zero by the end of the term. Each payment usually contains both interest and principal. The payment may stay fixed, but its composition changes: interest generally declines and principal generally increases.
For a $250,000 loan at 6.50% over 30 years, the monthly payment is about $1,580.17. The schedule determines how much of each payment goes to interest and how much reduces the balance.
Compound interest is a growth mechanism
Compound interest occurs when earned or accrued interest becomes part of the amount used to calculate later interest. An investment account is a familiar example: if $10,000 earns 6% and interest remains in the account, the second year can earn interest on more than the original $10,000.
Compounding can also work against a borrower when unpaid interest is capitalized. Once unpaid interest is added to principal, later interest may be charged on the larger balance.
Why ordinary amortized loans still use periodic interest
A standard monthly amortization formula uses a periodic interest rate, but regular payments prevent the loan from simply growing at the compound rate. Interest is calculated for the period, the payment is applied, and principal is reduced. The schedule therefore combines periodic interest mathematics with a repayment stream.
The distinction matters because saying a loan “compounds monthly” does not by itself tell you how quickly it will be repaid. The payment amount and timing are equally important.
A simple side-by-side comparison
| Feature | Amortization | Compound interest |
|---|---|---|
| Main purpose | Repay a balance over time | Describe interest-on-interest growth |
| Typical balance direction | Down | Up if no offsetting withdrawals/payments |
| Key inputs | Principal, rate, term, payment timing | Principal, rate, compounding frequency, time |
| Common use | Mortgages, installment loans, notes | Savings, investments, capitalized balances |
When the distinction matters most
The difference becomes especially important with deferred-interest arrangements, negative amortization, adjustable rates, and investments used to compare “pay down debt versus invest.” In those situations, build both cash flows explicitly rather than comparing two quoted annual rates without considering payment timing.
How to avoid mixing the two concepts in a comparison
A common mistake is to compare the total interest on an amortized loan with the future value of a compound-interest account and assume the quoted rates are directly comparable. They are not unless the cash flows are also aligned. The loan has regular payments that remove money from the balance; the investment may have deposits, withdrawals, or no cash flow at all.
Build each side using its actual timing. For a debt comparison, list the required payment dates and any extra principal. For an investment comparison, list deposits and the compounding convention. Then compare ending wealth or total cash paid over the same horizon. This produces a more meaningful result than comparing “6% loan” with “6% investment” as though the percentages describe identical processes.
The same discipline helps with deferred-interest and negative-amortization products. Ask whether interest is paid each period, left unpaid, or capitalized into principal. That answer determines whether the balance is amortizing, compounding upward, or doing some of both at different times.
Build the numbers instead of estimating them
Use the free online schedule calculator for a complete amortization table. If you need to edit individual payment dates, amounts, rates, or notes and save the revised loan, use Amortization Pro for Windows.