How an Amortization Schedule Works
An amortization schedule is the month-by-month story of a fixed-rate loan: the payment never changes, but what it buys does. Early on it mostly rents the debt (interest); later it mostly retires it (principal). Reading the table well turns it from paperwork into a decision tool.
The Two-Line Mechanism
| Each month | interest = balance × monthly rate · principal = payment − interest |
|---|
That is the whole engine. The payment is fixed by the amortization formula (see the loan interest guide for its derivation); each month the current balance determines the interest slice, and whatever remains of the payment reduces the balance. A falling balance means a falling interest slice — so the principal slice must grow. The curve is self-accelerating.
A Worked Slice (10,000 @ 8%, 36 months, payment 313.36)
| Month | Interest | Principal | Balance after |
|---|---|---|---|
| 1 | 66.67 | 246.70 | 9,753.30 |
| 2 | 65.02 | 248.34 | 9,504.96 |
| 18 | 37.17 | 276.20 | 5,298.65 |
| 36 | 2.08 | 311.29 | 0.00 |
Month 1 is 21% interest; month 36 is under 1%. On a 30-year mortgage the same shape stretches further — early payments can be over 70% interest, which is why the mortgage calculator's totals surprise first-time buyers.
Using the Table for Decisions
Two practical reads. Early payoff: the remaining-balance column is your payoff quote at any month — everything after it in the interest column is what early payoff saves. Extra principal: an extra payment removes balance that would have accrued interest every remaining month, so the earlier it lands, the more rows it shrinks. The yearly summary in our loan calculator — and the term comparison in the loan calculator guide — both fall out of this one table.
Frequently Asked Questions
Why is most of my early payment interest?
Interest each month is the balance times the monthly rate, and the balance is largest at the start. Same
payment, big balance: interest dominates until the balance falls.
Do extra payments change the schedule?
Yes — extra amounts go straight to principal, which reduces every future month's interest. The earlier the
extra payment, the more months of interest it eliminates.
Is amortization the same for mortgages and personal loans?
The mathematics is identical — only the size, term, and rate differ. A 30-year mortgage just stretches the
same schedule across 360 rows.