Amortization is the schedule by which a loan is paid down to zero over its term using a level payment. The payment stays the same; what changes is how it is divided.
The mechanics
Each month:
- Interest is calculated on the current balance — annual rate divided by twelve, multiplied by the balance
- That interest comes out of your payment first
- Whatever remains reduces the principal
- Next month, the balance is smaller, so the interest is smaller, so more goes to principal
The effect compounds gently in your favour, but it starts slowly.
A real schedule
$400,000 at 6.5%, 30-year fixed. Payment: $2,528.27.
| Month | Interest | Principal | Balance |
|---|---|---|---|
| 1 | $2,166.67 | $361.61 | $399,638 |
| 12 | $2,144.53 | $383.74 | $395,529 |
| 60 | $2,030.93 | $497.34 | $374,444 |
| 120 | $1,840.54 | $687.73 | $339,105 |
| 216 | $1,373.11 | $1,155.16 | $252,342 |
| 360 | $13.62 | $2,514.65 | $0 |
Note month 60: after five years of payments totalling more than $151,000, the balance has fallen by about $25,600. That is not a mistake — it is what charging interest on a large balance looks like.
Why this matters for extra payments
An extra $500 put against principal in month one removes that $500 from the balance for the remaining 359 months. The interest it would have generated — compounding at 6.5% — never happens.
The same $500 in year 25 saves interest on only five remaining years. Same dollars, dramatically different effect.
This is why the standard advice is that extra principal is most valuable early. It is also why refinancing into a fresh 30-year term resets you to the steepest part of the curve, which can raise lifetime interest even at a lower rate.
What amortization is not
It is not a penalty, and it is not the lender front-loading their profit. If you paid interest only on the average balance over the term, the total would be similar. The schedule simply reflects that you owe the most at the beginning.
Interest-only loans, balloon structures and negative amortization products behave differently — and the last of these is why “negative amortization” became a term the general public learned in 2008.
Common questions
On a $400,000 loan at 6.5% over 30 years it is month 233 — about year 19.4. Higher rates push the crossover later still.
The same formula applies, but the shorter term forces far more principal into each payment, so the crossover happens almost immediately.
General information, not advice on your specific situation. Guidelines, limits and pricing change, and vary by lender, programme and state. Nothing here is a commitment to lend or an offer of credit. For tax or legal questions, consult a qualified professional.
Sree Basireddy · Mortgage Loan Officer, NMLS 2705737 · Loan Factory, Inc., NMLS 320841 · Equal Housing Opportunity