What Amortization Means
To amortize a loan is to repay it in regular instalments that cover both interest and principal, so that the balance reaches exactly zero on the final payment. The word comes from the Old French amortir — to kill off — which is a fair description of what the schedule does to the debt.
The payment stays constant, but its composition does not. Interest is charged on the balance outstanding, and the balance falls every month, so the interest portion shrinks and the principal portion grows. The result is a schedule that feels stagnant at the start and accelerates sharply at the end.
How Each Payment Is Split
For every period:
Interest = Balance × monthly rate
Principal = Payment − Interest
New balance = Balance − Principal
On a $300,000 loan at 6.5% over 30 years, the payment is $1,896.20. Watch how the split moves:
| Payment # | Interest | Principal | Principal share |
|---|---|---|---|
| 1 | $1,625.00 | $271.20 | 14.3% |
| 12 | $1,608.40 | $287.81 | 15.2% |
| 60 | $1,523.20 | $373.01 | 19.7% |
| 120 | $1,380.41 | $515.80 | 27.2% |
| 180 | $1,182.95 | $713.25 | 37.6% |
| 240 | $909.90 | $986.30 | 52.0% |
| 300 | $532.33 | $1,363.87 | 71.9% |
| 360 | $10.22 | $1,885.99 | 99.5% |
The first payment is 86% interest. It takes until payment 219 — more than eighteen years in — before principal exceeds interest. This is not a fee structure or a trick; it is the direct consequence of charging interest on an outstanding balance that starts large.
Why Early Extra Payments Are Worth So Much More
An extra dollar of principal paid in month one cancels every interest charge that dollar would have generated for the remaining 359 months. The same dollar paid in month 350 cancels ten months of interest. The saving from an extra payment is therefore roughly proportional to how much time is left.
The same $300,000 loan at 6.5%, with a constant extra monthly amount:
| Extra per month | Total payment | Payments | Total interest | Interest saved |
|---|---|---|---|---|
| $0 | $1,896.20 | 360 (30.0 yrs) | $382,633 | $0 |
| $100 | $1,996.20 | 312 (26.0 yrs) | $321,639 | $60,995 |
| $200 | $2,096.20 | 277 (23.1 yrs) | $279,185 | $103,449 |
| $300 | $2,196.20 | 250 (20.8 yrs) | $247,518 | $135,115 |
| $500 | $2,396.20 | 210 (17.5 yrs) | $202,874 | $179,759 |
| $1000 | $2,896.20 | 153 (12.8 yrs) | $141,471 | $241,162 |
An extra $200 a month — about 10% more — removes roughly six years and a sixth of the total interest. The relationship is strongly non-linear, so the first increments matter most.
The Biweekly Trick
Paying half the monthly amount every two weeks produces 26 half-payments a year, which equals 13 monthly payments rather than 12. That single extra payment per year typically shortens a 30-year mortgage by four to five years. It works because the extra money lands as principal, not because of the payment timing.
Note that many "biweekly payment programs" charge a setup and per-payment fee for something you can do free by simply paying one-twelfth extra each month. Confirm too that your lender applies extra amounts to principal rather than holding them as a prepaid future payment.
What Amortization Schedules Are Used For
- Tax — mortgage interest is deductible for U.S. filers who itemize, and the schedule shows the deductible amount each year.
- Refinancing decisions — comparing the remaining interest on the current loan against a new one, including its closing costs.
- Selling — the schedule gives the payoff balance at any future date.
- PMI removal — the point where the balance falls to 80% of the original value is read directly from the schedule.
- Accounting — businesses split each payment between interest expense and liability reduction.
Amortizing vs. Non-amortizing Loans
| Type | Payment covers | Balance at maturity |
|---|---|---|
| Fully amortizing | Interest and principal | Zero |
| Interest-only | Interest only | The full original principal |
| Balloon | Interest and some principal | A large lump sum remains |
| Negative amortization | Less than the interest due | Higher than the original principal |
Negative amortization loans were a significant contributor to the 2008 mortgage crisis: borrowers made affordable payments while their balances grew, and the reset to a fully amortizing payment was unaffordable.
Frequently Asked Questions
Does an extra payment lower my monthly payment?
No. It shortens the term instead. Some lenders offer a recast, which recalculates the payment on the reduced balance, usually for a fee of a few hundred dollars.
How do I make sure extra money goes to principal?
Tell the lender explicitly, in writing where possible. Many servicers default to applying extra funds to the next scheduled payment, which does nothing for the interest.
Why does my lender's schedule differ by a few dollars?
Day-count conventions and rounding. Some lenders compute interest on actual days elapsed rather than a flat twelfth of the annual rate, which shifts each row slightly. The totals stay very close.
Should I pay off the mortgage early or invest?
Compare the mortgage rate against the after-tax return you expect from investing. Paying off a 6.5% mortgage is a guaranteed 6.5% return; investing may beat it but is not guaranteed. Risk tolerance and the value you place on being debt-free decide the rest.