Understanding Amortization
Learn how mortgage amortization works, why so much of your early payment goes to interest, and how to use this knowledge to save tens of thousands of dollars.
What Is Amortization?
Amortization comes from the Latin word meaning "to kill" — and that is literally what you are doing to your debt with each payment. A mortgage is a fully amortizing loan, which means each of your equal monthly payments is calculated so that the loan balance reaches exactly zero on the final payment date.
The key insight: your payment amount stays constant, but the composition of that payment changes every single month. Early payments are mostly interest. Late payments are mostly principal. The shift is gradual and mathematically precise.
The Math Behind Amortization
The monthly payment for a fixed-rate mortgage is calculated using this formula:
Once the payment is set, every month works like this:
- Calculate interest: Outstanding Balance × Monthly Rate
- Subtract interest from payment: the remainder reduces principal
- New balance = Old balance − Principal paid
- Repeat for next month on the new (lower) balance
Why Early Payments Are Mostly Interest
This is the part most borrowers find frustrating — and it makes perfect mathematical sense once you understand it.
In month 1, your entire $300,000 balance is outstanding. At 7%, one month of interest on $300,000 is $1,750. Your payment is $1,995.91. So only $245.91 goes toward reducing the balance. Your balance after payment 1: $299,754.09.
In month 2, you owe interest on $299,754.09 — which is $1,748.57. Now $247.34 goes to principal. Infinitesimally less goes to interest, but the shift is tiny.
This compounds slowly. It takes until roughly payment 153 (month 12.75 — year 13) before more than half of each payment goes to principal on a 30-year loan at 7%.
$300,000 loan at 7%, 30-year fixed — key milestones
| Payment 1 — interest | $1,750.00 (87.7% of payment) |
| Payment 1 — principal | $245.91 (12.3% of payment) |
| Payment 153 — interest | $994.47 (49.8% of payment) |
| Payment 153 — principal | $1,001.44 (50.2% of payment) |
| Payment 360 — interest | $11.59 (0.6% of payment) |
| Payment 360 — principal | $1,984.32 (99.4% of payment) |
| Total interest paid (30 years) | $418,527 |
How to Read an Amortization Schedule
An amortization schedule is a table with one row per payment period. Each row contains:
- Payment number — Which installment this represents (1 through 360 for a 30-year loan)
- Payment date — When the payment is due
- Total payment — The fixed amount you pay each month
- Interest paid — How much of this payment covers interest charges
- Principal paid — How much of this payment reduces the balance
- Remaining balance — What you still owe after this payment
Many schedules also show cumulative interest and cumulative principal paid to date. Looking at the cumulative interest column at different points in the schedule is a sobering exercise — it shows exactly how much the bank has collected from you so far.
Use the Amortization Calculator to generate and download your complete payment schedule with year-by-year and month-by-month views.
The Power of Extra Payments
Because every dollar of principal paid now eliminates future interest on that dollar for the rest of the loan term, extra payments are disproportionately powerful early in the loan.
Impact of extra payments — $300,000 loan, 7%, 30-year fixed
| No extra payments | 30 years / $418,527 total interest |
| $100 extra/month | 26.3 years / $338,079 interest — saves $80,448 |
| $200 extra/month | 23.6 years / $272,837 interest — saves $145,690 |
| $500 extra/month | 19.1 years / $174,272 interest — saves $244,255 |
| One extra payment/year | 26.1 years / $335,609 interest — saves $82,918 |
When making extra payments, always specify that the extra amount should be applied to principal. Some lenders will apply unspecified extra payments to future scheduled payments rather than immediately reducing the balance. Check your loan servicer's policy and confirm on your next statement.
Biweekly Payments — A Simple Trick
Instead of making 12 monthly payments per year, pay half your monthly payment every two weeks. Because there are 52 weeks in a year, you make 26 half-payments — equivalent to 13 full monthly payments. That one extra payment per year, applied entirely to principal, can shave 4–5 years off a 30-year mortgage at typical rates.
Many servicers offer biweekly payment programs, though some charge a fee to set them up. You can replicate the effect by simply dividing your monthly payment by 12 and adding that amount to each monthly payment as extra principal.
Amortization vs. Interest-Only Loans
Some mortgages (less common today) have an interest-only period, typically 5–10 years, during which you pay only interest and the balance does not decrease at all. After the interest-only period ends, the loan re-amortizes over the remaining term, causing a significant payment jump. These loans make sense for certain investors and high-income borrowers who invest the payment difference, but they carry significant risk for typical homeowners.