Amortization Schedule Explained (With Real Examples)
How each mortgage payment splits between principal and interest — and why extra payments save tens of thousands.
What "Amortization" Actually Means
Amortization is the process of paying off a loan through scheduled fixed payments over time. Each payment contains two parts:
- Principal: the portion that reduces the balance you owe
- Interest: the portion that pays the lender for letting you borrow
The fixed monthly payment stays constant throughout the loan, but the split between principal and interest shifts dramatically over time.
How The Split Works: $300,000 Mortgage At 6.5% Over 30 Years
Monthly payment: $1,896. Same amount every month for 360 months. But the split:
| Month | Interest | Principal | Balance |
|---|---|---|---|
| 1 | $1,625 | $271 | $299,729 |
| 60 (year 5) | $1,521 | $375 | $280,629 |
| 120 (year 10) | $1,371 | $525 | $252,655 |
| 180 (year 15) | $1,163 | $733 | $214,104 |
| 240 (year 20) | $873 | $1,023 | $160,558 |
| 300 (year 25) | $468 | $1,428 | $85,848 |
| 360 (year 30) | $10 | $1,886 | $0 |
In month 1, $1,625 of your $1,896 payment is interest — 86%. By month 120 (year 10), interest is 72%. Only after month 240 does principal become the larger share. For the first 15 years of a 30-year mortgage, you're paying the bank far more than you're reducing your own balance.
Why The Split Changes
Interest is calculated each month on the remaining balance. Since the balance barely moves in the early months, the interest charge barely moves. But each month you pay off a little more principal, the balance shrinks slightly, and next month's interest charge shrinks slightly. Over 360 iterations, the effect snowballs.
The Total Interest Shocker
Total cost over 30 years on the $300,000 example:
- Total paid: $682,632
- Principal: $300,000
- Interest: $382,632
You pay $382,632 in interest on a $300,000 house. Over 30 years, the interest alone exceeds the house price. This is why amortization understanding matters so much.
The Power Of Extra Payments
Because interest compounds on remaining balance, paying down principal early has outsized effects on total cost. On the same $300,000, 6.5%, 30-year mortgage:
- $0 extra/month: 30 years, $382,632 total interest
- $100 extra/month: 25 years 3 months, $307,000 total interest (saves $75,632 + 4.75 years)
- $200 extra/month: 22 years 1 month, $257,000 total interest (saves $125,632 + 7.9 years)
- $500 extra/month: 16 years 10 months, $173,000 total interest (saves $209,632 + 13.2 years)
The reason extra payments work so well: every extra dollar applied to principal in year 1 avoids 29 more years of compounding interest on that dollar.
One Extra Payment Per Year Trick
Make 13 payments per year instead of 12 (easiest: split your regular payment in half and pay every two weeks — gives 26 half-payments = 13 full payments annually).
On the $300,000 example, this saves $95,000 in interest and knocks ~5 years off the loan with minimal lifestyle impact. Most mortgage servicers support this as a "biweekly payment" option.
Amortization Schedule For Other Loans
Auto loan example: $30,000, 7%, 60 months. Payment $594/month.
- Month 1: $175 interest + $419 principal
- Month 30: $106 interest + $488 principal
- Month 60: $3 interest + $591 principal
Shorter loans front-load interest less than mortgages. By month 30 of a 60-month auto loan, you're already paying more principal than interest. On a mortgage that flip happens around year 20 of 30.
Why Negative Amortization Is Dangerous
Some interest-only or "payment option" mortgages allow payments smaller than the monthly interest charge. The shortfall is added to the balance — your debt grows every month. This is called negative amortization, and it wiped out many homeowners in 2008. If a loan offers a "low payment option" below the standard amortization amount, read the fine print carefully.
Reading An Amortization Schedule
Any full amortization schedule has five columns:
- Payment number (or date)
- Payment amount (fixed)
- Interest paid this month
- Principal paid this month
- Remaining balance
The sum of columns 3 and 4 always equals column 2. The remaining balance in column 5 equals the previous month's balance minus column 4.
Tax Implications (US)
US mortgage interest is tax-deductible if you itemise (subject to the $750,000 loan cap under current law). The amortization schedule tells you exactly how much interest you paid in any given year — useful for tax filing. In early years, when interest payments are highest, the deduction is most valuable.
UK equivalent: mortgage interest on residential primary residence is not tax-deductible. Interest on buy-to-let mortgages is partially deductible via the 20% tax credit (since 2020 changes).
Refinancing Resets The Schedule
When you refinance, the amortization schedule starts over. If you are 10 years into a 30-year mortgage (so 20 years of payments remaining, mostly principal now) and refinance into a new 30-year loan, you are back at month 1 — mostly interest. This can increase total interest paid if you're not careful.
A 15-year refinance or extra payments on the original loan often beats refinancing to reset the clock.
The Bottom Line
Amortization is not a mystery — it's arithmetic that compounds in favor of lenders during the early years of any loan. Understanding this is why extra early payments save far more than late-in-life payments, why 15-year mortgages beat 30-year ones for total cost, and why refinancing is a double-edged sword. Always look at the full schedule before signing any long-term loan.