The Quick Answer
Loan amortization is a repayment method where each fixed monthly payment is split between interest and principal. Early in the loan, most of the payment covers interest. Over time, the interest portion shrinks and more goes toward reducing the balance.
The monthly payment formula is:
M = P × [r(1 + r)^n] / [(1 + r)^n − 1]
Where P is the loan amount, r is the monthly interest rate, and n is the total number of payments.
A $250,000 loan at 6.5% for 30 years has a monthly payment of $1,580.17. Over the full term you pay $318,861 in interest — more than the original loan amount.
What "Amortized" Actually Means
An amortized loan is designed so that a fixed payment, repeated every month for the entire term, pays off the loan exactly by the last payment. The word comes from the Latin admortire — "to kill" — because each payment kills off a piece of the debt.
The key mechanism: each month, interest is charged on whatever balance remains. Since the balance shrinks with every payment, the interest charge also shrinks. Because the total payment stays fixed, the principal portion grows automatically.
This is different from:
- Interest-only loans, where you pay just the interest and the balance never changes
- Balloon loans, where small payments are followed by one large payment at the end
- Simple interest loans, where interest does not compound on the remaining balance in the same way
The Formula, Step by Step
M = P × [r(1 + r)^n] / [(1 + r)^n − 1]
Let's calculate the monthly payment on a $200,000 loan at 7% for 30 years.
- P = 200,000 (loan amount)
- r = 0.07 / 12 = 0.005833 (monthly interest rate)
- n = 30 × 12 = 360 (total payments)
Plug in:
M = 200,000 × [0.005833 × (1.005833)^360] / [(1.005833)^360 − 1]
M = 200,000 × [0.005833 × 8.1165] / [8.1165 − 1]
M = 200,000 × 0.04735 / 7.1165
M = 200,000 × 0.006653
M = $1,330.60
The monthly payment is $1,330.60. Over 360 payments, you will pay $479,017 total — of which $279,017 is interest.
Walking Through an Amortization Schedule
Here are the first 6 months of that $200,000 loan at 7%:
| Payment # | Payment | Principal | Interest | Remaining Balance |
|---|---|---|---|---|
| 1 | $1,330.60 | $164 | $1,167 | $199,836 |
| 2 | $1,330.60 | $165 | $1,166 | $199,671 |
| 3 | $1,330.60 | $166 | $1,165 | $199,506 |
| 4 | $1,330.60 | $166 | $1,164 | $199,339 |
| 5 | $1,330.60 | $167 | $1,163 | $199,172 |
| 6 | $1,330.60 | $168 | $1,162 | $199,004 |
Notice: in month 1, only $164 of the $1,330.60 payment reduces the balance. The rest — $1,167 — is interest. That is 87.7% interest, 12.3% principal.
Now compare the last 6 months of the same loan:
| Payment # | Payment | Principal | Interest | Remaining Balance |
|---|---|---|---|---|
| 355 | $1,330.60 | $1,293 | $38 | $5,225 |
| 356 | $1,330.60 | $1,300 | $30 | $3,924 |
| 357 | $1,330.60 | $1,308 | $23 | $2,617 |
| 358 | $1,330.60 | $1,315 | $15 | $1,301 |
| 359 | $1,330.60 | $1,323 | $8 | — |
| 360 | Final | — | — | $0.00 |
By the end, almost the entire payment goes to principal. The interest portion is negligible because the remaining balance is small.
Why Early Payments Are Mostly Interest
This is the most common source of confusion with amortized loans. The reason is straightforward:
Interest each month = remaining balance × monthly rate
When your balance is $200,000 and the monthly rate is 0.5833%, the interest charge is $1,167. Your $1,330.60 payment covers that interest first, and only the leftover $164 reduces the balance.
When the balance has dropped to $5,000, the interest charge is only $29. Almost all of the $1,330.60 goes to principal.
This is not a trick or hidden fee — it is the mathematical consequence of charging interest on a declining balance with a fixed payment.
How Loan Term Affects Total Interest
The term length has a dramatic effect on total cost. Here is a $300,000 loan at 6.5%:
| Term | Monthly Payment | Total Interest | Total Paid |
|---|---|---|---|
| 10 years | $3,407 | $108,878 | $408,878 |
| 15 years | $2,613 | $170,388 | $470,388 |
| 20 years | $2,238 | $237,183 | $537,183 |
| 30 years | $1,896 | $382,633 | $682,633 |
The 30-year option has the lowest monthly payment but costs $273,755 more in interest than the 10-year option. The 15-year term is often a practical middle ground — the monthly payment is $717 higher than 30-year, but total interest drops by $212,245.
How Interest Rate Affects Total Cost
On a $300,000 loan for 30 years:
| Rate | Monthly Payment | Total Interest |
|---|---|---|
| 4.0% | $1,432 | $215,609 |
| 5.0% | $1,610 | $279,767 |
| 6.0% | $1,799 | $347,515 |
| 6.5% | $1,896 | $382,633 |
| 7.0% | $1,996 | $418,527 |
| 8.0% | $2,201 | $492,467 |
Each 1% increase in rate adds roughly $60,000–$75,000 in total interest on a $300,000 loan over 30 years.
Extra Payments: Small Amounts, Large Impact
Extra principal payments reduce the balance ahead of schedule. Since interest is calculated on the balance, every dollar of extra principal saves interest for the remaining life of the loan.
Example: $250,000 at 6.5% for 30 years. Base monthly payment: $1,580.17.
| Extra Monthly | Years Saved | Interest Saved | Total Paid |
|---|---|---|---|
| $0 (base) | — | — | $568,861 |
| $100 | 4.3 years | $62,437 | $506,424 |
| $200 | 7.0 years | $101,952 | $466,909 |
| $500 | 12.8 years | $180,432 | $388,429 |
Adding just $100 per month — about $3.33 per day — saves over $62,000 and takes more than 4 years off the loan.
Timing matters. An extra $1,000 payment in year 1 saves more interest than the same $1,000 payment in year 20, because the interest savings compound over more remaining years.
Bi-Weekly Payments
A bi-weekly payment plan means paying half the monthly amount every two weeks. Since there are 52 weeks in a year, you make 26 half-payments — equivalent to 13 full payments instead of 12.
That one extra payment per year goes entirely to principal. On a $250,000 loan at 6.5%, switching to bi-weekly payments:
- Saves approximately $80,000 in total interest
- Pays off the loan roughly 4–5 years early
- Requires no increase in per-paycheck cost (if you are paid bi-weekly)
Refinancing and the Amortization Reset
Refinancing replaces your existing loan with a new one — new rate, new term, new amortization schedule. This can be beneficial when rates drop, but there is a catch:
If you are 10 years into a 30-year mortgage and refinance into a new 30-year term, you restart the amortization clock. Even at a lower rate, the total interest paid over 40 years (10 on the old + 30 on the new) can exceed what you would have paid on the original loan.
To avoid this: When refinancing, consider matching or shortening the remaining term. If you have 20 years left, refinance into a 20-year or 15-year term rather than starting over at 30.
Common Mistakes
Focusing only on the monthly payment
A longer term or interest-only period lowers the monthly payment but increases total cost. Always check total interest paid, not just the monthly number.
Ignoring the early-year interest ratio
Making extra payments in the first 5 years of a loan has far more impact than the same extra payments in year 25. If you have extra money to apply, earlier is better.
Comparing loans without matching terms
A 15-year loan at 6% is not directly comparable to a 30-year loan at 6.5%. Use total interest paid and total cost to compare on equal footing.
Forgetting about closing costs when refinancing
Refinancing has upfront costs (typically 2–5% of the loan). If you plan to sell the home within a few years, the savings from a lower rate may not cover the closing costs. Calculate the break-even point before deciding.
What This Calculator Does Not Do
- It assumes a fixed interest rate for the entire term (adjustable-rate loans will differ)
- It does not include property taxes, insurance, or PMI (which affect the total housing payment but not the loan amortization itself)
- It does not account for extra payments (it shows the standard schedule)
- It rounds to the nearest cent, so the final payment may differ slightly in practice
Try It Yourself
Use the loan amortization schedule calculator to generate a full payment-by-payment breakdown for any loan. Enter your amount, rate, and term to see exactly how each payment splits between principal and interest — and how much total interest you will pay.
You can also use the loan calculator for a quick payment estimate, or the mortgage calculator for a mortgage-specific view.