- The payment stays the same, but the split between interest and principal changes every month.
- Interest is charged on the remaining balance, so it is highest at the start.
- Extra payments go straight to principal and cut future interest.
- A shorter term means a higher payment and much less total interest.
What amortization means
An amortized loan is repaid in equal installments over a set term. Mortgages, auto loans and most personal loans work this way.
Two terms are useful here:
- Principal is the amount you borrowed and still owe.
- Interest is the charge for borrowing, worked out as a percentage of the remaining principal.
Every payment is split between these two parts. The total payment does not change on a fixed-rate loan, but the mix does. That shifting mix is what an amortization schedule shows.
How the fixed payment is calculated
The payment is set so that the balance reaches exactly zero on the last installment. The standard formula is:
Payment = P × r / (1 - (1 + r)^-n)
- P is the amount borrowed.
- r is the rate per period. For monthly payments, divide the yearly rate by 12.
- n is the total number of payments.
Example: $10,000 at a 12% yearly rate over 12 months. The monthly rate is 1%, or 0.01, and there are 12 payments.
- (1.01)^-12 = 0.887449
- 1 - 0.887449 = 0.112551
- $10,000 × 0.01 = $100
- $100 / 0.112551 = $888.49
You rarely need to do this by hand, but it helps to know that the payment depends on only three things: the amount, the rate and the number of payments.
How each payment is split
The split follows the same three steps every period:
- Multiply the remaining balance by the periodic rate to get the interest due. For monthly payments, the periodic rate is the yearly rate divided by 12.
- Subtract that interest from the fixed payment. What is left is the principal portion.
- Subtract the principal portion from the balance. This is the new balance for next period.
Take the same $10,000 loan with its $888.49 payment and 1% monthly rate.
In month 1, interest is $10,000 × 1% = $100.00. The principal portion is $888.49 - $100.00 = $788.49. The new balance is $10,000 - $788.49 = $9,211.51.
In month 2, interest is charged on $9,211.51, so it drops to $92.12. That leaves $796.37 for principal.
Reading an amortization schedule
A schedule lists every payment with its interest portion, principal portion and remaining balance. Here are selected rows for the same $10,000 loan.
| Month | Payment | Interest | Principal | Balance |
|---|---|---|---|---|
| 1 | $888.49 | $100.00 | $788.49 | $9,211.51 |
| 2 | $888.49 | $92.12 | $796.37 | $8,415.14 |
| 3 | $888.49 | $84.15 | $804.34 | $7,610.80 |
| 6 | $888.49 | $59.78 | $828.71 | $5,149.20 |
| 12 | $888.49 | $8.80 | $879.69 | $0.00 |
Read across a row to see where one payment goes. Read down the interest column to see it shrink, and down the principal column to see it grow. The final payment is often a few cents different because of rounding.
Total payments come to about $10,662, so the loan costs about $662 in interest. You can build a full schedule for any loan with the amortization calculator.
Why early payments are mostly interest
Interest is always charged on what you still owe. At the start you owe the most, so the interest portion is at its largest. This is not a penalty or a trick. It is simply the result of step 1 above.
The effect is much stronger on long loans. Consider a $200,000 loan at 6% over 30 years. The monthly rate is 0.5% and the payment is $1,199.10.
- First month interest: $200,000 × 0.5% = $1,000.00.
- First month principal: $1,199.10 - $1,000.00 = $199.10.
About 83% of the first payment is interest. The balance falls slowly for years, then faster and faster toward the end. This is why the amount owed on a long loan can seem to barely move in the early years.
The mortgage calculator shows this curve for home loans, including the point where principal overtakes interest.
What extra payments do
An extra payment is money paid on top of the required amount. When it is applied to principal, the balance drops right away. Every future interest charge is then calculated on a smaller balance, so more of each regular payment goes to principal too.
On the $200,000 loan above, adding $100 to every monthly payment would clear the loan in about 295 months instead of 360. That is more than five years sooner, and it saves roughly $49,000 in interest.
A few practical points:
- Extra payments have the most effect early, when the balance is highest.
- Check how the lender applies extra money. It should reduce principal, not just prepay the next installment.
- Some loans charge a fee for paying early. Read the loan terms first.
The loan payoff calculator estimates the time and interest saved for any extra amount.
Shorter terms vs longer terms
The term is the length of the loan. A shorter term spreads the principal over fewer payments, so each payment is higher. But the balance falls faster, and interest has less time to build up.
| $200,000 at 6% | 30 years | 15 years |
|---|---|---|
| Monthly payment | $1,199.10 | $1,687.71 |
| Total paid | $431,676 | $303,788 |
| Total interest | $231,676 | $103,788 |
The 15-year loan costs $488.61 more each month and saves about $127,888 in interest. Shorter loans are also often offered at lower rates, which would widen the gap further.
The trade-off is flexibility. A lower required payment leaves more room in a monthly budget, while a higher one builds ownership faster. Running both versions through a loan calculator makes the difference concrete.
Try it with your own numbers
- Amortization CalculatorYear-by-year schedule, with extra payments.
- Loan Payoff CalculatorHow fast extra payments clear a loan.
- Mortgage CalculatorMonthly payment with tax, insurance and HOA.
- Loan CalculatorPayment and total interest for any fixed-rate loan.
Frequently asked questions
What is an amortization schedule?
An amortization schedule is a table that lists every payment on a loan. For each payment it shows how much goes to interest, how much goes to principal and what balance remains. It lets you see the total interest cost and how quickly the amount owed falls over the life of the loan.
Why does my loan balance go down so slowly at first?
Interest is charged on the remaining balance, which is largest at the start. A big share of each early payment is used to cover that interest, leaving only a small part to reduce the balance. As the balance falls, the interest portion shrinks and the principal portion grows, so the payoff speeds up.
Do extra payments lower my monthly payment?
Usually not on a standard fixed-rate loan. The required payment stays the same, and extra money shortens the loan and reduces total interest instead. Some lenders will recalculate the payment on request after a large lump sum. How extra payments are handled varies, so check the loan terms.
Is amortization the same for all loans?
No. Fixed-rate installment loans follow the level-payment pattern described here. Interest-only loans, loans with a large final payment and revolving credit such as cards work differently. Loans with a changing rate are re-amortized when the rate moves, so the payment and the split can shift during the term.
This guide is general education, not financial advice. Rules of thumb vary by lender, industry and country.