FinanceUpdated May 17, 2026🕐 4 min read
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How Loan Amortization Works
Loan amortization is the process of paying off a loan through regular fixed payments over a set term. Each payment covers the interest owed for that period and reduces the principal by the remainder. As the balance falls, each successive payment covers less interest and more principal — until the final payment brings the balance exactly to zero.
When you take out an amortising loan — a mortgage, car loan, or standard personal loan — you make the same fixed payment every month for the entire term. What changes is how that payment is split between interest and principal.
In the early months, most of your payment covers interest because the outstanding balance is large. Interest is calculated as a percentage of the remaining balance, so when the balance is 200.000 at 4 percent annual rate, the first month's interest is 200.000 x (0,04 / 12) = 667. If your monthly payment is 1.056, only 1.056 - 667 = 389 goes toward reducing the principal that month.
In the second month, the principal is 200.000 - 389 = 199.611. The interest charge is 199.611 x (0,04 / 12) = 665. Now 391 goes to principal. Each month the interest charge falls slightly and the principal reduction grows slightly. After 300 months the balance reaches exactly zero.
This is amortization: a fixed payment that gradually shifts from being mostly interest to being mostly principal as the balance declines.
The monthly payment formula
Formula
M = P \times \frac{r(1+r)^n}{(1+r)^n - 1}
Monthly payment equals the principal multiplied by the monthly rate times the compound factor, divided by the compound factor minus one. This is the fixed payment amount that, applied every month for n months at rate r, will reduce the balance from P to exactly zero.
MMonthly payment — the fixed amount paid every month
PPrincipal — the original loan amount at the start of the term
rMonthly interest rate as a decimal. Divide the annual rate by 12. At 4% annual: r = 0,04 / 12 = 0,00333.
nTotal number of monthly payments. A 25-year mortgage has n = 25 x 12 = 300.
Why you pay mostly interest at the start
The front-loading of interest is not a feature designed by lenders to maximise their income. It is a mathematical consequence of the amortization structure. Interest is calculated on the outstanding balance, and the balance is largest at the beginning of the loan.
On a 200.000 mortgage at 4 percent over 25 years:
Month 1: payment is 1.056. Interest portion is 667. Principal portion is 389.
Month 60 (year 5): interest portion is 613. Principal portion is 443.
Month 150 (year 12,5): interest portion is 527. Principal portion is 529. At this midpoint the payment is roughly equal between interest and principal.
Month 300 (year 25): interest portion is 4. Principal portion is 1.052.
The transition is gradual and continuous. The total interest paid over the full 25-year term at these numbers is approximately 116.800 on a 200.000 loan — which is 58 percent of the original principal paid in interest alone. This is why the loan term is such an important decision. A 20-year term at the same rate produces a higher monthly payment but saves tens of thousands in total interest.
Result: Monthly payment: 1.056 | Total paid: 316.800 | Total interest: 116.800
Monthly rate r = 0,04 / 12 = 0,003333. Using the formula: M = 200.000 x (0,003333 x (1,003333)^300) / ((1,003333)^300 - 1) = 200.000 x (0,003333 x 2,712) / (2,712 - 1) = 1.056. Total paid: 1.056 x 300 = 316.800. Total interest: 316.800 - 200.000 = 116.800. That is 116.800 paid purely for the time value of borrowing 200.000 over 25 years.
Result: Monthly payment: 290 | Total paid: 17.400 | Total interest: 2.400
Monthly rate r = 0,06 / 12 = 0,005. M = 15.000 x (0,005 x (1,005)^60) / ((1,005)^60 - 1) = 290. Total interest of 2.400 is 16 percent of the loan amount, far lower than the mortgage example. Shorter terms and lower rates produce far less total interest. The car loan pays off 58 percent less total interest as a proportion of principal than the 25-year mortgage above.
Example 3Impact of extra monthly payments
Given: 200.000 mortgage | 4% | 25 years | Extra payment: 200 per month from the start
Result: Saves approximately 28.000 in interest | Pays off 5 years early
Adding 200 per month to the principal reduces the balance faster than the standard schedule. Lower balance means less interest each period, which means more of the next payment goes to principal, which reduces the balance further. This compounding effect on debt repayment works in the borrower's favour. The 200 monthly extra payment, representing 19 percent extra over the standard 1.056 payment, saves approximately 24 percent of total interest and cuts 5 years from a 25-year term.
Build your amortization schedule
Enter your loan amount, rate and term to see the full month-by-month payment breakdown showing interest and principal for every payment.
Sample amortization schedule — 10.000 at 5%, 12 months
Month
Payment
Interest
Principal
Remaining Balance
1
856
42
814
9.186
2
856
38
818
8.368
3
856
35
821
7.547
6
856
25
831
5.059
9
856
13
843
2.538
12
856
4
852
0
Important
Amortization schedules assume every payment is made on time and in full. Missing a payment or paying late changes the schedule, increases the total interest paid, and may trigger default penalties. On variable-rate loans, the monthly payment or remaining term adjusts when the rate changes. Always confirm the exact terms with your lender.
Common mistakes
✗ Choosing a longer loan term to get a lower monthly payment without calculating total interest
✓ A 200.000 loan at 4 percent over 25 years costs 116.800 in total interest. The same loan over 20 years costs 91.200 in interest. The 25-year term saves 261 per month but costs an extra 25.600 in total. Always calculate the total cost, not just the monthly payment.
✗ Making extra payments without specifying they should reduce the principal
✓ Tell your lender explicitly that extra payments are to be applied to the principal balance. Without this instruction, some lenders apply extra payments to future scheduled payments instead, which does not reduce the balance or save interest.
✗ Refinancing without calculating the break-even period
✓ Refinancing resets the amortization. You start again paying mostly interest on the new loan. If you refinance a 15-year-old 25-year mortgage into a new 25-year mortgage, you add 15 years of payments. Calculate the break-even: divide the refinancing costs by the monthly saving to find how many months before you benefit.
✗ Assuming the midpoint of the loan term is where half the principal has been repaid
✓ Due to front-loading, at the midpoint of a 25-year mortgage at 4 percent, approximately 35 percent of the principal has been repaid, not 50 percent. The principal repayment accelerates in the second half of the loan term.
Special loan structures
Interest-only periods
Some loans have an initial period where payments cover only interest and no principal is repaid. The balance stays unchanged during this period. When the repayment period begins, the remaining principal is amortised over the remaining term, producing higher payments than a standard amortising loan from the start.
5 years interest-only, then 20 years amortising the full principal
Balloon payment loans
A balloon loan has lower regular payments than a fully amortising loan because it does not fully repay the principal over the term. A large lump-sum payment is due at the end. The monthly payments may only cover interest, or partially amortise the loan, with the remainder due as the balloon.
Monthly payment of 500 for 5 years, then a balloon payment of 50.000
Variable rate recalculation
When the interest rate on a variable-rate loan changes, the amortization schedule must be recalculated. Typically the monthly payment changes to keep the remaining term constant. Some lenders instead keep the payment fixed and adjust the remaining term. Always clarify which method applies when your rate changes.
Rate rises from 3% to 5% on 150.000 remaining balance — payment increases by approximately 170 per month
Methodology
All payment calculations use the standard amortization formula M = P x r(1+r)^n / ((1+r)^n - 1) with monthly compounding. Interest is calculated on the outstanding balance at the start of each period. The schedule assumes payments are made on the same date each month with no late fees or penalties applied.
Results are for illustrative purposes. Your lender may use slightly different calculation conventions, rounding methods, or payment date rules.
Why do I pay so much interest at the start of a mortgage?
Because interest is calculated on the outstanding balance, and the balance is at its maximum at the start of the loan. On a 200.000 mortgage at 4 percent, the first month's interest is 200.000 x (0,04/12) = 667. As you reduce the balance each month, the interest charge falls and more of each fixed payment goes toward principal. The process is gradual but continuous throughout the full term.
Does making extra payments reduce the monthly payment?
For most standard amortising loans, extra payments reduce the outstanding balance and therefore the total interest paid and the remaining loan term, but the fixed monthly payment does not change unless you formally refinance or restructure the loan. The benefit of extra payments is a shorter term and lower total interest cost, not a lower required monthly payment.
What happens if I miss a payment?
Missing a payment typically triggers a late fee. The missed interest is added to the outstanding balance, increasing future interest charges. Repeated missed payments can damage your credit score, trigger default proceedings, and in the case of a mortgage, ultimately lead to repossession. Always contact your lender immediately if you cannot make a payment — lenders typically have hardship provisions.
What is the difference between amortization and depreciation?
Amortization in a loan context refers to paying off a debt over time through regular payments. Depreciation refers to the reduction in the accounting value of a physical asset over its useful life. Both spread a cost over time, but amortization is about debt repayment while depreciation is an accounting treatment for asset value reduction.