How mortgage amortization works, with worked examples

Mortgage amortization is the process of paying off a loan through regular fixed payments, where each payment covers that month's interest and uses the rest to reduce the balance. Because interest is calculated on the outstanding debt, early payments are mostly interest and later payments are mostly principal. Understanding how mortgage amortization works lets you compare loan terms, predict your remaining debt at any point and judge whether overpaying is worth it.

The examples below use euros and a hypothetical interest rate of 4% per year. They are illustrations, not market rates; the maths works the same in any currency and at any rate.

How mortgage amortization works: interest first, then principal

A standard repayment mortgage (also called an annuity or capital-and-interest mortgage) has a fixed monthly payment for as long as the interest rate stays the same. Every month, the lender does three things:

1. Calculates interest on the current balance: balance × annual rate ÷ 12.
2. Takes that interest out of your payment.
3. Uses whatever is left of the payment to reduce the balance.

Next month the balance is a little lower, so the interest is a little lower, so a little more of the same payment goes to principal. Repeat this a few hundred times and the balance reaches exactly zero at the end of the term.

The annuity formula behind the monthly payment

The fixed payment is not a guess. It comes from the annuity formula:

M = P × i ÷ (1 − (1 + i)^−n)

• M = monthly payment
• P = loan amount (principal)
• i = monthly interest rate (annual rate ÷ 12)
• n = number of monthly payments (years × 12)

Worked example

Take a €200,000 loan over 25 years at a hypothetical 4% per year:

• i = 0.04 ÷ 12 = 0.003333
• n = 25 × 12 = 300
• M = 200,000 × 0.003333 ÷ (1 − 1.003333^−300) = €1,055.67 per month

Over the full term you pay 300 × €1,055.67 ≈ €316,702. Subtract the €200,000 you borrowed and the total interest is about €116,702.

Some lenders and countries use slightly different conventions, for example different compounding periods or day-count methods, so your contract figure may differ by a few euros. The principle is identical. If you want to skip the arithmetic, the mortgage calculator produces the payment, total interest and a full yearly schedule for any loan.

The first two payments, step by step

Here is what happens inside the first two payments of the example loan:

Payment	Opening balance	Interest (balance × 4% ÷ 12)	Principal (payment − interest)	Closing balance
1	€200,000.00	€666.67	€389.01	€199,610.99
2	€199,610.99	€665.37	€390.30	€199,220.69

In month one, about 63% of the payment is interest. The principal portion grows by roughly €1.30 between the two months. That sounds tiny, but it compounds: by the end of the term almost the whole payment is principal.

A yearly amortization schedule

Rolling the monthly calculation forward gives the yearly picture for the same €200,000 loan at a hypothetical 4% over 25 years. Each year you pay €12,668 in total (12 × €1,055.67), rounded to the nearest euro below.

Year	Interest paid	Principal repaid	Balance at year end
1	€7,913	€4,755	€195,245
5	€7,090	€5,578	€174,209
10	€5,857	€6,811	€142,719
15	€4,352	€8,316	€104,269
20	€2,514	€10,154	€57,322
25	€270	€12,398	€0

Three things stand out:

• The crossover comes late. The principal part of the payment only exceeds the interest part from around payment 93, so in year eight.
• Half the debt takes more than half the term. The balance first drops below €100,000 after 186 payments, about 15.5 years into a 25-year loan.
• After ten years you still owe over 70%. €142,719 of the original €200,000 remains. This matters if you plan to sell or refinance early.

How to calculate the remaining balance at any point

You do not need to build the whole schedule to know what you will owe. The outstanding balance after k payments is:

B = P × (1 + i)^k − M × ((1 + i)^k − 1) ÷ i

For the example loan after 5 years (k = 60):

• (1 + i)^60 ≈ 1.2210
• P × 1.2210 ≈ €244,199
• M × (1.2210 − 1) ÷ 0.003333 ≈ €69,990
• B ≈ €244,199 − €69,990 = €174,209

That matches the schedule. Knowing your future balance is useful for planning a refinance, estimating equity when you sell, or tracking how your loan to value ratio falls over time.

What changes the amortization: term, rate and overpayments

Loan term

Stretching the term lowers the monthly payment but raises the total interest, because the balance stays higher for longer. Same €200,000 at a hypothetical 4%:

Term	Monthly payment	Total interest
20 years	€1,211.96	€90,871
25 years	€1,055.67	€116,702
30 years	€954.83	€143,739

Going from 25 to 30 years saves about €101 a month but costs about €27,000 more interest over the life of the loan. Going to 20 years costs about €156 a month more but saves almost €26,000.

Interest rate

The rate affects both the payment and how slowly the balance falls at the start. Same €200,000 over 25 years, with purely hypothetical rates:

Hypothetical rate	Monthly payment	Total interest
2%	€847.71	€54,313
4%	€1,055.67	€116,702
6%	€1,288.60	€186,581

Notice that doubling the rate from 2% to 4% more than doubles the total interest. At higher rates, a bigger share of each early payment is swallowed by interest, so principal repayment starts even more slowly.

If your rate is fixed only for a few years, the schedule is recalculated when the rate changes: the remaining balance is amortized over the remaining term at the new rate. That is why your balance at the end of a fixed period is the number to watch.

Overpayments

Any extra payment goes straight to principal, which removes interest on that amount for every remaining month. In the example, paying an extra €100 a month from the start:

• shortens the loan from 300 to 259 payments (about 3 years and 5 months earlier)
• cuts total interest from about €116,702 to about €98,682, a saving of roughly €18,000

You paid in €100 × 259 = €25,900 extra and saved €18,000 of interest on top of clearing the debt sooner. Early overpayments are the most powerful, since they have the longest time to work.

Before you overpay, check your loan agreement. Many lenders cap penalty-free overpayments, some charge early repayment fees, and some reduce the monthly payment instead of the term unless you ask otherwise. Rules differ between countries and lenders, so confirm with your lender or a mortgage adviser.

Why amortization matters for property investors

For a buy-to-let property, amortization has a practical side effect: the principal part of your payment is cash leaving your account, but it is not a cost. It builds equity. That is why a property can show a profit on paper while producing little or negative cash each month. The article on rental property cash flow walks through this with numbers, and the buy to let investing guide shows where financing fits into the wider strategy.

It also explains why some investors choose loans that do not amortize at all. The trade-offs are covered in interest only vs repayment mortgage.

Common mistakes

• Comparing only monthly payments. A lower payment from a longer term can cost tens of thousands more in total interest. Compare total interest and the balance at the end of any fixed-rate period.
• Assuming the balance falls evenly. It does not. After half the term you still owe well over half the loan. Plan sales and refinancing with a real schedule.
• Forgetting that rate changes reset the maths. A new rate means a new payment and a new split between interest and principal.
• Overpaying without reading the terms. Fees or caps can wipe out the benefit. Also keep an emergency buffer rather than putting every spare euro into the mortgage.
• Treating principal as an expense. For landlords, only the interest part is a financing cost. Mixing the two distorts profit and, in many countries, tax calculations. Ask a tax adviser how interest is treated where you live.

A quick checklist for reading your own schedule

• Find your exact loan amount, rate, term and payment date in the contract.
• Check whether the rate is fixed, and until when.
• Note the balance you will owe at the end of the fixed period.
• Check how much you can overpay without fees, and whether overpayments shorten the term or lower the payment.
• Recalculate whenever the rate, term or balance changes.

Key takeaways

• A repayment mortgage uses a fixed payment from the annuity formula: M = P × i ÷ (1 − (1 + i)^−n).
• Each payment covers interest on the current balance first; the rest reduces the debt.
• Early years are interest-heavy. In the example, half the debt is only gone after about 15.5 of 25 years.
• Shorter terms and lower rates cut total interest sharply. Early overpayments do too, if your contract allows them.
• Run your own numbers in the mortgage calculator before you commit to a term.