How Loan Amortization Works (With a Worked Example)

When you take out a car loan, personal loan or mortgage, your monthly payment usually stays the same from the first month to the last. What changes, quietly and dramatically, is what that payment is buying. That process is called amortization, and understanding it is the single best way to understand what a loan really costs.

What amortization means

To amortize a loan is to pay it off gradually through scheduled, equal payments. Each payment does two jobs:

Pays the interest that built up on the balance since the last payment.
Reduces the principal, the amount you still owe, with whatever is left over.

Because interest is charged on the remaining balance, the interest part is largest at the start and shrinks every month. The principal part does the opposite. The payment stays flat; the mix inside it slides from mostly interest to mostly principal.

The formula behind the payment

The fixed payment comes from the annuity formula:

M = P × r(1 + r)ⁿ ÷ [(1 + r)ⁿ − 1]

P: the amount borrowed
r: the monthly interest rate (annual rate ÷ 12 ÷ 100)
n: the number of monthly payments

You don’t need to calculate it by hand. The loan calculator does it instantly. But it helps to know that the formula is built so the balance lands on exactly zero after the last payment.

A month-by-month example

Take a $25,000 loan at 7.5% APR over 5 years. The monthly rate is 0.625% and there are 60 payments, giving a payment of $500.95.

Month	Interest	Principal	Balance after
1	$156.25	$344.70	$24,655.30
2	$154.10	$346.85	$24,308.45
3	$151.93	$349.02	$23,959.43
30	$87.99	$412.96	$13,664.84
60	$3.11	$497.84	$0.00

Look at month 1: almost a third of the payment is interest. By month 60, interest is barely three dollars. Over the whole loan you pay $5,056.92 in interest, and most of it in the first half.

Why early payments are “mostly interest”

This surprises a lot of borrowers, especially on mortgages. On a 30-year home loan at 6.5%, the first payment can be more than 80% interest. It isn’t a trick by the lender. It falls straight out of the maths: a large balance generates a large interest charge, and the fixed payment has to cover that first.

The practical consequences:

Equity builds slowly at first. After five years of a 30-year mortgage, you may have repaid less than 10% of the principal.
Selling or refinancing early means you’ve paid a lot of interest relative to the balance you’ve cleared.
Extra payments early on are powerful. Every extra dollar goes straight to principal and stops generating interest for the rest of the term. See How to Pay Off a Loan Faster: 7 Strategies That Save Interest for how much this saves.

How to read an amortization schedule

An amortization schedule is simply the full table above, for every payment. In our calculator you can view it by year (totals of 12 payments) or by month. Useful things to look for:

The crossover point: the first period where the principal portion exceeds the interest portion. On the example loan it happens in month 1, because the rate is moderate and the term short. On a 30-year mortgage at 7% it takes around 20 years.
Interest per year: important if your interest is tax-deductible, and handy for budgeting.
Balance at a future date: if you plan to sell a car in three years, the balance column tells you roughly what you’ll still owe.

What changes the schedule

Change	Effect on payment	Effect on total interest
Higher rate	Higher	Much higher
Longer term	Lower	Higher
Larger down payment	Lower	Lower
Extra monthly payment	(same)	Lower, loan ends early

A longer term is the most common trap. It makes the monthly figure look affordable while the total cost climbs. Compare the total interest line, not just the monthly payment.

Amortizing vs. other loan types

Not every loan amortizes. Interest-only loans charge only interest for a period, so the balance doesn’t fall at all. Balloon loans have small payments and a large lump sum at the end. Credit cards use a revolving balance with a minimum payment. The schedule in this guide applies to standard fixed-rate instalment loans. For variable-rate loans, the schedule is recalculated each time the rate changes. See Fixed vs Variable Interest Rates: Which Loan Is Right for You?.

Key takeaways

Your payment is fixed, but its split between interest and principal changes every month.
Interest is front-loaded because it’s charged on a balance that starts high.
Extra payments made early have the biggest impact.
Always compare loans by total interest and APR, not monthly payment alone.

Try it yourself below: change the term from 5 to 7 years and watch the total interest jump.