The Power of Compound Interest: How Compounding Builds Wealth Over Time

Put plainly, compounding is the process of earning returns on your returns. Each year, the earnings your money generates are added to your balance, and the following year you earn returns on that larger amount. Over long periods, that snowball effect can turn modest, regular contributions into a meaningful sum.

This guide explains what compound interest is, how it differs from simple interest, how it applies to shares and ETFs (which work a little differently from a bank account), and the practical habits that help everyday Australian investors put it to work. Any advice included in this article is general in nature and has been prepared without considering your objectives, financial situation or needs. Before acting on this advice, you should consider whether it's appropriate to you, having regard to your objectives, financial situation or needs

What is compound interest?

Compound interest means earning interest not just on the money you originally put in (the principal), but also on the interest you have already earned. In a savings account, for example, your interest is added to your balance, and the next interest payment is calculated on that bigger balance. You earn interest on your interest — and the cycle repeats.

The Australian Government's Moneysmart describes it well: the longer you save, the more you earn, so the lesson is to start as early as you can and keep going. A handy mental shortcut is the Rule of 72: divide 72 by your annual return to estimate how many years it takes to double your money. At an illustrative 7% per year, that is roughly 72 ÷ 7 ≈ 10 years to double.

Compound interest vs simple interest

Simple interest is paid only on your original principal. Compound interest is paid on your principal plus everything you have earned so far. Over a year or two the difference is small. Over decades it is dramatic.

Here is an illustrative example. Imagine $10,000 earning 7% per year for 10 years. (This figure is illustrative only — it is not a forecast, and real returns vary and can be negative.)

Same starting amount, same rate — but compounding earns roughly $2,670 more over the decade, purely because the earlier earnings keep working. Stretch the time horizon to 20, 30 or 40 years and the gap widens enormously.

The two things that supercharge compounding: time and rate

Compounding rewards time above almost everything else. The earlier you begin, the more compounding cycles your money goes through. Consider two illustrative investors who each invest $200 a month at an assumed 7% per year, compounded monthly.

Ali contributed only $24,000 more than Lou, yet finished with more than double the balance — simply by starting ten years earlier. That is the power of giving compounding more time. (Again, 7% is illustrative; it is not guaranteed, and past performance is not a reliable indicator of future returns.)

The rate of return matters too — higher-returning assets compound faster — but higher potential returns generally come with higher risk and more short-term ups and downs. Choosing investments that suit your goals and risk tolerance matters more than chasing the highest headline number.

Does compound interest apply to shares and ETFs?

Strictly speaking, shares do not pay interest — so it is more accurate to talk about compound returns. With shares and ETFs, compounding happens in two main ways:

• Reinvested dividends and distributions. When a company or fund pays income, you can take it as cash or reinvest it to buy more shares or units. Those extra units then earn income of their own next time around.

• Reinvested capital growth. As the value of your holdings grows, future percentage gains apply to a larger base.

A practical way to automate the first one is a dividend reinvestment plan (DRP), which many ASX-listed companies and ETFs offer. DRPs are typically commission-free — your income buys additional shares or units without you paying brokerage on the reinvestment — which keeps more of your money compounding.

One honest caveat: unlike a bank's stated interest rate, market returns are not fixed and not guaranteed. Share prices and distributions move up and down, and in some years your balance can fall. Compounding amplifies long-term growth, but it cannot remove the risk that comes with investing. Investing carries risk, including the loss of capital.

How to make compounding work for you in Australia

You do not need a large sum or perfect timing to benefit. A few simple habits do most of the heavy lifting.

1. Reinvest your income

Take dividends and distributions as more shares or units rather than cash where it suits your goals. Opting into a DRP, or manually reinvesting, keeps the snowball rolling instead of melting away as spending money.

2. Invest regularly

Investing a set amount on a regular schedule — known as dollar-cost averaging — adds fresh money for compounding to work on and smooths out the price you pay over time. Selfwealth's Auto-Invest feature lets you set up recurring orders so this happens automatically.

3. Keep your costs low

Fees compound against you in the same way returns compound for you, so every dollar saved on brokerage is a dollar that can keep growing. Selfwealth charges flat $9.50 brokerage per trade on Australian and US shares, with no account-keeping fees. Your ASX shares are CHESS-sponsored, meaning you hold direct legal ownership under your own Holder Identification Number (HIN) — there is no custodian in between.

4. Give it time and stay invested

Compounding does its best work over years and decades, not weeks. History suggests that time in the market tends to beat timing the market — staying invested through the ups and downs gives compounding the runway it needs.

Ready to put time on your side? You can open a Selfwealth account — individual, joint, company, trust or SMSF — and start building your portfolio. New to investing? Our guide to how to start investing with just $500 is a good place to begin.

The flip side: when compounding works against you

Compounding is not always your friend. The same maths that grows your investments also grows debt. High-interest debt such as credit cards and some buy-now-pay-later arrangements compounds against you. Using the Rule of 72, a balance at 20% interest can roughly double in under four years (72 ÷ 20 ≈ 3.6) if it is left to run. Clearing expensive debt is often one of the most powerful financial moves you can make.

Inflation works in a similar direction. If your money grows at 4% but prices rise at 3%, your real return — your growth in purchasing power — is only about 1%. It is worth thinking in real, after-inflation terms when you plan for the long run.

Common mistakes that interrupt compounding

Withdrawing your earnings instead of reinvesting them, which resets the snowball.

• Sitting entirely in cash for long periods, which can feel safe but may not keep pace with inflation.

• Trying to time the market, which often means missing the best days and interrupting growth.

• Chasing returns that look too good to be true, which usually carry outsized risk.

Starting late — the most common one. The second-best time to start is today.

Frequently asked questions

What is the power of compound interest?

It is the way earnings generate further earnings over time. Because you earn returns on both your original money and your accumulated returns, your balance can grow at an accelerating rate the longer it stays invested.

How is compound interest different from simple interest?

Simple interest is calculated only on your original principal. Compound interest is calculated on your principal plus all the interest already earned, so it grows faster over time.

Do shares and ETFs earn compound interest?

Not interest as such — shares and ETFs compound through reinvested dividends or distributions and reinvested capital growth. Unlike a bank's interest rate, these returns vary and are not guaranteed.

How can I work out compound interest?

The formula is A = P(1 + r/n)nt, where P is the principal, r the annual rate, n the number of times it compounds per year, and t the number of years. For a quick estimate of doubling time, use the Rule of 72. The Moneysmart compound interest calculator lets you model different scenarios.

How do I start compounding my investments in Australia?

Open a share-trading account, invest regularly (for example via Auto-Invest), reinvest your dividends or distributions, keep your costs low, and give it time. Starting small and staying consistent matters more than starting big.

Can compound interest work against me?

Yes. On debt such as credit cards, interest compounds on what you owe, so balances can grow quickly. Inflation also erodes the real value of money over time. Both are worth factoring into your plans.