Compound Interest Explained — How Your Money Grows

What Is Compound Interest?

Compound interest is interest earned on interest. When you invest or save money, you earn interest on your original amount — but then that interest also starts earning interest. Over time, this creates an exponential growth effect. Albert Einstein reportedly called it the eighth wonder of the world, and for good reason — over long periods, compound interest can turn modest savings into substantial wealth.

How It Works in Practice

If you invest $10,000 at a 7% annual return, after one year you have $10,700. In year two, you earn 7% on $10,700 — not just the original $10,000 — giving you $11,449. After 10 years without adding any extra money, that $10,000 grows to about $19,672. After 30 years, it becomes over $76,000. The key driver is time — the longer your money compounds, the more dramatic the growth.

Why Superannuation Is the Best Example

Superannuation is the most powerful example of compound interest available to Australians. Your employer contributes 12% of your salary, which is invested and earns returns. Those returns generate further returns. A 25-year-old earning $70,000 who retires at 67 could accumulate over $700,000 in super — even before considering additional contributions — simply through the power of compounding over four decades. Starting just five years later can reduce the final balance by over $150,000.

How Compound Interest Works Against You

Compound interest also works against you when you owe money. Credit card debt compounding at 20% per year can quickly spiral out of control. A $5,000 credit card debt paid only at the minimum rate could take over 30 years to clear and cost more than $15,000 in interest. This is why paying down high-interest debt should be a priority before building savings. Starting to save and invest early, even in small amounts, can make a dramatic difference over time — see our young adult financial checklist for tips on getting started.

How Compound Interest Works

Compound interest is often called the eighth wonder of the world. It means you earn interest not only on your original investment (the principal) but also on the interest that accumulates over time. The longer your money is invested, the more powerful the compounding effect becomes.

For example, if you invest $10,000 earning 7% per year: after 10 years you have $19,672 (without adding any more money), after 20 years you have $38,697, and after 30 years you have $76,123. More than half the growth comes in the later years as the compounding effect accelerates.

The Rule of 72

The rule of 72 is a simple way to estimate how long it takes to double your money. Divide 72 by the annual return rate. For example: 72 divided by 7% = approximately 10.3 years to double. At 10% per year, money doubles every 7.2 years. At 5%, it takes about 14.4 years.

This rule also works in reverse — to estimate the return needed to double your money in a given time period. If you want to double your money in 10 years, you need an annual return of approximately 7.2% (72/10 = 7.2).

Applying Compound Interest to Your Finances

Compound interest is most powerful when you start early. Investing $5,000 per year from age 25 to 35 (total $50,000) and then stopping would grow to approximately $600,000 by age 65 at 7% return. But investing $5,000 per year from age 35 to 65 (total $150,000) would grow to only about $500,000 — even though you invested three times as much.

Compound interest works against you on debt. Credit card debt at 20% interest doubles in just 3.6 years. This is why paying down high-interest debt should be a priority before investing.

Frequently Asked Questions

How is compound interest different from simple interest?
Simple interest is calculated only on the principal amount. Compound interest is calculated on the principal plus accumulated interest. Over time, the difference becomes dramatic — $10,000 at 7% simple interest for 30 years earns $21,000. The same amount at compound interest grows to $76,123.

Does compound interest apply to super?
Yes. Superannuation is one of the best examples of compound interest. Your super contributions plus investment earnings generate returns, and those returns generate further returns. This is why starting super contributions early is so important.

How often is interest compounded?
It depends on the product. Savings accounts may compound daily or monthly. Investments may compound when dividends are reinvested. Super funds compound when investment earnings are credited to your account. More frequent compounding means slightly faster growth.

Can compound interest make me rich?
Combined with regular saving and time, compound interest can build significant wealth. A regular savings plan of $500 per month from age 25 to 65 at 7% would grow to approximately $1.2 million. The key is consistency and time.

The Rule of 72 and Other Mental Models

The Rule of 72 is a simple way to estimate how long it takes an investment to double at a given annual rate of return. Divide 72 by the expected annual return to get the approximate number of years. For example, at 6% per year: 72 ÷ 6 = 12 years to double. At 8%: 72 ÷ 8 = 9 years. At 10%: 72 ÷ 10 = 7.2 years. This rule works for any compounding investment — shares, super, property. The flip side is that the same rule applies to debt: a credit card charging 18% interest will double your debt in just 4 years (72 ÷ 18 = 4) if you don't pay it off.

Another useful concept is the "difference of 1%." An extra 1% in annual returns may not seem significant, but over 30 years on a $100,000 investment, 6% vs 7% compounded annually produces a difference of approximately $160,000 — the 1% extra return generates over 50% more wealth in dollar terms. This is why minimising fees (which reduce your net return) is so important. A 0.50% fee may seem small, but over decades it consumes a significant portion of your potential returns.

Compounding in Superannuation — Real Numbers

Superannuation is where compounding has its most powerful effect for most Australians, due to the long investment horizon and tax-advantaged environment. Consider a 30-year-old with a $50,000 super balance earning $70,000 per year with the employer contributing 11.5% Super Guarantee. Assuming 7% annual returns (after fees and taxes), the super balance at age 60 could reach approximately $1.1-$1.2 million. Of that, only about $250,000 would come from contributions — the remaining $850,000+ is investment earnings, mostly from compounding in later years.

This highlights two key insights. First, every dollar of fees saved today has a massive impact on your final balance — reducing fees from 1% to 0.5% could add $100,000+ to your retirement balance. Second, additional contributions early in your career are disproportionately valuable. An extra $10,000 contributed at age 30 could grow to approximately $75,000 by age 60 at 7% returns. The same $10,000 contributed at age 50 would grow to only about $20,000. This is why starting to save and invest early — even small amounts — is the single most powerful financial decision you can make.