What Is Compound Interest?
Compound interest is earning interest on your interest. When you invest money, you earn a return. Next period, you earn a return on your original investment plus on last period's return. The effect is exponential โ it starts slowly, then accelerates, then becomes overwhelming.
Albert Einstein is often (probably incorrectly) credited with calling compound interest the eighth wonder of the world. Accurate attribution aside, the principle is genuinely remarkable, and the mathematics are unforgiving in both directions โ it builds wealth over time and it destroys financial health when it's working against you through debt.
Try the numbers yourself with our Compound Interest Calculator.
A Simple Example
Imagine two Australians, Alex and Jordan. Both invest $5,000 per year and earn 7% annually (roughly the long-run return from a diversified share portfolio).
- Alex starts at age 25 and invests for 40 years until age 65
- Jordan starts at age 35 and invests for 30 years until age 65
Alex contributes $200,000 total (40 ร $5,000). Jordan contributes $150,000 total (30 ร $5,000). Alex contributed $50,000 more.
At retirement:
- Alex's portfolio: approximately $1,068,000
- Jordan's portfolio: approximately $472,000
Alex ends up with more than double Jordan's balance by contributing for 10 extra years. The extra $50,000 in contributions produced an extra $546,000 in wealth. That's the power of time in the market.
The Rule of 72
A useful mental shortcut: divide 72 by your expected annual return to find how many years it takes to double your money.
- At 4%: 72 รท 4 = 18 years to double
- At 7%: 72 รท 7 = ~10 years to double
- At 10%: 72 รท 10 = 7.2 years to double
A 30-year-old with $50,000 at 7% will see that become $100,000 by 40, $200,000 by 50, and $400,000 by 60 โ without adding another cent. That's three doublings in 30 years.
Compounding Inside Superannuation
For Australians, superannuation is one of the most powerful compound interest vehicles available โ it's tax-advantaged, professionally managed, and has a decades-long time horizon. The compulsory Super Guarantee means most workers are already participating, but few think about it in compounding terms.
Use the Super Balance Growth Calculator to project your super balance at retirement based on your current balance, salary, and contribution rate. Increase the contribution rate by even 2โ3% and the difference at retirement is typically six figures.
Frequency of Compounding Matters
Compounding can occur annually, monthly, weekly, or daily. The more frequently interest compounds, the faster your balance grows. Most savings accounts compound monthly or daily. Investment returns (shares) are typically modelled annually, but dividend reinvestment and price growth compound continuously in practice.
Compounding Working Against You: Debt
The same maths that builds wealth through investing destroys it through high-interest debt. A credit card balance at 20% interest compounds just as relentlessly. $10,000 in credit card debt with minimum payments can take 20+ years to clear and cost you more than double the original balance in interest.
Use the Savings Goal Calculator to plan how to build savings, and treat high-interest debt as the mathematical inverse of investing โ eliminating it is the guaranteed equivalent of earning the debt's interest rate as a return.
Practical Steps to Harness Compounding
- Start now, not later. Time is the variable that matters most. A 5-year head start is worth more than a 1% higher return over a full investment lifetime.
- Automate contributions. Regular, automatic investments prevent the behavioural tendency to time the market or miss months.
- Reinvest dividends. Don't take investment income as cash โ reinvest it so it compounds alongside your capital.
- Minimise fees. A 1% annual management fee on an investment returning 7% costs you roughly 14% of your final balance over 30 years. Low-cost index funds are a common solution.
- Don't interrupt compounding. Withdrawing from investments resets the compounding clock on that money.
For a thorough, Australian-focused guide to building wealth through long-term investing, investing books on Amazon AU cover portfolio construction, superannuation strategy, and the psychology of staying the course through market volatility.