Compound interest is the mechanism by which investment returns generate further returns — earning interest on interest, or growth on growth. It is the fundamental driver of long-term wealth accumulation, and understanding how it works helps Australian investors appreciate why time in the market matters more than almost any other factor.
What Is Compound Interest?
Simple interest pays interest only on the original principal. Compound interest pays interest on both the original principal and all previously earned interest.
Simple interest example: $10,000 at 7% for 5 years = $10,000 + (7% × $10,000 × 5) = $13,500
Compound interest example: $10,000 at 7% compounded annually for 5 years:
| Year | Opening balance | Interest earned | Closing balance |
|---|---|---|---|
| 1 | $10,000 | $700 | $10,700 |
| 2 | $10,700 | $749 | $11,449 |
| 3 | $11,449 | $801 | $12,250 |
| 4 | $12,250 | $858 | $13,108 |
| 5 | $13,108 | $918 | $14,026 |
Compound interest: $14,026 vs simple interest: $13,500. Over 5 years, the difference is $526. Over 30 years, it becomes transformative.
The Compound Interest Formula
$$A = P \times \left(1 + \frac{r}{n}\right)^{n \times t}$$
Where:
- $A$ = Final amount (principal + interest)
- $P$ = Principal (starting amount)
- $r$ = Annual interest rate (decimal — 7% = 0.07)
- $n$ = Number of compounding periods per year
- $t$ = Time in years
For annual compounding ($n = 1$), this simplifies to:
$$A = P \times (1 + r)^t$$
Compounding Frequency
How often interest compounds affects the final result:
| Compounding frequency | $10,000 at 7% after 20 years |
|---|---|
| Annually | $38,697 |
| Quarterly | $39,575 |
| Monthly | $39,869 |
| Daily | $40,027 |
In practice, the difference between monthly and annual compounding is modest — the far bigger factor is the rate of return and time invested.
For share market investments, compounding works through:
- Price appreciation (the share price grows year on year)
- Dividends reinvested (dividends buy more shares, which earn more dividends)
The Rule of 72
A simple mental shortcut: divide 72 by the annual return rate to estimate how many years to double your money.
| Return rate | Years to double |
|---|---|
| 4% | 18 years |
| 6% | 12 years |
| 7% | 10.3 years |
| 8% | 9 years |
| 10% | 7.2 years |
At 7%, your money doubles approximately every 10 years.
Long-Term Compounding: Australian Examples
$50,000 invested at 7% annually:
| Years | Value |
|---|---|
| 10 | $98,358 |
| 20 | $193,484 |
| 30 | $380,613 |
| 40 | $748,985 |
After 40 years, $50,000 has grown to nearly $750,000 — without adding a single dollar of additional savings.
$500/month invested from age 25 to 65 at 7%:
- Total contributed: $240,000
- Final portfolio value: ~$1,320,000
- Interest/growth earned: ~$1,080,000
The power: you earned more from compounding than you contributed yourself.
Why Starting Early Matters More Than Amount
Scenario 1: Anna invests $10,000/year from age 25–35 (10 years), then stops. Total contributed: $100,000.
Scenario 2: Ben invests $10,000/year from age 35–65 (30 years). Total contributed: $300,000.
Assuming 7% annual return, by age 65:
- Anna: ~$1,068,000
- Ben: ~$944,000
Anna contributed one-third as much as Ben but ends up with more — because of the additional 10 years of compounding at the start.
Compounding in Australian Superannuation
Super is the most powerful compounding vehicle for most Australians:
- Earnings inside super (accumulation phase) are taxed at only 15% — meaning more remains to compound
- Concessional contributions (employer + salary sacrifice) are taxed at 15%, not your marginal tax rate
- Pension phase: 0% tax on earnings — full compounding with no tax drag
The combination of tax concessions and mandatory employer contributions (11.5% in FY2024–25) makes super Australia’s primary long-term compounding engine.
Compound Interest vs Compound Returns (Shares)
Savings accounts and term deposits pay compound interest — a guaranteed rate on your balance. Share market investments earn compound returns — which include price growth and dividends, but are not guaranteed and vary year to year.
The mathematics is the same — but shares carry volatility risk that bank accounts do not. Over long time horizons (20+ years), diversified share portfolios have historically outpaced savings accounts by a significant margin (MSCI World, ASX 200 data).
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Frequently Asked Questions
How does compound interest work in Australia? Compound interest (or compound returns in share investing) means you earn returns on your existing returns — not just your original investment. A $10,000 investment at 7% earns $700 in year 1, then earns 7% on $10,700 in year 2, and so on. Over 30 years at 7%, $10,000 becomes $76,123 — with $66,123 coming from compounding.
What is the best compound interest account in Australia? For guaranteed compound interest, high-interest savings accounts and term deposits at banks like ING, Macquarie, and online banks offer the highest rates. For long-term compounding with higher expected returns (and higher risk), diversified share ETFs in a brokerage account or super are more commonly used for wealth-building.
How do I calculate compound interest in Australia? Use $A = P \times (1+r)^t$ for annual compounding, or $A = P \times (1 + r/n)^{nt}$ for other frequencies. Alternatively, ASIC’s MoneySmart compound interest calculator (moneysmart.gov.au) provides an interactive tool with AUD examples.
This article provides general financial information only. Compound interest calculations are illustrative — actual investment returns will vary. For advice tailored to your situation, speak with a licensed financial adviser through the ASIC financial advisers register or MoneySmart.