Visualise the power of compound interest on Australian savings accounts, term deposits, super, or any investment over time.
| Amount |
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| Year | Opening Balance | Deposits | Interest Earned | Closing Balance |
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Albert Einstein reportedly called compound interest the "eighth wonder of the world." The key insight is that you earn interest not just on your principal, but on all previously accumulated interest.
Simple interest: I = P × r × t. Compound interest: A = P × (1 + r/n)^(n×t). The difference grows dramatically over time — $10,000 at 6% for 30 years gives $18,000 simple interest vs $60,226 compound (monthly).
High-interest savings accounts and term deposits in Australia compound interest monthly or quarterly. Superannuation compounds tax-effectively inside the fund at a concessional 15% tax rate during accumulation.
Divide 72 by the interest rate to estimate how many years it takes to double your money. At 6% p.a., your money doubles in approximately 12 years (72 ÷ 6 = 12). At 9%, about 8 years.
In Australia, interest earned in savings accounts and term deposits is taxable at your marginal rate. Consider tax-effective structures like superannuation for long-term compound growth.
Compound interest is interest calculated on both your original principal AND the interest already accumulated. Unlike simple interest (which only ever calculates on the original amount), compound interest accelerates growth over time because each period's interest itself earns interest in subsequent periods.
Albert Einstein is often (if apocryphally) credited with calling compound interest "the eighth wonder of the world." Whether he said it or not, the mathematical truth is undeniable: compound interest turns modest consistent saving into substantial wealth over time — and it is the fundamental engine behind superannuation, investment returns, and long-term savings growth.
| Scenario | Principal | Rate | After 10yr | After 20yr | After 30yr |
|---|---|---|---|---|---|
| Simple interest | $10,000 | 7% p.a. | $17,000 | $24,000 | $31,000 |
| Compound interest (annual) | $10,000 | 7% p.a. | $19,672 | $38,697 | $76,123 |
| Compound (monthly) | $10,000 | 7% p.a. | $20,097 | $40,388 | $81,220 |
At 30 years, compound interest produces $45,123 more than simple interest on the same $10,000 at the same rate — from the same initial investment, simply because of how interest accumulates on itself.
The standard compound interest formula is:
A = P × (1 + r/n)^(nt)
Where: A = final amount | P = principal (starting amount) | r = annual interest rate (decimal) | n = compounding periods per year | t = time in years
For example: $20,000 at 6% p.a. compounded monthly for 5 years: A = $20,000 × (1 + 0.06/12)^(12×5) = $20,000 × (1.005)^60 = $20,000 × 1.3489 = $26,978
| Year | Balance | Interest Earned That Year |
|---|---|---|
| Year 1 | $21,064 | $1,064 |
| Year 2 | $22,185 | $1,121 |
| Year 5 | $25,837 | $1,298 |
| Year 10 | $33,379 | $1,677 |
| Age (starting 35) | Balance | Total Contributed | Total Growth |
|---|---|---|---|
| 40 | $143,566 | $100,000 | $43,566 |
| 50 | $428,318 | $200,000 | $228,318 |
| 60 | $1,059,287 | $300,000 | $759,287 |
| 67 | $1,699,544 | $370,000 | $1,329,544 |
This example illustrates the extraordinary power of compound growth in superannuation — contributions of $370,000 over 32 years generate $1.33 million in investment growth alone at 7.5% p.a.
The Rule of 72 is a quick mental calculation for estimating how long it takes to double money at a given compound interest rate: Years to double = 72 ÷ Annual interest rate (%)
| Interest Rate | Years to Double | Example |
|---|---|---|
| 3% (low savings account) | 24 years | $50,000 → $100,000 in 24 years |
| 5% (term deposit, 2026) | 14.4 years | $50,000 → $100,000 in 14.4 years |
| 7% (balanced super fund) | 10.3 years | $50,000 → $100,000 in 10 years |
| 9% (growth investment) | 8 years | $50,000 → $100,000 in 8 years |
| 12% (high-growth equity) | 6 years | $50,000 → $100,000 in 6 years |
More frequent compounding produces modestly higher returns at the same annual rate. For $10,000 at 6% over 10 years:
| Compounding Frequency | Final Balance | Difference from Annual |
|---|---|---|
| Annual | $17,908 | — |
| Quarterly | $18,061 | +$153 |
| Monthly | $18,194 | +$286 |
| Daily | $18,221 | +$313 |
The difference in compounding frequency matters less than the rate itself and the time invested. Starting 5 years earlier at the same rate has a far greater impact than switching from annual to daily compounding.
What is the difference between simple and compound interest?
Simple interest is calculated only on the original principal throughout the entire period. Compound interest is calculated on the principal plus all accumulated interest — meaning interest earns interest. Over long periods, the difference is dramatic: $10,000 at 7% for 30 years yields $31,000 in simple interest versus $76,123 with annual compound interest.
How often do Australian savings accounts compound?
Most Australian high-interest savings accounts compound monthly or annually. More frequent compounding gives slightly higher effective returns. Daily compounding gives approximately the same result as monthly for most practical purposes — the difference is about $27 per year on a $10,000 account at 5%.
What is a realistic compound interest rate for Australian investments?
For high-interest savings accounts in June 2026: 5.0–5.2% p.a. For balanced super funds (long-run historical): approximately 6.5–8% p.a. net of fees. For the ASX 200 total return (including dividends, long-run average): approximately 9–10% p.a. Use the rate appropriate to your actual investment when modelling.
How does compound interest affect superannuation?
Super is compound interest in action over a career. Employer SG contributions (12%) and investment returns compound tax-advantageously at 15% earnings tax inside super. Starting contributions early is more valuable than contributing larger amounts later. A 25-year-old starting super earns on average 40 extra years of compound growth versus someone starting at 45 with the same annual contribution.
What is the Rule of 72?
Divide 72 by the annual compound interest rate to estimate how many years it takes to double your money. At 6% annual return, money doubles in approximately 12 years. At 9%, about 8 years. At 3%, about 24 years. The rule works best for rates between 3% and 20%.
Does compound interest apply to debt as well?
Yes, and this is why credit card debt is so damaging. A $10,000 credit card balance at 20% p.a. compounding monthly grows to $22,196 in 4 years without any payments. Compound interest works against you when you owe money just as powerfully as it works for you when you invest.
What is the effective annual rate vs nominal rate?
The nominal rate is the stated annual rate. The effective annual rate accounts for compounding frequency. A 6% nominal rate compounded monthly has an effective annual rate of 6.168% — because 12 monthly compounds at 0.5% each actually produce slightly more than a single 6% annual compound. Most Australian product disclosures quote nominal annual rates.
How does extra saving frequency affect compound growth?
Adding regular contributions to a compound interest investment dramatically accelerates growth. Adding $500 per month to a $10,000 starting balance at 7% annual compound growth for 20 years produces approximately $288,000 — versus just $38,697 without the monthly contributions. The regular contributions matter more than the initial balance in long-run wealth building.