Compound Interest
Calculator Australia 2026

How Compound Interest Works

Compound interest is interest earned on both your original investment and on the interest that has already been added to your balance. The formula is A = P(1 + r/n)^(nt) where A is the final amount, P is the principal, r is the annual interest rate, n is the number of times interest compounds per year, and t is the number of years. When you also make regular contributions, each deposit begins earning its own compound interest from the moment it is added, creating a snowball effect that accelerates over time. The key insight is that growth is exponential rather than linear. In the early years the interest earned may seem modest, but as the balance grows, each compounding period adds a larger absolute amount of interest than the last.

For example, investing $10,000 initially plus $500 per month at 7.5% annual return compounded monthly over 20 years grows to approximately $326,000. Your total contributions would be $130,000, meaning compound interest earned you roughly $196,000 — more than you put in. This is the power of compounding over time. In the first year you earn about $1,200 in interest. By year 10, your annual interest exceeds $10,000. By year 20, you are earning over $22,000 per year in interest alone — more than three times your annual contributions. The longer you stay invested, the harder your money works for you.

Simple Interest vs Compound Interest

Simple interest is calculated only on the original principal amount, so it grows linearly. If you invest $10,000 at 7% simple interest, you earn $700 every single year regardless of how long you hold the investment. After 20 years you would have $24,000 in total. Compound interest, by contrast, calculates interest on the growing balance each period. After year one you have $10,700, after year two $11,449 (because 7% of $10,700 is $749, not $700), and after year three $12,250. Each year the interest earned is larger than the last because the base amount keeps growing. After 20 years at 7% compounded annually, your $10,000 grows to approximately $38,697 — over $14,600 more than with simple interest. Over 30 years the gap becomes even more dramatic: $24,000 with simple interest versus $76,123 with compound interest. This is why compound interest is sometimes described as the most powerful force in finance. Every Australian savings account, term deposit, and superannuation fund uses compound interest, which is why consistent long-term investing produces such remarkable results.

The Rule of 72

The Rule of 72 is a mental shortcut that tells you approximately how many years it takes for your money to double. Simply divide 72 by your annual interest rate. At 4% (a typical savings account), your money doubles in about 18 years. At 6%, roughly 12 years. At 7.5% (a common long-term super return), about 9.6 years. At 10% (historical share market returns), approximately 7.2 years. The rule works in reverse too: if you want to double your money in 8 years, you need a return of about 9% per year (72 / 8 = 9). This makes it easy to evaluate investment options without a calculator. For instance, if a term deposit offers 5% and the share market has historically returned 10%, the Rule of 72 shows that shares double your money roughly twice as fast (7.2 years vs 14.4 years), though with considerably more volatility along the way. The rule is most accurate for interest rates between 2% and 15%.

Compound Interest and Australian Super

Superannuation is perhaps the best illustration of compound interest for most Australians. With the employer Superannuation Guarantee at 11.5% of ordinary time earnings (increasing to 12% from 1 July 2025), plus long-term investment returns averaging 7-8% per year for balanced and growth options, super balances benefit enormously from decades of uninterrupted compounding. Consider a 25-year-old earning $80,000 per year. With 11.5% employer contributions ($9,200 per year or roughly $767 per month) growing at 7.5% per year over 40 years to age 65, their super balance reaches approximately $2.1 million. Of that, only about $368,000 came from employer contributions. The remaining $1.7 million — more than 80% of the final balance — was generated by compound investment returns. This is why starting early matters so much. A 35-year-old in the same scenario with only 30 years of compounding would accumulate roughly $940,000. The extra 10 years of compounding more than doubles the final balance without a single extra dollar of contributions. Salary sacrifice contributions amplify the effect further, and the concessional tax rate of 15% on super contributions makes it one of the most tax-effective ways to build wealth in Australia.