Compound Interest Calculator Australia — See Your Money Grow (2026)

Compound interest is the reason a $10,000 investment growing at 8% for 40 years becomes ~$217,000 — not the $42,000 simple interest would give you. Each year's earnings join the base, so next year's earnings are calculated on a bigger pile. Over decades, the difference becomes enormous.

The mechanics

Compound interest grows a balance by applying the rate to the new total each period, not just the original principal. The formula:

A = P × (1 + r/n)^(n × t)

Where P is the starting balance, r is the annual rate (as a decimal), n is the number of compounding periods per year, and t is the time in years.

Two things drive the result: rate and time. Rate is linear — a 1% increase delivers a meaningful but predictable bump. Time is exponential — doubling the horizon more than doubles the result, because the later years are doing the heaviest lifting.

Time matters more than rate

This is the single most important thing about compounding: starting earlier almost always beats getting a higher rate.

Starting balance	Rate	Years	End balance
$10,000	6%	40	~$103,000
$10,000	8%	40	~$217,000
$10,000	6%	25	~$43,000
$10,000	8%	25	~$68,000

A 2% higher rate over the same 40 years delivers ~$114,000 of extra balance. But shortening the horizon by 15 years at the higher rate costs ~$149,000. Time wins. This is the case for starting super contributions, kid's investment accounts, or long-horizon savings goals as early as possible — the math compounds in a way intuition doesn't.

With regular contributions

Most real-world compounding scenarios involve adding money over time — paycheck contributions to super, monthly transfers to an investment account, regular savings deposits. The math gets a bit more involved, but the principle is the same: every contribution gets to compound for the time remaining until the end of the horizon.

The earliest contributions matter the most. A $5,000 deposit in year 1 of a 30-year, 8%-return scenario grows to ~$50,000. The same $5,000 deposit in year 25 only has 5 years to grow — it ends at ~$7,400. The first contribution earns roughly 7× more than an identical contribution made 24 years later.

A practical illustration:

• $500/month for 40 years at 7% return → ~$1.31 million end balance, ~$240,000 contributed, ~$1.07 million from compounding.
• $1,000/month for 20 years at 7% return → ~$521,000 end balance, ~$240,000 contributed, ~$281,000 from compounding.

Same total contributions, half the timeframe. The first scenario produces 2.5× more wealth — purely from giving the money more time.

Compounding frequency: less important than you'd think

You'll see savings accounts advertised as "compounded daily" and term deposits as "interest paid annually". In practice, the difference is small over long horizons.

For a 7% nominal annual rate over 30 years:

• Annual compounding: ~7.00% effective rate, balance grows ~7.6× from initial
• Monthly compounding: ~7.23% effective rate, balance grows ~8.1×
• Daily compounding: ~7.25% effective rate, balance grows ~8.2×
• Continuous compounding: ~7.25% effective rate, balance grows ~8.2×

Spending energy choosing between monthly and daily compounding usually misses the bigger lever — picking the right product (savings vs term deposit vs ETF vs super) and starting earlier. A 1% higher underlying return swamps any compounding-frequency optimisation.

Inflation: the missing variable

A 7% return sounds great until you realise inflation eats some of it. The Reserve Bank targets 2–3% inflation over the medium term. So a 7% nominal return is closer to a 4.5% real return — i.e., 4.5% growth in actual purchasing power.

Over 30 years at 4.5% real, $1 grows to roughly $3.75 in today's dollars. Over the same 30 years at 7% nominal, $1 grows to ~$7.61 — but those future dollars buy less. Both views are useful: nominal for matching what you'll see on statements, real for actually understanding what the money will buy.

For long-horizon planning, default to real return (your assumed rate minus 2.5%). It removes the illusion of bigger numbers and tells you what you can actually afford.

Tax (the second missing variable)

Inside super, fund earnings are taxed at 15%. Outside super, you're taxed at your marginal rate on interest (savings, term deposits) and on dividends, with a 50% CGT discount on shares held over 12 months.

Quick rules of thumb for Australian investors:

• Savings/term deposit at 5%, 30% marginal tax: ~3.5% after-tax. Use 3.5% in this calculator for realistic outcomes.
• Index fund at 8% total return, 30% marginal tax, half from dividends and half from gains held >12 months: ~6.4% after-tax. Use 6.4%.
• Super at 7% gross fund return: ~6.0% after-tax inside the fund.

For a fairer projection, use the after-tax rate in the calculator above. Actual results vary with your specific tax situation, but the ballpark is more realistic than gross figures.

The rule of 72

A shortcut: 72 ÷ rate (in %) ≈ years to double the money.

• At 4%: doubles in 18 years
• At 6%: doubles in 12 years
• At 8%: doubles in 9 years
• At 12%: doubles in 6 years

Useful for back-of-envelope planning. A 30-year-old contributing to super at a real return of 6% can expect their starting balance to double 2.5 times before retirement — i.e., roughly 6× the initial amount, before adding any new contributions.

For more specific scenarios, plug your actual numbers into the calculator above. For savings-specific projections (deposits at a known rate), see the Savings Account Calculator and Term Deposit Calculator.