Compound Interest Calculator

The maths behind the number on your screen

The calculator above is doing the same thing a financial calculator has always done, just faster and without the tiny buttons. The formula is A = P(1 + r/n)^nt, where A is your final balance, P is your initial investment, r is the annual rate as a decimal, n is how many times a year it compounds, and t is the number of years. Feed in $10,000 at 7% p.a. compounded annually for 10 years and you get $10,000 x (1.07)¹⁰ = $19,672. No extra deposits, just $10,000 left alone earning $9,672.

Add $500 a month in regular contributions to that same scenario and the final balance jumps to roughly $106,637. You put in $70,000 across the decade ($10,000 up front plus $500 x 120 months), and the calculator hands back an extra $36,637 that you never actually deposited. That gap is pure compounding, and it's bigger than most people expect because every contribution starts earning its own return the moment it lands, not just the original lump sum. If you want the plain-English version of why this works, our what is compound interest guide breaks down the mechanics without the algebra.

Simple interest pays a flat $700 a year, every year, on the original $10,000. Compound interest pays 7% of whatever the balance has grown to, so each year's dollar amount is bigger than the last. That gap only widens the longer you leave it running.

Why contribution frequency quietly changes your result

Here's a lever most people never touch: even if the total amount you contribute each year stays exactly the same, splitting it into smaller, more frequent deposits gets you a bigger final balance. Money contributed earlier in the year starts compounding sooner, so weekly beats monthly, and monthly beats one annual lump sum.

That's roughly a $10,900 gap between annual and weekly contributions of the exact same yearly total, over 20 years at 8% p.a. If your broker or savings account supports weekly or fortnightly auto-invest, switching to it is a genuinely free upgrade, no extra money required, just better timing. This is essentially dollar cost averaging at work: use the calculator above to model your actual contribution schedule, not just the annual total, and see what it's really costing (or earning) you.

Realistic return rates for Australian investors

The return rate you enter drives more of the outcome than any other input, so it pays to ground it in something real rather than a hopeful guess. The ASX and the RBA both publish long-run data that puts the ASX All Ordinaries at roughly 9% to 10% p.a. nominal, including dividends reinvested. A diversified ETF portfolio blending Australian and international shares is more commonly modelled at 7% to 8% p.a. High-interest savings accounts and term deposits, being far lower risk, sit well below that.

These are historical averages and planning assumptions, not guarantees. Past performance doesn't predict future results, and any given year can land well above or below these long-run figures.

Common mistakes when using a compound interest calculator

• Using nominal returns without adjusting for inflation. An 8% return sounds great, but at 2.5% to 3% inflation your real purchasing power is growing closer to 5% to 5.5%. Subtract your expected inflation rate before entering a return figure if you want today's dollars, not future dollars.
• Forgetting fees. A low-cost ETF barely dents the outcome, but a managed fund or super option charging 1% to 1.5% p.a. can cost you tens of thousands of dollars over a few decades. Subtract the total annual fee from the rate you enter.
• Ignoring tax. This calculator shows gross growth. Returns held outside super are taxed at your marginal rate (with a CGT discount on long-held assets), and super earnings are taxed at 15% inside the fund. Trim your return rate by a point or two to approximate an after-tax figure.
• Assuming contributions are perfectly consistent. Life happens. A 30-year projection at $500 a month is a best case. Run the numbers again at 70% of that contribution to see the realistic range.
• Over-optimising for the best-case rate. Entering 12% because of one great year you read about isn't planning, it's hoping. Always run a conservative scenario alongside your base case.

How to stress-test your projection

One number from the calculator isn't a plan. A range is. Run the same contribution amount and time period three times: a conservative 5% p.a. (roughly a balanced super fund or a high-interest savings account), a base case of 7.5% p.a. (a reasonable long-run assumption for a diversified ETF portfolio), and an optimistic 10% p.a. (in line with the long-run ASX average, though never guaranteed). If your conservative scenario still gets you where you need to be, your plan is solid. If you need the optimistic number to work, you're carrying more risk than you might realise.