Compound Interest Calculator (Australia)

What this calculator does

This is two tools in one. In “How much will it grow?” mode you set a starting balance, a regular contribution and a return rate, and it projects what your savings could be worth after a chosen number of years. In “How long to reach a goal?” mode you flip the problem around: you name a target — say a $120,000 house deposit — and it tells you roughly how many years of saving will get you there. Either way it splits the final figure into what you put in versus what compounding added on top, and draws a simple chart so you can see the snowball build.

Why compounding matters

Compound interest is the reason saving early beats saving hard later. When your return is reinvested rather than spent, the next period’s return is calculated on a bigger balance — so you earn returns on your returns. In the early years the line looks almost straight, because your contributions are doing most of the work. But give it a decade or two and the curve bends upward sharply, because interest starts to dwarf the money you’re adding. That bend is the whole game: time in the market does the heavy lifting, which is why most planners say the best day to start was years ago and the second-best day is today.

Exactly how it’s calculated

For a one-off lump sum the formula is the classic A = P × (1 + r/n)^n×t, where P is the starting balance, r is the annual return as a decimal, n is how many times a year it compounds, and t is the number of years.

Because most people also add money regularly, the calculator uses the future value of an annuity on top of the lump sum:

FV = P × (1 + i)^N + PMT × [ (1 + i)^N − 1 ] ⁄ i

Here i = r ⁄ n is the rate per compounding period, N = n × t is the total number of periods, and PMT is your contribution converted to a per-period amount. If you choose start-of-period contributions (an annuity-due), the contribution term is multiplied by an extra (1 + i) because each payment gets one more period to grow. The “time to reach a goal” mode rearranges the same equation and steps forward period by period until your balance crosses the target, then reports the answer in years and months.

Two optional levers refine the result. An annual fee is subtracted straight off the return rate (a 7% return with a 0.50% fee compounds at 6.50%), which is how ongoing investment costs quietly erode growth. The inflation toggle discounts the final balance back into today’s dollars by dividing by (1 + inflation)^t, so you can see real buying power rather than a big nominal number that just looks impressive.

The Australian specifics

The default 7% return mirrors the kind of long-run assumption used in MoneySmart-style projections for a diversified growth portfolio — broadly in line with historical Australian and global share returns after fees but before inflation and tax. It is an assumption, not a promise: real-world returns are bumpy, and a high-interest savings account or term deposit in 2025–26 is more likely to be around 4.5–5.5%. The inflation default of 2.5% sits squarely in the middle of the Reserve Bank of Australia’s 2–3% target band, which is the band the RBA sets interest rates to maintain over time.

If you’re saving for a home, a 20% deposit is the usual benchmark because it lets you sidestep Lenders Mortgage Insurance, though schemes like the Home Guarantee Scheme can let eligible buyers in with as little as 5%. Remember to budget for stamp duty and other purchase costs on top of the deposit itself — those vary by state and price, so set your target a little higher than 20% of the purchase price to be safe.

The key caveats

This is a planning estimate, not a forecast. It assumes a constant return every period, which never happens in reality — markets zig-zag, and a run of poor early years can dent the outcome more than the average suggests. The figures are pre-tax: in Australia, interest and investment income are added to your assessable income and taxed at your marginal rate, and capital gains tax may apply when you sell. The result also ignores any contribution caps, account limits, or product-specific rules. Treat the number as a friendly ballpark to motivate a plan — then confirm the specifics with your bank, your adviser or the ATO before you rely on it.