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How Compound Interest Actually Works
And Why Starting Late Costs More Than You Think
Compound interest is one of those concepts everyone's heard of and almost nobody has actually modelled. Here's what's happening under the hood, what drives it, and why time in the market beats almost everything else.
The difference between interest and compound interest
Simple interest is straightforward. You put $10,000 in an account at 5% per year. Every year you earn $500. After 20 years you've earned $10,000 in interest and your balance is $20,000.
Compound interest is different because the interest you earn becomes part of the balance that earns the next round of interest. In year one you earn $500 on your $10,000. In year two you earn 5% on $10,500, which is $525. The year after that, 5% on $11,025 - which is $551.25. Each year the base grows, and the interest earned on that base grows with it.
After 20 years at the same 5%, your $10,000 becomes $26,533 - not $20,000. That extra $6,533 is the compounding effect. And that's without adding another dollar to the account.
The three variables that drive everything
Rate, time, and contributions. Not in equal proportion.
Rate
The annual interest or return rate. Higher is obviously better, but chasing yield above sustainable rates is how people end up in risky products. A reliable 6-7% over decades outperforms a volatile 15% that resets periodically.
Time
The most powerful variable and the hardest to recover once it's gone. Ten extra years at the start of a savings period matters more than almost any other change you can make to the model.
Contributions
Regular additions to the principal accelerate compounding significantly. Even small consistent contributions outperform larger irregular ones over long periods because of the time each dollar gets to compound.
Why time beats rate: increasing your return rate from 5% to 7% is valuable. But starting ten years earlier at 5% typically produces a larger outcome than starting ten years later at 7%. Time compounds. You can't buy it back.
The cost of waiting in real numbers
What a 10-year delay actually does to an outcome.
Take two people. Both invest $500 per month at 7% per year. Person A starts at 25. Person B starts at 35.
Person A contributes for 40 years and ends up with roughly $1.3 million at 65. Person B contributes for 30 years and ends up with roughly $567,000. Person A put in $60,000 more in contributions - but ended up with $733,000 more in the account. The extra $673,000 beyond the additional contributions is the compounding effect of those ten extra years.
Those numbers are illustrative - your actual outcome depends on your return rate, contribution amount, fees, and tax treatment. But the shape of the relationship is consistent: time is disproportionately valuable, and delays are disproportionately expensive.
Compounding frequency - does it matter?
Daily vs monthly vs annually - here's the honest answer.
Banks and investment products compound interest at different frequencies - some daily, some monthly, some quarterly, some annually. More frequent compounding does produce a higher return, but the difference is smaller than most people expect.
At 5% annual interest on $10,000, the difference between annual compounding and daily compounding over one year is about $12.67. Over 20 years it's more meaningful but still secondary to rate and time. If a product is offering daily compounding as a headline feature, look past it to what the actual rate is.
For savings accounts, the comparison metric that matters is the comparison rate (or effective annual rate), which accounts for compounding frequency and fees. Use that to compare products, not the advertised rate alone.
Where to find competitive savings rates in Australia
The rate your savings earn is the other half of the equation.
Modelling compound interest is only useful if your money is actually earning a competitive rate. Savings accounts at the major banks often pay significantly less than what's available through online banks, credit unions, and challenger banks.
Comparison sites like Canstar, RateCity, and Finder publish current savings rate tables updated regularly. The highest rates in Australia typically come with conditions - bonus rates that apply only if you deposit a minimum amount per month, don't make withdrawals, or meet other criteria. Read the conditions before switching.
Canstar →
Savings account comparison with star ratings and rate history. Filter by institution type and conditions.
RateCity →
Real-time rate comparisons across Australian savings products. Includes bonus rate conditions clearly.
Finder →
Savings account reviews with editorial ratings and rate alerts you can set up for products you're watching.
This content is general information only and not personal financial advice. Consider your own circumstances before making financial decisions.
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