Compound Interest Explained: How Your Money Grows (with Examples)

Compound interest is interest calculated on both the initial principal and the accumulated interest from previous periods. Unlike simple interest, which is calculated only on the original amount, compound interest creates a snowball effect where your money grows at an accelerating rate over time.

The short version: The formula is A = P(1 + r/n)^(nt), where P is the principal, r is the annual interest rate, n is the number of compounding periods per year, and t is the number of years. For example, $10,000 invested at 7% annual interest compounded annually grows to $10,700 after one year, $11,449 after two years, and $19,672 after ten years. The $9,672 in growth is more than the original investment and accelerates each year because you're earning interest on an increasingly larger balance.

This is why Einstein reportedly called compound interest the eighth wonder of the world.

The single most important factor in compound interest is time. Starting early, even with smaller amounts, almost always beats starting later with larger amounts.

Consider two investors: Alice invests $5,000 per year from age 25 to 35 (10 years, total invested $50,000) and then stops contributing. Bob invests $5,000 per year from age 35 to 65 (30 years, total invested $150,000). Assuming 7% annual returns, Alice's portfolio at age 65 is approximately $602,000.

Bob's portfolio at age 65 is approximately $505,000. Alice invested one-third as much money but ended up with more because her money had an extra 10 years to compound. This example illustrates why financial advisers consistently emphasise starting to save and invest as early as possible.

Real talk — Every year you delay costs you significantly more than the amount you would have invested. That catches a lot of people off guard.

Compound interest works both ways — it grows your savings but also grows your debt. On savings, it's your best friend.

A $20,000 high-interest savings account at 5% earns $1,000 in the first year, $1,050 in the second year, and $1,102.50 in the third year — each year earning interest on the previous year's interest. On debt, compound interest is your enemy. A $10,000 credit card balance at 20% grows by $2,000 in the first year if unpaid, $2,400 in the second year, and $2,880 in the third — the debt accelerates just as savings do.

This is why paying off high-interest debt before investing is usually the optimal strategy. Eliminating a 20% credit card balance is equivalent to earning a guaranteed 20% return on your money, which no investment can reliably match.

Interest can compound at different frequencies: daily, monthly, quarterly, or annually. More frequent compounding results in slightly higher returns because interest is calculated on a larger balance more often.

One thing people miss: For a $10,000 investment at 7%: annual compounding yields $10,700 after one year; monthly compounding yields $10,723; and daily compounding yields $10,725. The difference is small over short periods but grows over decades. Most Australian savings accounts compound daily and credit interest monthly.

Term deposits typically compound at maturity or at intervals you choose. Share market investments don't compound in the traditional sense, but reinvesting dividends achieves a similar effect — buying additional shares that generate their own dividends, creating a compounding cycle. Use our Compound Interest Calculator to visualise how different principal amounts, rates, and compounding frequencies affect your wealth over time.

To harness compound interest for wealth building, follow these principles. First, start as early as possible — even small amounts benefit enormously from time.

Second, be consistent — regular contributions (weekly, fortnightly, or monthly) into savings or investments create a steady compounding base. Third, reinvest returns — don't withdraw interest or dividends; let them compound. Fourth, minimise fees — investment fees reduce your effective return and therefore reduce compounding.

A 1% annual fee on a $200,000 portfolio costs $2,000 per year that's no longer compounding. Fifth, be patient — compound interest is a long-term game; the most dramatic growth occurs in the later years. A $100,000 investment at 7% takes 10 years to reach $197,000, but only another 10 years to reach $387,000 and another 10 to reach $761,000.

Heads up — The growth is exponential, not linear. Use our Compound Interest Calculator and Savings Goal Calculator to model your specific scenario.