What Is Compound Interest?
Compound interest is interest calculated not just on your original principal, but also on the interest you've already earned. In other words, your interest earns interest. Over time, this creates an accelerating growth effect that can dramatically increase the value of money — whether you're saving, investing, or carrying debt.
It's often described as one of the most powerful concepts in personal finance, and for good reason: the longer compound interest works, the more dramatic its effects become.
Compound Interest vs. Simple Interest
With simple interest, you earn interest only on your original deposit. For example, $1,000 at 5% simple interest earns $50 per year, every year — for $500 total over 10 years.
With compound interest, your interest is added to the principal each period, and future interest is calculated on this growing balance. The same $1,000 at 5% compounded annually grows to over $1,628 after 10 years — an extra $128 compared to simple interest, entirely from compounding.
The Formula
The compound interest formula is:
A = P × (1 + r/n)^(nt)
- A = Final amount
- P = Principal (starting amount)
- r = Annual interest rate (as a decimal)
- n = Number of times interest compounds per year
- t = Time in years
The more frequently interest compounds (daily vs. monthly vs. annually), the faster growth occurs — though for most savings accounts, this difference is modest.
The Rule of 72
A useful shortcut: divide 72 by your annual interest rate to estimate how many years it takes for your money to double. At 6% annual return, your money doubles roughly every 12 years. At 9%, every 8 years. This rule illustrates why higher returns and longer time horizons matter so much.
How Compounding Works In Your Favour
Savings Accounts
Even modest interest in a high-yield savings account benefits from compounding. The key is consistency: leaving money in place and adding to it regularly maximises the compounding effect.
Investment Portfolios
In a diversified investment portfolio, compound growth comes from reinvested dividends and capital appreciation. A long-term investor who reinvests all dividends will accumulate significantly more wealth than one who takes dividends as cash — even if the underlying returns are identical.
When Compounding Works Against You
The same mechanism that builds wealth can destroy it when applied to debt. Credit cards typically compound interest daily or monthly at high rates. If you carry a balance and only make minimum payments, the interest on your debt compounds rapidly — meaning you can end up paying far more than you originally borrowed.
This is why high-interest debt should be eliminated as a financial priority before focusing on investing. The "guaranteed return" of paying off a 20% APR credit card is hard to beat.
The Time Factor: Why Starting Early Matters
| Starting Age | Monthly Contribution | Annual Return | Value at Age 65 |
|---|---|---|---|
| 25 | $200 | 7% | ~$525,000 |
| 35 | $200 | 7% | ~$243,000 |
| 45 | $200 | 7% | ~$101,000 |
These figures are illustrative estimates using standard compound growth calculations.
The difference between starting at 25 versus 35 is stark — not because of different contributions, but because of 10 extra years of compounding. This is why financial advisors consistently emphasise starting as early as possible, even with small amounts.
Practical Takeaways
- Start investing early — even small amounts benefit enormously from time.
- Reinvest dividends — don't take them as cash if you're in a growth phase.
- Eliminate high-interest debt — compound interest on debt is compounding working against you.
- Be consistent — regular contributions amplify compounding's effect.
- Leave investments alone — time is the most important ingredient.
Final Thoughts
Compound interest rewards patience, consistency, and time above all else. Whether you're building a savings account, growing an investment portfolio, or working to eliminate debt, understanding how compounding works gives you a clear strategic advantage. The best time to start was yesterday — the second best time is today.