How Compound Interest Actually Works
Compound interest is the single most powerful force in personal finance — and the most misunderstood. It is interest earned on both your original money and on the interest that money has already earned. Each year your balance grows, the next year's interest is calculated on a larger base, so the growth snowballs. The mathematics are simple, but the human brain struggles to grasp exponential growth intuitively, which is why most people start saving too late. The year-by-year table above is the clearest way to see it: notice how the interest earned each year grows larger than the year before, even though your monthly contribution stays the same.
What Is Compound Interest?
Compound interest is the return you earn on returns. If you save R1,000 at 10% for one year, you earn R100 in interest and end up with R1,100. In year two, you earn 10% on the full R1,100 — that is R110, not R100 — bringing your balance to R1,210. In year three, you earn 10% on R1,210, which is R121. Each year the interest gets larger because it is calculated on a larger base, and that is the snowball effect. Over 20 or 30 years the effect becomes dramatic: at 10% over 30 years, R1,000 grows to R17,449 — more than 17 times the original amount, with 94% of that final figure being interest rather than your contribution.
What Return Rate Should I Use?
The rate you choose matters more than any other input. For cash savings in a high-interest account, 6–8% is realistic. For a balanced unit-trust or ETF portfolio, 8–11% nominal is a sensible long-run assumption, but use 4–6% real (after inflation) if you want the answer in today's Rands. For equities only, 10–13% nominal is the long-run historical average but with significant year-to-year volatility. Always use a real-return figure if you are planning for a goal more than 5 years away — a 12% nominal return that leaves you with 5% real purchasing power is much less impressive than the headline suggests. The calculator lets you toggle between nominal and real returns.
Is the Projection Guaranteed?
No. The calculator assumes a constant annual return every year, which no real investment delivers. In practice, returns vary year by year — sometimes sharply. A balanced portfolio might return 18% one year and -8% the next, while averaging 9% over a decade. The projection is a useful planning figure that shows the order of magnitude you can expect, but actual outcomes will differ, sometimes significantly. For short-term goals (under 3 years), prefer cash or low-volatility options where the variance is much smaller. For longer horizons, the variance smooths out and the projection becomes more reliable as a planning tool.
How Is This Different From the Retirement Savings Calculator?
The retirement calculator is purpose-built for retirement: it includes the SARS s11F tax deduction on contributions (which only applies to pension, provident, and retirement annuity funds), uses a 4% safe-withdrawal rate to derive monthly income, and assumes a long time horizon. This compound interest calculator is a pure savings-growth tool that works for any goal — a deposit, a car, a holiday, a wedding, a child's education — without tax wrappers or retirement-specific assumptions. If your goal is specifically retirement, use the retirement calculator; for everything else, this is the right tool.
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Disclaimer: Finance Atlas is not a registered Financial Services Provider (FSP). This calculator provides general guidance for educational purposes only and does not constitute financial, tax or legal advice. Always confirm current SARS rates and your personal tax position with a registered tax practitioner.