Compound interest is often called the “eighth wonder of the world” — and for good reason. Understanding how compound interest works can help South Africans make smarter investment decisions, whether they are saving for retirement, a home deposit, or their children’s education.
What Is Compound Interest?
Compound interest is interest calculated on both the initial principal and the accumulated interest from previous periods. Unlike simple interest, which only earns on the principal, compound interest creates an exponential growth effect over time.
The Compound Interest Formula
The formula for compound interest is: A = P(1 + r/n)^(nt), where P is the principal, r is the annual interest rate, n is the number of times interest is compounded per year, and t is the number of years.
South African Context
South African banks typically compound interest daily or monthly on savings accounts. The South African Reserve Bank’s repo rate directly affects the interest rates offered by commercial banks. When the repo rate rises, savings rates tend to follow.
Real-World Example
If you invest R100,000 at 8% compounded monthly for 10 years, your investment grows to approximately R221,964. The same investment at simple interest would only reach R180,000 — a difference of over R41,000.
Tax Implications
In South Africa, interest income up to R23,800 per year (for individuals under 65) is tax-free. Amounts above this threshold are subject to income tax at your marginal rate. This makes tax-free savings accounts (TFSAs) particularly attractive.
Use Our Calculator
Try our Compound Interest Calculator to see how your investments can grow over time with different interest rates and compounding frequencies.