You have probably heard the phrase compound interest. The idea is simple: money you leave invested can earn a return, and those earnings can then earn further returns.
That can make time a valuable part of a savings plan. It is not magic, and it does not make investment risk disappear. The result depends on the return actually earned, fees, tax, inflation, contributions and withdrawals.
What is compound interest?
With compound interest, interest is added to your starting amount, also called the principal. Future interest is then calculated on that larger balance. In everyday language, it is interest on interest.
For investments such as shares or unit trusts, compound growth is often the more accurate term. Reinvested distributions and changes in investment value can contribute to growth, but they are not necessarily fixed interest payments. Market values can fall as well as rise.
A simple R10,000 example
Imagine a single R10,000 deposit earning a constant 10% a year, compounded annually, with no further deposits or withdrawals. This is an illustration, not a product quotation or forecast.
- Year 1: R10,000 × 1.10 = R11,000. Interest earned: R1,000.
- Year 2: R11,000 × 1.10 = R12,100. Interest earned: R1,100.
- Year 3: R12,100 × 1.10 = R13,310. Interest earned: R1,210.
The rate has not increased. The rand amount grows because the same positive rate is applied to an increasing balance.
What happens over a longer period?
| Years | Compound balance | Simple-interest balance |
|---|---|---|
| 1 | R11,000 | R11,000 |
| 5 | R16,105 | R15,000 |
| 10 | R25,937 | R20,000 |
| 20 | R67,275 | R30,000 |
| 30 | R174,494 | R40,000 |
Amounts are rounded to the nearest rand. Both examples assume the same R10,000 starting amount and constant 10% annual rate, with no additions, withdrawals, fees or tax. They show nominal rand amounts, without adjusting for inflation.
After 30 years, the compound balance is approximately 17.45 times the starting amount. That does not mean every investment will multiply by that amount: the assumed return is doing a great deal of work in this example.
Simple and compound interest compared fairly
Simple interest is calculated on the original principal. At 10%, R10,000 earns R1,000 a year, giving R40,000 after 30 years if all that interest is kept but does not itself earn interest.
At the same positive annual rate, leaving earnings to compound produces a larger balance over multiple years. It is not accurate to say compound interest “wins every time” regardless of the interest rate, term, fees, risk or cash flow. Compare the actual terms of the alternatives.
The formula, without the intimidation
A = P × (1 + r/n)nt
- A: the future balance.
- P: the initial amount.
- r: the nominal annual rate as a decimal; 10% is 0.10.
- n: the number of compounding periods per year.
- t: the number of years.
This formula assumes a constant nominal rate and no additional cash flows. Monthly contributions, changing rates, fees and withdrawals require a fuller calculation. Ask whether a quoted rate is nominal or effective before comparing accounts.
Starting earlier buys more time
Under the same simplified 10% assumptions, R10,000 invested for 30 years becomes about R174,494. The same amount invested for only 20 years becomes about R67,275. Ten extra years make a substantial difference in this illustration.
Starting later is still worthwhile. Focus on an affordable contribution and a realistic plan rather than feeling that you have missed your opportunity.
Four things that can change the result
- Fees: Under a simplified constant net-return comparison, R10,000 growing at 8% for 30 years becomes about R100,627, rather than R174,494 at 10%. This shows sensitivity to net returns; it is not a fee quotation.
- Inflation: A larger future balance may buy less than its rand amount suggests. Compare growth with the rising cost of what you need.
- Withdrawals: Money taken out no longer participates in future growth. Essential spending still matters; build an accessible cash buffer rather than forcing every rand into a long-term product.
- Investment risk: Returns vary. A 10% loss followed by a 10% gain takes R10,000 to R9,900, not back to R10,000.
Reinvest earnings when it fits your goal
For an accumulation goal, reinvesting interest and distributions can support future compounding. Someone who needs an income may instead need to draw some earnings. “Always reinvest” is too absolute: the right choice depends on your cash needs, time horizon and plan.
Where do TFSAs and retirement annuities fit?
Qualifying South African tax-free investments provide returns free of South African income tax, dividends tax and CGT. For the 2026/27 tax year, the annual contribution limit is R46,000 and the lifetime contribution limit is R500,000, across your accounts. Withdrawals do not restore used contribution room; reinvestment after withdrawal counts as a new contribution. Growth retained inside the account does not itself use contribution room. SARS: Tax-Free Investments
Retirement funds also have tax advantages for investment growth, but contribution, access and benefit-tax rules apply. An RA is a retirement arrangement, not an unrestricted savings account. Tax advantages do not guarantee returns or remove fees. For the detailed rules, read the GFS RA guide and the GFS TFSA guide.
A practical place to start
Choose a goal, record when you need the money, identify an affordable monthly amount and compare costs and access rules. Set a review date. Compounding is most useful when it is part of a plan that you can maintain.
Practical GFS resources
Use these optional educational resources to prepare questions and organise your information. They do not replace a professional recommendation or legal document.
- Retirement Planning Guide — R69.99: saving examples and the retirement choices to discuss with your adviser
- Retirement Capital Lifetime Explorer — R49.99: test the effect of returns, fees, inflation and withdrawals in educational retirement-income scenarios; it is not a guaranteed-growth calculator
- Monthly Budget Planner — Free: identify an affordable contribution and keep track of monthly commitments
- FTSE/JSE All Share Index Made Clear — Free: understand share-market indices and why market returns fluctuate
Browse the GFS Product Library. Prices checked 2 November 2026.
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General South African financial education, checked 2 November 2026. Your financial adviser should consider your personal circumstances, needs, goals and financial information, discuss the relevant benefits, costs, risks and restrictions with you, and recommend products only where they are suitable. Wills, trusts and estate arrangements also require advice from an appropriately qualified legal and tax professional. No return, tax saving or legal outcome is guaranteed.