Last updated 23 September 2026 · Written by Kgothatso Moleke · General information, not financial advice
The short version: compound interest is interest earning interest. It does almost nothing for the first few years, which is why most people give up on it, and then it does something close to absurd. R500 a month for 40 years at 9% becomes R2.34 million, of which only R240,000 came out of your pocket.
Simple versus compound, in one line each
Simple interest is calculated on the original amount only. R10,000 at 10% earns R1,000 a year, every year, forever.
Compound interest is calculated on the original amount plus everything it has already earned. R10,000 at 10% earns R1,000 in year one, then R1,100 in year two, then R1,210 in year three — because you are now earning interest on the interest.
Over one year the difference is nothing. Over thirty it is everything, and that gap is the single most important idea in personal finance.
Why it looks like it is not working
Here is R500 a month at 9%, which is a realistic long-term return for a diversified investment, not a savings account:
| After | You put in | It is worth | Growth |
|---|---|---|---|
| 10 years | R60,000 | R96,757 | R36,757 |
| 20 years | R120,000 | R333,943 | R213,943 |
| 30 years | R180,000 | R915,372 | R735,372 |
| 40 years | R240,000 | R2,340,660 | R2,100,660 |
Look at the last column. Doubling the time from 10 to 20 years does not double the growth — it multiplies it nearly six times. Going from 30 to 40 years adds R60,000 of your own money and R1.37 million of growth.
That is why the first decade feels pointless. After ten years you have R96,757 against R60,000 contributed, and it is reasonable to look at that and think the effort is not worth it. It is also exactly the moment most people stop. The entire payoff sits in the years after the point where quitting feels rational.
Ten years costs more than you think
Two people save R500 a month at 9% until they are 65. One starts at 25 and one starts at 35. The first ends with R2,340,660. The second ends with R915,372.
Ten years of R500 is R60,000. That R60,000 of delay cost R1.4 million. Not because the second person did anything wrong afterwards — they contributed for thirty unbroken years — but because the money they never put in early never got its forty years of compounding.
The practical version of that: starting small now beats starting properly later. R200 a month today is worth more than R1,000 a month in eight years’ time.
It works just as hard against you
The same mechanism runs in reverse on debt, and this is where it touches most people’s lives long before any investment does.
Take R20,000 on a credit card at 22%. Pay only the minimum — typically around 5% of the balance — and you clear it in roughly nine years and eight months, having paid about R30,947. Pay a fixed R800 a month instead and you are done in under three years, having paid R27,000.
The minimum payment is designed to shrink as the balance shrinks, which keeps you in debt for as close to a decade as possible. Paying a fixed amount rather than the minimum is the single highest-return decision available to most households, and it requires no extra money — only that you stop letting the payment fall.
The two things that decide the outcome
Time, far more than amount. You cannot make up for a late start by contributing more, because the missing ingredient is years and no amount of money buys those back.
The rate, more than it looks. R500 a month for 30 years at 7% gives R609,985. At 9% it gives R915,372. At 10%, R1,130,244. Two percentage points is worth over R300,000 — which is why fees matter so much on long-term investments, since a 2% annual fee comes straight off that rate.
An honest caveat about the numbers
Every figure above assumes a steady return, and real investments do not deliver one. Some years are strongly positive, some are negative, and the average only appears over long periods. The 9% used here is a plausible long-run nominal return for a diversified equity-heavy investment — it is not a promise, and a bank savings account will not produce it.
Inflation also matters: R2.34 million in forty years will not buy what R2.34 million buys today. The comparison that stays true regardless is the relative one — starting earlier beats starting later, and a lower fee beats a higher one, in every scenario.
Where to start
If you are carrying debt above roughly 15%, compounding is working against you faster than any investment will work for you. Clear that first — paying off a 22% card is a guaranteed 22% return, which no investment can promise.
Once that is handled, the order that generally works is a small emergency buffer first, then a tax-free savings account, then everything else. Check what you can commit without straining with the affordability calculator.
How these figures were worked out: monthly compounding, contributions made at the end of each month, no fees or tax deducted, and a constant return. Real returns vary year to year. Figures are nominal, not adjusted for inflation.