When I first started investing, someone told me compound interest would make me wealthy. I nodded along. I had no real idea what that meant. I thought interest was interest — the bank pays you a small percentage and that's that.
It took me a while to understand the actual mechanics. Once I did, it changed how urgently I thought about starting — and how patiently I thought about staying.
SIMPLE INTEREST VS COMPOUND INTEREST
Start with the difference. It's the key to understanding everything else.
Simple interest pays you a percentage of your original investment only. Every year, you earn the same fixed amount — your principal doesn't grow. It's like being paid a flat wage regardless of how long you've worked.
Compound interest pays you a percentage of your original investment plus all the interest you've already earned. Your earnings generate their own earnings. The pile gets bigger, and then it earns interest on the bigger pile. That's the magic.
| Scenario | Starting Amount | Rate | After 10 Years | After 20 Years |
|---|---|---|---|---|
| Simple interest | R10,000 | 10%/year | R20,000 | R30,000 |
| Compound interest | R10,000 | 10%/year | R25,937 | R67,275 |
Same starting amount. Same interest rate. After 20 years, compound interest delivers more than double the outcome. That gap widens dramatically the longer you hold.
HOW IT ACTUALLY WORKS
Here's the mechanics in plain terms. You invest R10,000 at 10% per year.
Year 1: You earn 10% of R10,000 = R1,000. Total: R11,000.
Year 2: You earn 10% of R11,000 = R1,100. Total: R12,100. Notice — you earned R100 more than year 1, just because last year's interest is now also earning interest.
Year 3: You earn 10% of R12,100 = R1,210. Total: R13,310. The interest keeps compounding on itself, growing faster each year.
Example: R10,000 × (1 + 0.10)20 = R67,275
You don't need to memorise the formula. What matters is the intuition: every rand you earn starts working for you immediately. The longer you leave it, the faster it grows — because a bigger pile generates bigger interest.
THE REAL POWER: TIME
Time is the most important variable in compound interest. Not the rate. Not the amount. Time.
Here's what R500 per month looks like at 10% annual growth over different time horizons:
You contributed R180,000 over 30 years. The investment returned R1.13 million. R950,000 was created by compound growth alone — not by you putting in more money, but by time doing its work.
"The best time to start was yesterday. The second best time is today. Every month you wait is a month of compounding you'll never get back."
THE COST OF WAITING
This is the part that makes compound interest genuinely urgent. Consider two investors — both invest R500 a month at 10% annual growth:
Invests for 30 years.
Total at 55: R1.13M
Invests for 20 years.
Total at 55: R380K
Just 10 years earlier.
Extra outcome: R750K
Ten years of delay cost R750,000. Not because Kagiso invested more — they both put in R500 a month. Because Kagiso gave compound interest 10 more years to work.
WHERE COMPOUND INTEREST APPLIES IN SA
Compound interest isn't just a savings account concept. It shows up in every investment you make:
ETFs on the JSE — your units grow in value over time, and any dividends you reinvest buy more units, which generate more dividends. The compound effect applies to both price appreciation and income reinvestment.
Your TFSA — compound growth inside a TFSA is tax-free. SARS doesn't take 20% of your dividends or up to 18% of your capital gains. Every rand of growth stays in the pile, compounding at full speed.
Dividend reinvestment — when your ETF pays a dividend and you use it to buy more units rather than withdrawing it, you're adding fuel to the compound engine. Small reinvestments early on have an outsized effect decades later.
Fees are the enemy of compound interest. A 2% annual fee on a unit trust doesn't just cost you 2% — it compounds against you the same way growth compounds for you. Over 20 years, a 2% fee vs a 0.1% ETF fee on R100,000 costs you hundreds of thousands of rands in lost compound growth. Keep fees as low as possible.
FREQUENTLY ASKED QUESTIONS
It depends on the investment. Savings accounts may compound daily or monthly. ETFs compound continuously as the underlying assets grow in value and dividends are reinvested. The more frequently it compounds, the faster it grows — though the difference between daily and monthly compounding is relatively small over time. What matters far more is starting early and leaving it alone.
Yes — and this is the most important thing for beginners to understand. The amount matters far less than the time. R200 a month started at 25 is worth more at 55 than R500 a month started at 35. Start with whatever you can. The compound engine doesn't care about the size of your starting contribution — it cares about how long it has to run.
The JSE has historically delivered average annual returns of around 10–14% over long periods, before inflation. Broad market ETFs like the Satrix 40 track the top 40 JSE companies and have delivered competitive long-term returns. Past performance doesn't guarantee future returns — but broad diversification through ETFs gives you the best chance of participating in market growth over the long term. Always invest for the long term and don't expect linear, consistent returns year to year.
Three things: Start as early as possible. Invest inside a TFSA so growth is tax-free. Keep fees low by using index ETFs rather than actively managed funds. Those three steps, applied consistently over a long period, give compound interest the best possible environment to do its work.
Yes — withdrawing reduces the pile, which reduces what future interest compounds on. This is especially important inside a TFSA, where withdrawals don't restore your annual contribution limit. The money you take out is gone from your lifetime allowance permanently. Treat your TFSA as long-term, untouchable wealth — not a savings account to dip into.
MY PERSONAL TAKE
Understanding compound interest didn't make me feel excited about investing. It made me feel a low-grade urgency I hadn't felt before. Not panic — just the quiet realisation that every month I delay is a month I can never recover.
The maths is unforgiving. But it's also the most encouraging thing I've read about money — because it means you don't need to be lucky, or clever, or earn a huge salary. You just need to start, stay consistent, and give time the space to do the heavy lifting.
Start inside your TFSA. Keep fees low. Reinvest everything. And then leave it alone.
FURTHER READING
These are well-written breakdowns of compound interest from some of South Africa's major financial institutions — worth reading alongside this post for additional perspectives:
START YOUR COMPOUND JOURNEY
Open a TFSA on platforms like EasyEquities or Sygnia and let compound growth work tax-free.