The core idea
Compound interest means interest calculated on your original deposit plus all the interest that deposit has already earned. Each time interest is added, the base it's calculated on grows — so the next round of interest is calculated on a bigger number than the last.
Simple interest, by contrast, is always calculated on the original amount only. If you deposit R10,000 at 8% simple interest, you earn exactly R800 every year, forever. With compound interest at the same rate, you earn R800 in year one, then a bit more in year two because you're now earning 8% on R10,800, not R10,000.
Why the compounding frequency matters
The "annual rate" quoted on a savings account or loan isn't the whole story — how often that interest is actually added to your balance changes the real return. Interest that compounds monthly grows faster than the same nominal rate compounding annually, because the gains get reinvested twelve times a year instead of once.
This is why two accounts advertising the same headline rate can produce different balances after a few years. The one that compounds more frequently — daily or monthly rather than annually — will edge ahead, even though the quoted rate looks identical on paper.
Why this matters more than people expect
Compounding is often described as 'slow at first, then fast.' In the early years, the interest you earn is small relative to your principal, so growth feels linear. Given enough time, the base keeps growing on itself and the curve bends sharply upward — which is why starting early, even with small amounts, consistently outperforms starting later with larger ones.
The same mechanism works in reverse on debt. Credit card balances and some loans compound too, which is exactly why unpaid interest snowballs if you only make minimum payments — you're paying interest on interest you haven't paid off yet.
Want to see this in action? Try the Compound Interest Calculator.
Frequently asked questions
What is the simplest definition of compound interest?
Interest earned on both your original deposit and on the interest you've already accumulated, rather than on the original deposit alone.
Is compound interest always better than simple interest?
It's better for savers and worse for borrowers. As a saver, compounding accelerates your growth. As a borrower with compounding debt, the same mechanism accelerates how fast you owe more if you don't pay it down.
Does compounding frequency really make a noticeable difference?
Over short periods the difference between monthly and annual compounding is small. Over 10-20+ years at meaningful balances, it adds up to a measurable amount — enough to be worth checking when comparing two accounts with similar headline rates.