Last updated 13 May 2026
Compound interest is one of the best tools created in the finance industry. Our compound interest calculator will help you estimate how much compound interest you can get if you contribute a certain amount of money and leave it to compound for a certain number of years.
Compound Interest Calculator
See how your money grows over time. Accounts for fees, regular contributions, and the full power of compounding.
Your Inputs
Your Results
| Year | Contributions | Growth | Total Value | Real Value |
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Enter your details on the left and click Calculate to see your projections.
How to Use The Compound Interest Calculator
Enter your starting amount (can be R0 if you're starting from scratch), your planned monthly contribution, the expected annual return rate, your investment period in years, and the annual fee (TER) of your chosen ETF. Click Calculate to see your projected growth year by year.
For a realistic baseline, try: R0 starting amount, R1,000/month, 12% return (conservative long-term average for a JSE/global ETF blend), 20 years, 0.25% fee. The results may surprise you.
What Is Compound Interest?
Compound interest (or compound growth) is returns earned on returns. When your investment grows, that growth itself starts earning returns in the next period. Over time, this creates exponential growth, and your money snowballs rather than growing in a straight line.
Here's the simplest possible example: R10,000 invested at 10% annual return with no additional contributions:
Compound Interest Calculator
See how your money grows over time, accounting for fees, contributions, and compounding.
| Year | Paid In | Growth | Total | Real |
|---|
Enter your details above and tap Calculate.
No additional contributions, the original R10,000 grew to R174,494 in 30 years purely through compounding. Now add R1,000/month on top of that, and at 10% over 30 years, you'd have over R2.28 million.
The Rule of 72
A useful mental shortcut: divide 72 by your annual return to estimate how many years it takes to double your money.
- At 8% return: 72 ÷ 8 = 9 years to double
- At 10% return: 72 ÷ 10 = 7.2 years to double
- At 12% return: 72 ÷ 12 = 6 years to double
- At 14% return: 72 ÷ 14 = 5.1 years to double
Why Starting Early Matters More Than Starting Big
Consider two investors at a 10% annual return:
Person A invests R1,000/month from age 25 to age 65 (40 years). Total contributions: R480,000.
Person B invests R2,000/month from age 35 to age 65 (30 years). Total contributions: R720,000 — 50% more than Person A.
Yet Person A ends up with approximately R6.3 million at 65, versus Person B's R4.5 million — despite Person A investing less money overall. That extra 10 years of compounding is worth more than 50% extra monthly contributions.
This is why the single best investment advice is simply: start now, even if it's small.
What Return Rate Should You Use?
. Here are the historical average returns for popular South African ETFs:
- Satrix Top 40: 14.05% (10-year average)
- Satrix S&P 500: 16.60% (since inception)
- Satrix MSCI World: 14.51% (since inception)
- Satrix Nasdaq 100: 22.47% (since inception)
Past performance doesn't guarantee future returns. For planning purposes, 10–12% is a reasonable conservative assumption for a diversified equity ETF over 20+ years.
Frequently Asked Questions
How often does compound interest compound?
This calculator compounds monthly, which is the most common frequency for investment returns. Some savings accounts compound daily, which gives marginally higher returns. ETFs effectively compound continuously as dividends are reinvested and prices reflect underlying earnings.
Does compound interest work with ETFs?
Yes. ETF returns compound in two ways: price appreciation (as the underlying companies grow) and dividend reinvestment (when dividends are paid back into the fund or you use them to buy more units). Over long periods, reinvested dividends can account for a significant portion of total returns.
How much should I invest monthly?
Any amount you can afford consistently. R500/month is a meaningful start. R1,000/month will build significant wealth over 20–30 years. The key is consistency — a monthly debit order for a small amount beats irregular lump sums every time. Use our Salary Calculator to understand how much you have available to invest.
To invest tax-free and supercharge your compound growth, open a TFSA. Use our TFSA Calculator to model growth specifically inside a tax-free account, or read our Complete Beginner's Guide to Investing.