Investment Growth Calculator: See What Consistent Saving Could Actually Become

There’s a particular kind of surprise that happens when you actually run the numbers on saving a few hundred rand a month for fifteen or twenty years. Most people assume it adds up to roughly what they put in, plus a bit. It rarely works that way. Compound growth does most of the heavy lifting once you give it enough time, and the gap between “money you contributed” and “money you end up with” gets bigger than intuition suggests. Our Investment Growth Calculator shows you that gap directly, a lump sum, a monthly contribution, a growth rate, and a term, and you’ll see exactly how much of your final balance is money you put in versus money the market added for you.

Here’s how the maths works, what a realistic growth rate actually looks like, and how South Africa’s tax-free savings account fits into the picture.

How Compound Growth Actually Works

The basic idea isn’t complicated: each month, your money earns a return, and that return gets added to your balance, which then earns its own return the following month. Do that for a few years and the growth starts compounding on itself rather than just adding up in a straight line.

What surprises people is how much of the total comes from growth rather than contributions once you stretch the timeline out. Take someone investing R20,000 upfront and R1,500 a month at a 9% average annual return for 15 years. They’ll have contributed R290,000 of their own money. Their balance at the end, though, comes to somewhere around R644,000. More than half of that final figure isn’t money they put in at all, it’s growth on growth, compounding quietly in the background the whole time.

That’s the entire case for starting early. The same monthly amount started five years later produces a meaningfully smaller final number, not because you contributed less overall, but because those extra five years of compounding never had the chance to happen.

Choosing a Realistic Growth Rate

This is where a lot of online calculators quietly mislead people, by defaulting to an overly optimistic number. What you enter in the “expected annual growth rate” field should reflect where your money is actually invested, not an aspirational figure.

• Cash and money market accounts typically return somewhere in the 6 to 8% range currently, tracking fairly close to interest rates.
• A balanced or moderate-risk unit trust might realistically average 8 to 10% a year over the long term, though individual years will swing well above or below that.
• A pure equity portfolio (like a JSE tracker fund) has historically averaged somewhere in the 10 to 12% range over long periods, but with far more volatility year to year, including years of outright losses.

None of these are guarantees, and the further out your projection runs, the more that single growth rate assumption drives the result. It’s worth running the calculator at two or three different rates, say a conservative 7% and a more optimistic 10%, just to see the range of outcomes rather than anchoring on one number.

Why Monthly Contributions Matter More Than the Lump Sum

A once-off lump sum feels satisfying to invest, but for most people building wealth over time, the monthly contribution does more of the work simply because it’s sustained. Our calculator also lets you model an annual increase to your monthly contribution, useful if you’re planning to bump up your debit order in line with a salary increase each year. Even a modest 5 to 6% annual step-up compounds nicely alongside the investment growth itself, since you’re not just earning more on the same contribution, you’re also contributing more each year.

Using a Tax-Free Savings Account

If part of what you’re modelling is a Tax-Free Savings Account (TFSA), our calculator has a toggle specifically for that, because TFSAs come with contribution limits that are easy to accidentally exceed if you’re not paying attention.

As of 1 March 2026, the annual contribution limit increased from R36,000 to R46,000, the first change to that limit since 2021. The lifetime contribution limit stayed at R500,000. A few things worth knowing:

• Both limits apply to contributions only, not growth. If your TFSA balance grows past R500,000 through investment returns, that’s completely fine, the cap only restricts what you’re allowed to put in.
• Any unused portion of your annual limit doesn’t carry over to the next year. If you only contribute R30,000 this tax year, you don’t get to add the unused R16,000 onto next year’s allowance.
• Exceeding either limit triggers a 40% penalty on the excess amount, which is steep enough that it’s worth double-checking your contributions rather than assuming you’re within bounds.
• Everything earned inside a TFSA, interest, dividends, and capital gains, is completely tax-free, which is what makes it such a powerful long-term compounding tool compared to an ordinary taxable account.

If you tick the TFSA box in the calculator, it’ll flag the year your contributions would breach either the annual or lifetime limit, so you can adjust your monthly amount before you actually hit that ceiling.

What This Calculator Doesn’t Account For

To keep the projection clear and usable, a few real-world factors aren’t built in, and it’s worth knowing what they are:

• Fees. Fund management fees, platform fees, and advice fees all eat into your real return. A fund charging 1.5% a year effectively reduces a 9% growth assumption to something closer to 7.5% in practice.
• Tax (outside a TFSA). Interest, dividends and capital gains in an ordinary investment account are all taxed differently, and that tax reduces your effective growth rate too.
• Inflation. The final value shown is in today’s rand terms for contributions, but doesn’t discount the future balance back to today’s purchasing power. R644,000 in 15 years won’t buy what R644,000 buys today.
• Volatility. Markets don’t move in a smooth straight line at a fixed percentage every year. The projection is a smoothed average, real returns will bounce around it.

Frequently Asked Questions

What growth rate should I actually use?

It depends entirely on what you’re invested in. Use a conservative rate for cash-based savings, and a more moderate long-term average for equity or balanced investments, then run a couple of different scenarios rather than relying on a single number.

Is the TFSA annual limit really R46,000 now?

Yes, as of 1 March 2026, up from R36,000. This was the first increase to the annual limit since 2021. The lifetime limit remains R500,000.

What happens if I contribute more than the TFSA limit?

SARS applies a 40% penalty tax on the amount over either the annual or lifetime limit, so it’s worth tracking your contributions carefully, especially if you hold TFSAs with more than one provider, since the limits apply across all of them combined, not per account.

Does this calculator account for market crashes or bad years?

No, it applies a smoothed average growth rate across the whole term. Real markets have good years and bad years mixed together. Over a long enough period the average tends to hold up reasonably well, but the path there is rarely as smooth as a straight-line projection suggests.

Related Calculators

Saving and investing usually sit alongside other financial decisions. A few tools on our Calculator hub that connect naturally with this one:

• Loan Calculator – compare the cost of borrowing against what the same money could grow into if saved instead
• SARS PAYE Calculator – work out your net income first, so you know what’s realistically available to invest each month
• SA UIF Contribution Calculator – understand your income safety net alongside your long-term investing plan
• SA Two-Pot Retirement Calculator – see how retirement savings rules interact with your broader investment strategy
• VAT Calculator – useful if you’re running a side business and reinvesting the profits

You can browse the full Calculator hub for everything else we’ve built.

A Final Word

The single biggest variable in almost every investment projection isn’t the growth rate, it’s time. Starting five years earlier with a smaller monthly amount often beats starting later with a bigger one, purely because compounding needs time more than it needs size. Run a few different scenarios through the calculator, be honest with yourself about a realistic growth rate rather than an optimistic one, and if you’re using a TFSA, keep an eye on those contribution limits so you don’t lose part of the benefit to an avoidable penalty.

This calculator and article are for general informational and illustrative purposes only and don’t constitute financial advice. Investment returns are not guaranteed and past performance doesn’t predict future results. Please consult a registered financial adviser before making investment decisions.