Compound Interest Calculator

The Snowball Effect of Compounding

Compound interest is the engine behind almost every long-term investment plan. The idea is simple but powerful: you earn returns on your original capital, and then you earn returns on those returns. Each period's growth is calculated on a bigger balance than the one before, so the curve starts gently and then bends sharply upward the longer you stay invested. This is why starting early matters far more than starting big — a modest monthly contribution left to compound for thirty years routinely beats a much larger amount invested for ten.

This calculator compounds monthly, which mirrors how most South African unit trusts, ETFs and retirement products actually grow. It adds your monthly contribution, applies one month of growth, and repeats — so the figure you see reflects regular investing rather than a single lump sum sitting still.

What Return Rate Should You Use?

There is no single correct number — returns depend entirely on what you invest in, and they are never guaranteed. As a rough guide for long horizons, South African cash and money-market funds tend to track close to the repo rate, income and bond funds sit a little higher, and diversified equity portfolios have historically delivered something in the region of inflation plus five to seven percent over multi-decade periods, albeit with significant ups and downs along the way. If you are unsure, it is wiser to model a conservative return and be pleasantly surprised than to plan around an optimistic one.

Why the "Today's Money" Figure Matters

A future balance of two million rand sounds life-changing, but if it arrives in thirty years, inflation will have quietly reduced what it can buy. That is why this tool includes an inflation field: the "value in today's money" line discounts your future balance back to present-day buying power, so you can judge the result against what things cost now rather than what they might cost decades from today. For South African planning, modelling inflation somewhere in the Reserve Bank's 3–6% target band is a sensible default.

A Tax-Smart Place to Compound

Where you hold an investment affects how much of this growth you keep. Inside a Tax-Free Savings Account, all interest, dividends and capital gains are exempt from tax — which means the compounding shown here is not eroded by tax along the way. If you have not used your annual allowance, our TFSA calculator models the same growth within the current SARS contribution limits.