What is compound interest and how does it work?

With simple interest, the return is calculated on the original amount only: 5,000 at 7% per year earns 350 each year, unchanged for every year of the term. With compound interest, each period’s return is added to the balance, and the next period’s return is calculated on the larger balance, so the balance grows at a compounding rate rather than a flat one.

The difference can look small in the first year but widens steadily with time. Compounding rewards patience and long horizons, while simple interest stays the same no matter how long the money is invested.

Four inputs determine how your investment grows: the initial amount you start with, the periodic contribution you add regularly (monthly, quarterly or annually), the annual return as a percentage, and the compounding frequency (monthly, quarterly, semi-annually or annually).

For the same return, more frequent compounding adds a little more growth, and longer time horizons magnify the effect dramatically. These are exactly the fields in the compound interest calculator, and you can adjust each one to see its impact.

At the end of each period, the new balance equals the previous balance times (1 + period rate), plus any contribution. The period rate is the annual return divided by the compounding frequency; at 7% compounded monthly, the period rate is 7 ÷ 12, about 0.583% per month.

The same step is repeated for every period until the end of the term, producing a final balance, the total contributed, and the interest earned. You do not need to do this by hand, but the mechanism explains why the balance grows faster than what you actually deposited.

Mirroring the calculator’s example: an initial 5,000 dollars with a 200 dollar monthly contribution, a 7% annual return compounded monthly over 20 years.

Over the term you deposit 5,000 + (200 × 240) = 53,000 dollars. Yet the final balance reaches about 124,400 dollars — meaning compounding contributed roughly 71,400 dollars, more than your initial amount and contributions combined. That is the power of compounding over a long horizon.

Time is the strongest driver of compounding, because the interest you have already earned starts earning interest itself. That is why an early but modest investment often beats a later, larger one when the first runs for longer. Increasing the compounding frequency, say from annually to monthly, also improves the result for the same return.

Experiment with the calculator to see this yourself: compare the outcome after 10 years with the outcome after 20, and watch the gap widen as the horizon extends.

The calculator assumes a constant annual return for the whole term, with contributions added at the end of each period. Taxes, fees, inflation and market volatility are not included, and all of these affect the real-world result.

Real returns are never guaranteed and vary with the type of investment and market conditions; past performance does not predict future results. Treat the output as a planning estimate and consult a licensed financial professional before making investment decisions.

You can use compound interest in your favour in three ways: start early, because time matters most; add regular contributions, even small ones; and avoid frequent withdrawals that interrupt the compounding process.

Small, regular deposits accumulate surprisingly well over the years. Use the compound interest calculator to test different scenarios and compare how the horizon, contribution and return change the final result before you decide.