Compound interest is one of the most powerful concepts in finance. Unlike simple interest, which is calculated only on your original principal, compound interest is calculated on both your principal and on previously accumulated interest. The effect is that your money earns interest, and then that interest itself starts earning interest — creating exponential growth over time.
Three variables control how much wealth compounding can build for you: your contributions, your rate of return, and the time horizon. Of the three, time is by far the most powerful lever. A 25-year-old investing $300/month at a 7% return for 40 years will end up with dramatically more than a 35-year-old making the same contribution for 30 years — even though the difference in total contributed is modest. That is the practical reason 'start early' is the most consistent advice in personal finance.
Compounding frequency also matters, though less than most investors assume. Moving from annual to monthly compounding slightly increases your effective yield, but the gap between monthly and daily compounding is typically negligible compared to small changes in the underlying rate. The defaults on this calculator use monthly compounding, which closely approximates how most index funds and savings accounts actually work in practice.
Use this calculator to compare scenarios quickly: raise the contribution by $100/month, extend the horizon by five years, or lower your expected return by a percentage point. The chart helps build intuition for which levers matter most for your situation.