Compound interest means your growth starts earning its own growth. This free visualizer lets you set a starting amount, a monthly contribution, a number of years and an average yearly return, then shows the total balance next to the money you actually deposited.
Each year your earnings are added to your balance, and the following year that larger balance grows too. Over one or two years the effect is small. Over 30 or 40 years it is the single biggest driver of a big final balance — which is why starting as a teenager is such an advantage.
The long-run average for a broad US stock index has historically been roughly 7–10% a year before inflation, but no single year is average. Projections are estimates, not promises.