Enter a starting balance, an optional monthly contribution, and how many years you want it to grow. Pick a return assumption — or tap one of the preset buttons for a common benchmark. Everything compounds monthly. The return rates are illustrative, editable defaults (as of ~2026); the S&P 500 figure is a long-run nominal historical average, not a promise of future returns.

Simple interest pays you only on what you put in. Compound interest pays you on what you put in plus everything it has already earned — so each year’s growth starts from a bigger base than the year before. Over a few years the difference is modest. Over a few decades it is the whole story.

That is why the early dollars matter most: a contribution made at 30 has decades to multiply, while the same dollar added at 60 has only a handful of compounding cycles left. Time in the market — not timing it — is what does the heavy lifting. Adjust the years slider above and watch the “total growth” figure climb far faster than the “total contributed” figure.