Compound interest is the engine that makes small, regular investing into something serious — given enough time. Adjust the dials below and see the curve change.
Historical S&P 500: ~7% real return (after inflation)
Calculations assume monthly compounding and contributions made at the start of each month. All math runs locally in your browser; nothing is sent to a server.
The emerald curve is your total balance. The grey line is what you actually contributed. The gap between them — which grows wider every year — is interest doing its work. For a typical 30-year saver, more than half the final number is interest, not deposits.
The biggest lever isn't the rate or the principal. It's the years. Move the years slider from 20 to 30 and watch the curve quietly redraw — that's the entire case for starting early.