Starting balance, contributions, rate, and time in. A projection out, with the assumptions kept visible.
This assumes a constant annual return compounded monthly, which real markets never actually deliver in a straight line. It's a planning estimate, not a prediction, and it is not financial advice. Actual returns vary and can be negative in any given year.
Small differences in assumed annual return compound into large differences over long time horizons. Run this a few times with a more conservative rate before trusting a single optimistic projection, the gap between a 5% and a 9% assumption over 20 years is far larger than it looks over 1 year.