Every month your deposit goes in at the start of the month, and the whole balance grows by one twelfth of the annual return. Next month the new, slightly bigger balance does the same. That is compounding: the growth itself starts earning.
There is no safe single number. A broad stock index has averaged roughly 7% a year after inflation over long stretches, with bad years along the way. Cash and savings accounts sit far below that. Try a few rates and treat the answer as a range, not a promise.
At 7%, $200 a month becomes about $34,800 in 10 years, $104,800 in 20, and $245,400 in 30. The curve feels flat for years, then it is not. The full numbers are in the compound interest guide.
Estimates only, not financial advice. Real returns vary year to year, and taxes and fees are not included.
Figured Out