Editor's note: Calculations in this story have been revised by using a more accurate tool.

Young people are pretty confident about their financial futures. A recent survey from TD Ameritrade found that 53 percent of millennials expect to become millionaires at some point in their lives, if they aren't already.

While 23 percent believe they'll hit $1 million by age 50 or older, many expect to get there more quickly. A full 19 percent say they'll become a millionaire by age 40 and 7 percent predict it'll happen by 30.

But what if you want to speed up the process and reach that milestone in the next 15 years? It could be possible, if you're willing to work for it — and put away much more each month than the average American.

Using Bankrate's millionaire calculator, CNBC Make It found how much you'd have to save each month to become a millionaire by 2033. Here's how much you'll need to put away, depending on the amount you already have invested.