Millennials might be falling behind on saving and investing, but they're confident in their financial futures.

A new 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. The survey included responses from 1,519 Americans ages 21 to 37.

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 another 7 percent predict they'll become a millionaire by 30.

For millennials (or anyone) to actually reach that milestone, they need to start putting away a significant chunk of cash each month, beginning as early as possible. Here's how much you'd need to put in an an investment account in order to become a millionaire over certain periods of time, using Investopedia's millionaire calculator. All calculations assume no prior savings.