Here's how much you should be investing by your 30s, according to former Wall Street executive Sallie Krawcheck

In your 30s, you've (hopefully) started to check off important financial milestones: You're paying down debt, you have an emergency fund, and you're making monthly contributions to your 401(k).

And if you haven't already, you also want to start investing. But how much should you contribute?

The short answer: Anything you can, according to Sallie Krawcheck, a former Wall Street executive and founder and CEO of Ellevest.

But if you're waiting until your 30s to start investing, you're too late.

"Just do it, just do it, just do it!" Krawcheck told Business Insider. "Because it's costing you a fortune, especially women," she added, noting that women are less likely to invest. "It costs women tens of thousands, hundreds of thousands, sometimes millions of dollars over the course of their lives."

Contrary to the advice that you should wait until you have a substantial amount to invest, Krawcheck recommends that everyone start in their 20s, even if you only have $50 or $100 to put in, because it will start compounding right away.

"Over time [you will] have a return on that money you put in, and then you'll earn a return on that return, and then you'll earn a return on that return on that return," Krawcheck explains.

She's referring to compound interest, which is when the interest earned on an investment earns interest on itself. Because of it, a little money contributed today will ultimately earn more than a lot of money contributed tomorrow - so don't wait, even if you're only putting in 1% of your paycheck at first.