Retirement at 40? Isn’t 40 the new 30? Why would one want to retire at 40? Well, because once you have your finances in place, I suppose experiencing the wide world and seeing all it has to offer over the next 30 years of your life isn’t such a terrible thing. Or maybe that’s just my opinion.

David Cox, who runs a blog titled I Retired Young, told Time magazine “Start saving as early as possible. Even if it is not a big amount to start with, it gets you into the saving habit, and then as your career develops and your earnings grow, the amount you can save will increase as well.”

Cox retired at 47\. However, as the magazine notes, unless you’re in the top 5% of earners, merely saving enough to retire by age 40 will be difficult. That’s an understatement, of course. You’ll end up sacrificing nearly everything fun, save for necessities and any inexpensive leisure activity. Say hello to long walks.

Okay, at this point, I kind of share your skepticism. Retirement at 40 seems a pipe dream. We don’t all have big inheritances, we’re not all tech or business tycoons, and we’re not all celebrities. So should we even consider this?

Well, yes. But the thing to keep in mind is, you have to make your money work for you. That old fundamental doesn’t change. As Robert Kiyosaki put it in his book Retire Young Retire Rich, “If you want to retire young and retire rich, it is very important that your money be like a bird dog, going out every day and bringing home more and more assets.”

So let’s get to the math and the numbers we need to know for an early retirement. It is of course a given that you need huge savings to even consider retiring. Besides the savings, you’d also need a steady income stream so you don’t deplete said savings. A 2012 study by Aon Hewitt found that in order to retire comfortably, an average employee required savings 11 times that of his or her career pay. They also needed to replace a massive 85 percent of the annual salary.

However, this doesn’t stop the determined early retirer. Paul Schrodt noted in the article in Time that the general rule is saving so you can replace 80% of your pre-retirement income with investments or other sources of income. This means you’re likely to put away much more than people who exit the workforce at an older age. Because if you retire at 40, you’ll likely be retired for 40 to 50 years, as against the usual 20 to 30 years. Therefore, depending on how much you earn, you will need to, no prizes for guessing this, save half of each paycheck to retire early.