Most people can tell you when they want to retire.

Not everyone can tell you how much money they will need once they get there.

In fact, more than 75% of Americans haven't even run the numbers, according to the 2020 Four Pillars of the New Retirement study by investment company Edward Jones.

Picking a specific age means you are focusing on the wrong part of the equation, according to financial expert and radio show host Chris Hogan. 

"Retirement is not an age. It is a financial number," Hogan said. 

To make things more complicated, the amount you'll need is not a one-size-fits-all.

Hogan recommends you start by determining exactly how much money you need per year to fund your dream retirement. Once you have that yearly total, work your way backwards to figure out what size nest egg you need. 

Check out this video to see case studies of how much money you should save and learn Hogan's process for setting realistic goals in retirement. 

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Disclosure: NBCUniversal and Comcast Ventures are investors in Acorns.