Rob Gronkowski saved his entire $54 million career earnings in the NFL by following one simple money rule

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Rob Gronkowski is retiring from the NFL with his entire $54 million NFL career earnings.

• Rob Gronkowski is retiring from the NFL with his entire NFL career earnings - a reported $54 million, according to Spotrac.
• He was able to save this nest egg by following a simple money rule: Keeping it simple by saving surplus money.
• Gronkowski previously told Business Insider he only lived off his endorsement earnings.
• Gronkowski is in good company with his frugal lifestyle choices; some of the world's wealthiest people, like Warren Buffett and Mark Zuckerberg, are known for their frugal spending habits.

Rob Gronkowski announced on Sunday that he is retiring from the NFL.

The New England Patriots tight end has a nice nest egg to fall back on: his entire NFL career earnings, which reportedly total $54 million after nine seasons in the NFL, according to Spotrac.

Business Insider's Scott Davis previously reported that Gronkowski, age 29,  hadn't spent any of the money he made during his NFL career, instead spending only his endorsement earnings. He advised other players to follow a similar plan and watch their spending overall.

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"I'd just say keep it simple," Gronkowski said. "Keep it easy, and I'd say keep it simple. Get what you need to be comfortable, save the extra."

He continued: "It's a short career here in the league. The average is about three to four years, and when it's done, it's done. Be simple, manage it safe, so when it's done you've got a little base, a little foundation to live off."

Gronkowski's savings strategy isn't far from the financial habits of other successful millionaires and billionaires, many of whom also live relatively frugal lifestyles.

Some of the world's wealthiest people live very affordable lifestyles

Frugal lifestyles help millionaires get rich in the first place, according to Sarah Stanley Fallaw, director of research for the Affluent Market Institute. She coauthored "The Next Millionaire Next Door: Enduring Strategies for Building Wealth," in which she surveyed more than 600 millionaires in America.

During her research, she studied the characteristics most predictive of net worth. She found that six behaviors, which she called "wealth factors," are related to net-worth potential, regardless of age or income. One of those is frugality - a commitment to saving, spending less, and sticking to a budget.

"Spending above your means, spending instead of saving for retirement, spending in anticipation of becoming wealthy makes you a slave to the paycheck, even with a stellar level of income," she wrote.

Read more: Rob Gronkowski is retiring from the NFL with $54 million after saving all of his career earnings

Consider billionaire Warren Buffett, who still lives in the modest Omaha, Nebraska home he bought for $276,700 in today's money. Billionaire Richard Branson is famously frugal when it comes to buying luxury items, and several billionaires, like Mark Zuckerberg and Jeff Bezos, have been spotted driving affordable cars - Hondas, to be exact.

Tyra Banks, who has an estimated net worth of $90 million, saved so much money that she was able to buy a house at 20 years old. Her accountants even told her she needed to spend money and had her set up a frivolous account.  Meanwhile, Jay Leno, who has an estimated net worth of $250 million, had a similar strategy to Gronkowski: He's always tried to hold two jobs, spending money from one job while saving money from the other. He reportedly saved all of his money from hosting the "Tonight Show" - reportedly as much as $30 million per year, according to CNBC - and spent only the money he made from stand-up comedy touring.

When it comes to reigning in the paycheck, it looks like Leno and Gronkowski are on to something.