The surprisingly frugal habits of 8 extremely wealthy people

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Frugality is a subjective term. To the average Joe it could mean eating meals at home or scouring the internet for cheap flights.

But to a billionaire it means showing up to work in a T-shirt and jeans, driving a Toyota or Volkswagen, and, in some instances, foregoing the purchase of a private jet or lavish vacation home.

Surprisingly, some of the richest people on earth are incredibly frugal, each one with their own penny-pinching habits.

From eating lunch in the office cafeteria with their employees to residing in homes worth a fraction of what they could afford, these eight self-made billionaires — many of whom are also generous philanthropists — know the secret to keeping their net worth high.

Warren Buffett, chairman and CEO of Berkshire Hathaway, still lives in the same home he bought for $31,500 in 1958.

Net worth: $68.1 billion

The "Oracle of Omaha" is one of the wisest and most frugal billionaires around. Despite his status as one of the richest people on earth, he still lives in the same modest home he bought for $31,500 in 1958, doesn't carry a cellphone or have a computer at his desk, and once had a vanity license plate that read "THRIFTY," according to his 2009 biography. And when his friend of 25 years Bill Gates visits Omaha, Buffett picks Gates up from the airport himself.

Buffett also has a decidedly low-brow palate, known not just for investing in junk-food purveyors like Burger King, Dairy Queen, and Coca-Cola, but also for filling up on them as well. The Buffett diet includes five Cokes a day, as well as Cheetos and potato chips.

At his annual shareholder's meeting in 2014, Buffett explained that his quality of life isn't affected by the amount of money he has:

"My life couldn't be happier. In fact, it'd be worse if I had six or eight houses. So, I have everything I need to have, and I don't need any more because it doesn't make a difference after a point."

Mark Zuckerberg, founder and CEO of Facebook, drives a manual-transmission Volkswagen hatchback.

Net worth: $51.5 billion

Despite his status as one of the richest tech moguls on earth, Mark Zuckerberg leads a low-key lifestyle with his wife Priscilla Chan and their newborn daughter. The founder of Facebook has been unabashed about his simple T-shirt, hoodie, and jeans uniform.

"I really want to clear my life to make it so that I have to make as few decisions as possible about anything except how to best serve this community," Zuckerberg said.

The trappings of wealth have never impressed the 32-year-old, who in December 2015 announced he would donate 99% of his Facebook shares during his lifetime.

Zuckerberg chowed down on McDonald's shortly after marrying Chan in 2012 in the backyard of their $7 million Palo Alto home — a modest sum for such an expensive housing market and pocket change for a man worth more than $51 billion. In 2014, he traded in his $30,000 Acura for a manual-transmission Volkswagen hatchback.

Carlos Slim Helú, founder of Grupo Carso, has lived in the same six-bedroom house for more than 40 years.

Net worth: $31.6 billion

Rather than spending his fluctuating fortune, Carlos Slim funnels his billions back into the economy and his vast array of companies. He once mused to Reuters that wealth was like an orchard because "what you have to do is make it grow, reinvest to make it bigger, or diversify into other areas."

The 76-year-old is by far the richest man in Mexico, but he forgoes luxuries like private jets and yachts and reportedly still drives an old Mercedes-Benz. Slim runs his companies frugally, too, writing in staff handbooks that employees should always "maintain austerity in prosperous times (in times when the cow is fat with milk)."

The businessman has lived in the same six-bedroom house in Mexico for more than 40 years and routinely enjoys sharing home-cooked meals with his children and grandchildren. He's got a couple of known indulgences, including fine art — in honor of his late wife — and Cuban cigars, as well as an $80 million mansion in Manhattan, which he was trying to sell last spring.