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.
This is an update of an earlier version of this post.
Warren Buffett, chairman and CEO of Berkshire Hathaway, still lives in the same home he bought for $US31,500 in 1958.
Net worth: $US74 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 $US31,500 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.'
Carlos Slim Helú, founder of Grupo Carso, has lived in the same six-bedroom house for more than 40 years.
Net worth: $US65.3 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 77-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 previously tried to sell.
Azim Premji, chairman of Wipro Ltd., drives secondhand cars and always reminds employees to turn off the lights at the office.
Net worth: $US15.6 billion
India's wealthiest tech tycoon has also been called 'the bare-bones billionaire' and someone who 'makes Uncle Scrooge look like Santa Claus'.
The 71-year-old is worth more than $US15 billion, but that hasn't stopped him from jumping on one of India's three-wheel auto rickshaws to get home from the airport or keeping tabs on the toilet-paper usage at Wipro offices. Premji also flies economy, drives secondhand cars, and always reminds employees to turn off the lights at the office.
Mark Zuckerberg, founder and CEO of Facebook, drives a manual-transmission Volkswagen hatchback.
Net worth: $US70 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 young 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 33-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 $US7 million Palo Alto home -- a modest sum for such an expensive housing market and pocket change for a man worth more than $US70 billion. In 2014, he traded in his $US30,000 Acura for a manual-transmission Volkswagen hatchback.
Charlie Ergen, chairman of Dish Network, still packs a brown-bag lunch every day.
Net worth: $US19.9 billion
Charlie Ergen is a notoriously frugal business leader, but he also nickels and dimes in his personal life. The 64-year-old has said that his frugality hearkens back to his mother's childhood. 'My mum grew up in the Depression,' he told the Financial Times. 'I don't have a mahogany desk.'
The self-made billionaire packs a lunch of a sandwich and Gatorade before work every day and, until recently, he shared hotel rooms with colleagues during travel.
Amancio Ortega, founder of Inditex, eats lunch with his employees in the Zara headquarters cafeteria.
Net worth: $US82.3 billion
The founder of Zara is currently the third richest person in the world, but that probably won't affect his personal-spending habits. Ortega has led an extremely private life for years, often retreating to his quiet apartment in La Coruña, Spain, with his wife, frequenting the same coffee shop, and eating lunch with his employees in the Zara headquarters cafeteria.
Like fellow billionaire Mark Zuckerberg, the Spanish fashion magnate maintains a simple uniform. His consists of a blue blazer, white shirt, and grey pants that he wears every day. Some say the 81-year-old shouldn't be considered 'frugal' given his ownership of a $US45 million Bombardier private jet, but he doesn't travel often because he's too busy working.
Ingvar Kamprad, founder of IKEA, still flies economy and often rides the bus.
Net worth: $US43.8 billion
Kamprad is one of the richest people in Europe, but you wouldn't know it flying next to him in economy class or eating lunch with him in IKEA's cafeteria. Save for a flashy spending spree in the 1960s when he drove a Porsche and wore custom-made suits, the Swedish furniture-maker has been incredibly frugal -- some may even say 'cheap' -- with his billions, including driving a decades-old Volvo and frequently riding the bus.
The 91-year-old is worth more than $US43 billion, but when he moved home to Sweden in 2013 after spending 40 years in Switzerland -- where he was dodging Sweden's high taxes -- he happily returned to his modest one-story ranch home.
Judy Faulkner, founder of Epic Systems, says she's never been interested in 'living lavishly.'
Net worth: $US4.82 billion
The press-shy software programmer built Epic -- a private healthcare company that sells medical-records software -- from the ground up, launching in 1979 with about $US70,000 in capital.
Her company's success has made her a multibillionaire, but the 73-year-old has never been one to splurge. According to reports, Faulkner has had only two cars in the past 15 years and has lived with her husband in the same Madison, Wisconsin, suburb for nearly three decades.
In a May 2015 letter announcing her Giving Pledge membership and a promise to donate half of her fortune to charity, Faulkner wrote, 'I never had any personal desire to be a wealthy billionaire living lavishly' and said that, instead, she'll use her money to help others gain access to 'food, warmth, shelter, healthcare, education.'