YOU might think the lifestyle of a billionaire is made up of flashy cars, first-class trips and ostentatious designer clothing.

But the rich don't get rich by spending all their money.

Spending less than you earn is a simple way to grow your wealth but there are some super rich people take the idea to the extreme.

Reports this week said Keira Knightley paid herself a salary of just $50,000 last year, despite banking around $2.3 million through films and fashion commercial deals.

Here's a list of other frugal billionaires and multi-millionaires and their quirky habits, that see them living well below their means.

Warren Buffett

Legendary US investor Warren Buffett, who has an estimated net worth of $US54.6 billion, famously still lives in the same five bedroom house he bought 55 years ago for $US31,500.

Carlos Slim Helu

One of the world's richest men Carlos Slim Helu, who has an estimated net worth of $US67.2 billion, lives in the same modest house he has owned for 30 years, reports the UK Telegraph. His bedroom is said to be "the size of a Manhattan hotel room".

Mark Zuckerberg

Facebook founder Mark Zuckerberg wears the same thing every day.

"My drawer is about 20 of these grey t-shirts," he told NBC's Today last year.

He drives a $US30,000 Acura TSX and lives in a $US7 million house, which might sound like a lot of money but is well below the means of a man worth $US13.3 billion.

Chuck Feeney

Chuck Feeney, co-founder of Duty Free Shoppers who has donated his fortune of more than $US6 billion to charity, travels in economy class "because first class won't get me to my destination any faster". He doesn't own a house or a car and wears a $15 watch.

Ingvar Kamprad

Ikea founder Ingvar Kamprad also prefers to fly economy class, and prefers to book with budget airlines. He was once refused entry to a business award ceremony because he had come by bus.

And he isn't one for fine dining, with express.co.uk reporting he prefers to eat his favourite Swedish meatballs in Ikea cafeterias. He also reportedly swipes salt and pepper packets.

He also encourages his Ikea staff not to be wasteful, and tells people off if they don't use both sides of a sheet of paper or leave lights on when they leave a room.

T. Boone Pickens

US oil magnate T. Boone Pickens, worth $US1.2 billion, reportedly shops with a grocery list and only carries enough cash in his wallet to cover what he is about to buy for better budgeting.

James Caudwell

UK mobile phone mogul James Caudwell, worth more than $2 billion, cuts his own hair. Before his retirement he would save money on petrol and stay fit by riding his bike to work.

Azim Premji

Indian mogul Azim Premji, worth an estimated $US11.2 billion, drove a Ford Escape for eight years before he traded it with a Toyota Corolla. When he travels he prefers budget hotels over five-star accommodation. There is also a rumour he used paper plates during his son's wedding.

Graham Hill

Graham Hill, tech entrepreneur and founder of sustainability news website Treehugger.com, believes that having less stuff equals more happiness. He has given TED talks on his "life editing" strategies and he personally lives in a 39sq m apartment where his bed and home office fold out of the wall.