He is a best-selling author who has written and co-authored a number of books for which he has received significant royalties. For more than five years, his book "A Brief History of Time" was on the Sunday Times bestseller list, and it was translated into 35 languages. In 20 years, the book has sold over 10 million copies.

Hawking was awarded the Special Fundamental Physics Prize in 2012, which included a cash prize of $3 million.

The late physicist also worked on a number of films and made cameo appearances in films and television shows, for which he was compensated well. He is also said to have made a lot of money from the movie "The Theory of Everything," which starred Eddie Redmayne as a physicist.

Hawking was often invited to give speeches and lectures by a variety of institutions and organizations, which provided a steady stream of income.

Hawking owned an Aston Martin, according to Celebrity Life Cycle, though little is known about his immovable properties. His annual earnings are estimated to be around $3 million.

Though he leaves a $20 million estate behind, he has previously stated that wealth is significant but not the most important aspect of life. Hawking wrote in an op-ed for the Guardian that due to his health problems, the importance of money is different for him than it is for others.

Hawking was paralyzed due to a slow-progressing type of amyotrophic lateral sclerosis (ALS).

"I will be the last person to dismiss the importance of capital," he wrote in the op-ed. "However, while wealth has played an important practical role in my life, I have, of course, had a different relationship with it than most people."

"Paying for my treatment and job as a chronically disabled man is critical; acquiring possessions is not." Even if I could afford one, I'm not sure what I'd do with a racehorse or a Ferrari." As a result, I've come to regard money as a facilitator, a means to an end – whether it's for ideas, health, or welfare – but never as an end in and of itself."