Bill Gates is one of the world's biggest philanthropists and has given away billions of dollars of his money.

That's why so many people think that Gates' personal wealth is shrinking as he gives his money away.

But it isn't. Gates is astoundingly rich and getting richer all the time.

And he just joined a two-man club of people worth more than $100 billion.

Bill Gates is one of the world's biggest philanthropists. He and his wife, Melinda Gates, have given away a whopping $45.5 billion in their lifetimes through their foundation and other charities, according to The Chronicle of Philanthropy. Plus, he has vowed to give away the majority of his wealth to charity and founded the Giving Pledge, an organization that asks other wealthy people to do the same, with his close friend Warren Buffett.

That's why so many people think that Gates' personal wealth is shrinking as he gives his money away.

But it isn't. Gates is astoundingly rich and getting richer all the time.

Read more: I spent an uplifting day at the Bill & Melinda Gates Foundation and discovered what it's really like to work there

He's $9.5 billion richer now than he was one year ago, and his personal wealth is just cresting at the $100 billion mark, according to the Bloomberg Billionaire Index. It's amazing to contemplate that the rounding number — that half-billion dollars of the $9.5 billion — is itself half a billion dollars and represents a larger net worth than even the most well-off families will ever be worth. And in 2014, Gates was worth $76 billion. So he's up $24 billion in five years.

That $100 billion net worth puts him in rarified air even for a billionaire. It's an exclusive two-man club with that other superrich Seattle-area guy Jeff Bezos. Bezos is known as the richest man, worth about $146 billion.

Gates has, many times, been considered the No. 2 richest person before, some seven or so times out of the last 24 years.

But the truth is, we don't really know how rich Gates is.

Many of his investments are done through Cascade Investment, his firm run by a notoriously secretive money manager, Michael Larson. Those investments run the gamut from large stakes in public companies, such as Waste Management and AutoNation, to real estate and other business interests.

On top of that, the Bill and Melinda Gates Foundation also invests in some for-profit businesses, with those stakes controlled by either Bill, Melinda, or both of them. For instance, their foundation bought nearly 800,000 shares of Liquidia Technologies, a biotech company developing personalized, engineered drugs, according to Securities Exchange Commission filings. Liquidia is in the process of an initial public offering. Although, to be fair, such stakes are for the foundation and don't count toward Gates' personal wealth.

On the other hand, most of his fortune no longer comes from Microsoft, even though his Microsoft stock is doing well under CEO Satya Nadella.

Over the past 15 years, Gates sold off most of his Microsoft stake. In 2004, he had 1.1 billion shares of Microsoft. Today, he keeps about 103 million shares, which are worth about $12 billion. He hasn't been selling or acquiring more shares over the past couple of years, waiving both his cash salary and his share grants for his role as chairman.

But there are times when he winds up with a little more from Microsoft through his many business interests.

For instance, in April 2017, Microsoft acquired a company called Intentional Software, founded by a former longtime Microsoft engineer, Charles Simonyi, who worked on Excel and Word. Gates was an investor in Intentional through one of his investment companies, and Microsoft paid him $60 million when it bought the company, it said.

So while Gates spends his energy on charitable giving, his fortune is growing far faster than he can give it away.