Next month, Warren Buffett will celebrate his 92nd birthday. That puts him in a select group, considering the average lifespan of Americans is around 78 years. But Buffett also belongs to an even rarer group: He's one of the four wealthiest individuals in the U.S. 

Buffett's net worth currently tops $99 billion, according to Forbes. His fortune has more than tripled over the past two decades. And there's one stock that has made Buffett $62 billion richer since 2002.

No mystery here

The stock that's made Buffett a much wealthier man is, of course, Berkshire Hathaway (BRK.A 1.18%) (BRK.B 1.30%). Roughly 99% of the legendary investor's personal wealth is tied to the conglomerate. 

Sure, Buffett owns a handful of other stocks. He has publicly disclosed personal positions in real estate investment trust (REIT) Seritage Growth Properties and Wells Fargo in the past. However, all of his other holdings amount to a drop in the bucket compared to his stake in Berkshire. 

Looking back even further into the past, Berkshire has made Buffett a lot more than $62 billion. Buffett took control of the company in 1965 when he was only 35 years old. His estimated net worth at the time was $7 million.

How the fortune was made

Buffett didn't make billions of dollars by passively investing in Berkshire Hathaway. He led the company to make multiple acquisitions and investments through the years that contributed significantly to the stock's performance.

Berkshire's acquisition of Burlington Northern Santa Fe (BNSF), which closed in 2010, is arguably the most important deal for the company over the past 20 years. The transaction put Berkshire in the S&P 500. Buffett considers the railroad one of Berkshire's "four giants", with the business generating $6 billion of profit in 2021. 

Shrewd investments in other publicly traded companies also paid off handsomely. While Berkshire's portfolio has changed quite a bit since 2002, its stake in Coca-Cola has been a constant. The food and beverage stock has generated a total return of nearly 370% during the period.

Buffett became a fan of Apple more recently, initiating a position in the tech giant in the first quarter of 2016. Apple now ranks as the biggest holding, by far, in Berkshire's portfolio. It's been a massive winner for Buffett, delivering a total return of roughly 500% in a little over six years.

We can't leave out one other important key to Buffett's growing fortune: Berkshire's stock repurchases. The Oracle of Omaha wrote to Berkshire shareholders earlier this year about buybacks, "When the price/value equation is right, this path is the easiest and most certain way for us to increase your wealth." And as Berkshire's largest shareholder, Buffett's wealth increased nicely also with well-timed buybacks.

Learning from the master

There's one important lesson that every investor can learn from how Buffett grew his fortune so much over the past two decades: Buy stocks and hold them for the long term. Buffett would be worth even more today had not given a big chunk of his Berkshire shares to charity (an admirable move, by the way).

You don't have to be one of the world's wealthiest people to have a good chance at more than tripling your personal fortune over the next 20 years. Invest in a basket of stocks of well-run companies with solid growth prospects or an S&P 500 index fund and then wait. The approach that has worked so well for Buffett can also work for you.