There’s only one skill that successful self-made billionaires like Warren Buffett and Bill Gates Zuckerberg all have in common.

Well, there’s hire great people and also fire fast. There is know thy customer but build great product, too. Create a great business plan but also experiment and test everything. There is lead by example with openness and transparency but we know you can’t share everything . . . I guess.

There is so much advice out there to consume but I will believe that there is one and I mean just one common skill that every successful self-made billionaire shares.

My best mentor taught this to me in 1995, when I had been asked to lead a corporate venture arm of our $6 billion company. The internet was just starting to emerge; I had just helped launch MapQuest and now the company wanted me to leverage its relationships with every print publisher dreaming about this new content distribution platform. “Content is king” was the mantra. But the content was locked away in proprietary systems, so I would invest in companies that would help unlock and distribute the content gold.

How many companies did you visit or talk to this week?

What are the average Series A valuations you are seeing?

Where are your best investment leads coming from?

How much is it costing us to maintain these partner relationships in Boston and on Sand Hill Road?

My mentor then shared a piece of advice that his father had shared with him and, in question form, it went like this: “Do you know your numbers?” You can’t make great decisions without knowing your critical numbers like you know your kids birthdays. For a general manager of a typical business unit, it is your basic P&L statement line by line. For me, as the president of 77 Capital, it was the investment funnel as well as the high level P&L for each of our portfolio companies.

Warren Buffett once said, “Accounting is the language of business.” I would add that accounting is the way we measure business success. Knowing your numbers and being able to talk about them at a moment’s notice without cue cards or a glance at your phone or tablet is a skill every billionaire shares.

Think Mark Cuban does not know his average attendance or merchandise sales for the Dallas Mavericks? Wonder if Larry Ellison has his sales trends for every business unit at the ready? I would bet that Bill Gates could recite his numbers regarding his philanthropic endeavors at a level of detail that would surprise you.

Knowing your numbers is a fundamental skill that signals your employees, partners, vendors, and investors that you are on your game.