Buffett’s 90/10 is Wrong. Even Though it’s Right.

The title sounds like a contradiction. How can something be wrong and right simultaneously? Step back and look at the famous utterance from the Sage of Omaha, and it makes complete sense — to me, anyway.

Some background. In his 2013 shareholder letter, Warren Buffett laid out a simple inheritance plan for his wife: put 10% of the cash in short-term government bonds and 90% in a very low-cost S&P 500 index fund. On the surface, it’s the ultimate endorsement of passive indexing. But while the math is technically right, it’s dangerously wrong for the average person navigating a 30-year retirement.

Context is everything here. To put it bluntly: in Warren Buffett’s reality, 10% of net worth is in the region of $15,000,000,000. That’s enough zeros to live off your cash holdings for several lifetimes without losing a moment’s sleep over how long a bear market lasts before you can draw from equities again.

If that’s your reality, or you’re simply a very high net worth individual, then the 90/10 split, which was, to highlight, specifically crafted as an inheritance instruction for his wife, might be an excellent choice. For the average everyday retiree, I think it carries dangerous downside risk.

Consider a retiree with an account balance well above average: $2 million invested at the same ratio gives you exactly $200,000 in the cash-equivalent portion. At a $100,000-per-year lifestyle, that’s two years of runway before you’re forced to sell equities — regardless of where the market stands.

To be fair, two years has been enough on occasion. When COVID crashed the market in February 2020, the S&P 500 bottomed out in just 33 days and had fully recovered by August, well inside that two-year window. Similarly, after Black Monday in October 1987, the market, while deeply shaken, had clawed its way back within roughly two years. In both cases, our hypothetical retiree would have weathered the storm intact.

But those are the forgiving examples. The dot-com crash of 2000 took two and a half years just to reach its bottom, with full recovery stretching nearly five years. The 2008 financial crisis was even more unforgiving. Markets didn’t return to pre-crash highs for roughly six years. In either of those scenarios, our retiree with two years of cash would have been forced to sell equity positions at precisely the worst possible moment, locking in losses and damaging the portfolio’s ability to recover.

That’s the real danger hiding inside advice that sounds perfectly sensible — until, that is, you remove the $15 billion safety net underneath it.

Buffett’s advice isn’t wrong. It’s possibly just not yours. He was writing instructions for a billionaire’s inheritance, not a blueprint for the average retirement. The principle — low cost, passive indexing, minimal interference — is as sound as investing advice gets. But the allocation? That needs to be built around your reality: your balance, your spending, your ability to live while the market has a multi year temper tantrum. The Sage of Omaha and his wife can afford to wait out a six-year recovery. Whether you can too often comes down to one thing: how many zeros are in your account when the market decides to misbehave.

The bottom line is simple: The 90/10 rule is mathematically right for maximizing total wealth over 50 years, but it is operationally wrong for a human being who needs to buy milk and pay property taxes during a four-year bear market. So before you copy the most famous allocation in investing, ask yourself what Clint Eastwood might have asked: do you feel lucky? Because when the market drops 40% and your cash runs dry in month 25, luck is exactly what you’ll be relying on. Warren Buffett’s wife doesn’t need to feel lucky. She has $15 billion reasons not to. The question is whether you do too.