What is Warren Buffett's 90/10 Rule?
In his 2013 letter to Berkshire Hathaway shareholders, Buffett outlined the investment instructions he left in his will for his wife's trust:
"My advice to the trustee could not be more simple: Put 10% of the cash in short-term government bonds and 90% in a very low-cost S&P 500 index fund."
This is the 90/10 rule. It is aggressive for retirees, who are traditionally advised to hold 40% or 50% in bonds to protect capital. But Buffett argues that a low-cost stock index fund is more likely to build long-term purchasing power than bonds, which lose value to inflation. The 10% cash/bond buffer is simply there to cover living expenses during down years so the retiree never has to sell stocks during a market crash.
What is the 70/30 Rule?
If 90/10 feels too volatile, many financial planners recommend Buffett's classic 70/30 asset allocation rule (often used by pension funds and endowments).
Under the 70/30 split, you keep 70% of your portfolio in equities (stocks) for long-term growth and 30% in fixed income (bonds/cash) for stability. This offers a smoother ride than the 90/10 allocation while still providing enough equity exposure to outpace inflation.
How Long Will $1,000,000 Last Using the 4% Rule?
Now let us look at the math for a standard $1,000,000 nest egg. Under the famous 4% rule (originated by the Bengen study), a retiree withdraws $40,000 (4% of $1M) in the first year, and adjusts that withdrawal amount for inflation every subsequent year.
How long does this $1,000,000 last?
- In a 70/30 or 90/10 portfolio: Historically, there is a 95% to 98% probability that your portfolio will last at least 30 years. In fact, in the majority of historical scenarios, the retiree finishes the 30-year period with *more* money than they started with, thanks to compound growth in the stock market.
- In a cash-only or 100% bond portfolio: If you keep your money in a savings account or short-term treasury bills, inflation will eat your purchasing power. A $1M portfolio drawing $40,000 a year adjusted for a modest 3% inflation will run out in about 19 to 21 years.
Comparing Allocation Strategies on $1,000,000
| Strategy | Equity ($) | Fixed Income ($) | 30-Year Success Rate |
|---|---|---|---|
| 90/10 (Buffett Mrs. Trust) | $900,000 | $100,000 | ~96% |
| 70/30 (Balanced Pension) | $700,000 | $300,000 | ~98% |
| Cash/Treasury Only | $0 | $1,000,000 | ~5% |
The Core Lesson: Keep Costs and Tracking Tight
Buffett's instructions emphasize low fees. If you pay an advisor 1% and buy mutual funds with 1% fees, you are giving away half of your 4% safe withdrawal rate to service providers.
To maximize your success, minimize your fees by using simple, broad index funds and tracking your own expenses without paying for expensive subscription software. Keeping your overhead low is the most reliable way to guarantee your money lasts.