CODEXCard 7 of 12 · 12 Mental Models
Margin of safety
Build for the bad day.
A margin of safety is the buffer between what a plan needs to survive and what it is designed to handle. The phrase was made famous in investing by Benjamin Graham, Buffett's teacher, who advised buying only when the price sat well below a careful estimate of value. Munger embraced the idea and applied it widely.
The logic comes from engineering. A bridge built to carry exactly the expected traffic will fail on the day of an unusual load. Designers add capacity well beyond the average, because the cost of extra strength is small compared with the cost of collapse. In personal life, savings, spare time in a schedule and reserves of goodwill all play the same role.
The limit is building so much margin that you never act. Waiting until every conceivable risk is covered means missing reasonable opportunities and tying up resources that could be working. The useful margin is calibrated: large enough to get through a realistic bad case, but not so large that it becomes an excuse for doing nothing.
Margin of safety pairs naturally with the planning fallacy. Because estimates tend to be optimistic, deliberately adding slack is often the simplest correction available.
Example
You need to be at the airport by nine for a flight. The drive usually takes forty minutes, so leaving at eight-fifteen seems fine. Leaving at seven-forty-five instead covers roadworks, a slow car park and a long security queue. On a smooth day you wait with a coffee. On a bad day you still make the flight.
Try it this week
Add a realistic buffer to one deadline or budget this week, sized to the worst problem you have actually seen before.
From 12 Mental Models (Charles T. Munger · 1995).
