Margin of Safety
Buying well below your estimated value so that even if your judgment is wrong, the loss is limited — the core principle of value investing.
In plain terms
When designing a bridge you build it far stronger than the weight it must actually bear, so it does not collapse if estimates are off. Margin of safety is the same. If you value a stock at $10, you buy it not at $10 but around $6.
Then even if your value estimate is a bit off or unforeseen bad news comes, the loss is not large. It is not "buy when cheap" but "buy cheap enough to account for your own chance of being wrong."
What it tells you
Margin of safety comes from admitting "I too can be wrong." However much you analyze, the future is unknown, so you fill that uncertainty with a cushion in the price.
Benjamin Graham, the father of value investing, stressed it, and his student Warren Buffett named it one of the most important concepts in investing.
Formula
margin of safety = the gap (the cushion) between estimated fair value and the actual purchase price e.g. buying a stock you value at $10 for $6 gives a $4 margin of safety
What high or low means
The cheaper you buy below fair value, the bigger the margin of safety and the more room to withstand a wrong judgment. Buying near or above value leaves almost no margin, so a small bit of bad news produces a loss.
But "looks cheap" is not itself a margin of safety. If the value estimate is wrong, the margin is an illusion (which is why it matters to estimate value conservatively).
The pitfall of margin of safety is "there is no guarantee the value you assigned is right." If you believed you bought cheap but that value estimate was wrong, there was no margin from the start. So estimate value conservatively, not generously.
Buying just because it is cheap, using margin of safety as an excuse, is also risky. Cheap can have a reason (a value trap), so weigh both value and price.
"Margin of safety" is a concept Benjamin Graham, the father of value investing, stressed in his 1949 book The Intelligent Investor. His student Warren Buffett later named "margin of safety" the three most important words in investing.
The core is the humility of "I can be wrong." However good the analysis, it cannot perfectly predict the future, so you buy cheap enough to fill the room for misjudgment with price. It is the most solid foundation of value investing, tested over decades.
Metrics to read alongside
See it in real stocks
Search US stocks on Stocklore to see Margin and other financial metrics alongside the sector average.
This explanation is for information and reference only and is not a recommendation to buy or sell any security. Investment decisions and their consequences are your own.