Margin of safety: Benjamin Graham's most important idea
Every fair value is an estimate, and estimates miss. The margin of safety is the gap between fair value and the most you are willing to pay. It is your cushion for mistakes in your own assumptions. The more uncertain the company, the bigger it should be.
The idea
Benjamin Graham, Warren Buffett's teacher, made the margin of safety the core of his book "The Intelligent Investor". The thought is simple: you never know exactly what a company is worth. You can only estimate it with good reasons. So you don't buy at the estimated value, but well below it.
Engineers think the same way. A bridge is not built for exactly the weight it has to carry, but for a multiple of it. Not because the calculation is poor, but because reality rarely turns out the way it was planned.
The formula
- Fair value
- your estimate, for example from a DCF
- Margin of safety
- the discount in percent, for example 25% = 0.25
The other way round, you can work out for any price how big your margin of safety currently is: fair value minus price, divided by fair value. If the result is negative, you are paying more than you yourself consider reasonable.
The worked example
In the article on fair value, the made-up Example Inc. came to a fair value of €199.23 per share. This is the buy price at different margins of safety:
| Margin of safety | Discount | Buy price at most |
|---|---|---|
| 10% | €19.92 | €179.31 |
| 25% | €49.81 | €149.42 |
| one third | €66.41 | €132.82 |
| 50% | €99.62 | €99.61 |
What the cushion absorbs
Suppose your assumptions were too optimistic, and the true value of Example Inc. is 20% lower, at €159.38. What does that mean for you?
If you bought at the full fair value of €199.23, you paid 25% more than the stock is worth. The price would have to fall by 20% just for you to reach the true value.
If you bought with a 25% margin of safety at €149.42, you are still about 6% below the true value. Your mistake cost you nothing. That is exactly what the cushion is for.
How big should the cushion be?
There is no fixed number. What matters is how certain your assumptions are. For a large, stable company with a long, calm history, a smaller discount can be enough. For a young, fast-growing or highly volatile company it should be much bigger, because even small changes in growth move the fair value a lot.
A good rule of thumb: ask yourself how much you trust your own estimate. The more honestly the answer is "not much", the bigger the cushion.
The cushion and multiple models
The margin of safety protects best when the fair value itself is already well grounded. That is why it pays to calculate it in more than one way, for example with a DCF and a comparison based on multiples. If the results are close together, you can trust the average more. If they are far apart, that is a reason for a bigger cushion.
Three common mistakes
Your margin of safety, for every stock.
In Delta Metron you set your margin of safety yourself. The terminal calculates fair value with three models per company type and shows you right away from which price your cushion is reached – for more than 6,000 stocks. Sign up for free: The first 100 on the waitlist pay €5 instead of €10 a month at launch, for life.
Frequently asked questions
How big should my margin of safety be?
It depends on how certain your assumptions are. The more stable and predictable the company, the smaller the cushion can be; the more uncertain, the bigger. What matters is that you choose it deliberately.
Does a big margin of safety guarantee a profit?
No. It lowers the risk that a mistake in your estimate becomes expensive. Whether and when the price returns to fair value, nobody can predict.
What if the price never falls to my buy price?
Then you simply don't buy that stock – and look at others. There are thousands of companies. A watchlist helps you not to miss the moment when the cushion is reached.
Does the margin of safety work for ETFs?
Hardly. It requires that you can estimate a fair value. For a broad ETF with hundreds of companies, that is barely possible in a meaningful way.
Not investment advice. All content is for informational and educational purposes only. Example Inc. is made up.