What is a margin of safety?
However carefully you estimate a share’s value, nobody knows the future for certain. A margin of safety means buying well below your estimate of value, so that if the estimate is somewhat wrong or bad times arrive, the damage is limited. Benjamin Graham and David Dodd introduced the term in 1934, and Graham wrote that the margin of safety always depends on the price paid.
Warren Buffett explains it with a bridge: when you build one, you make sure it can carry 30,000 pounds, but you only drive 10,000-pound trucks across it. Investing follows the same rule: avoid putting money where you only come out ahead if every assumption turns out exactly right.
Say the trip to Dhaka airport usually takes an hour. But nobody catching a flight leaves exactly an hour before; allowing for traffic, rain or a breakdown, they give themselves two or three hours. Most days the extra time is not needed, but on the day it is, the flight is not missed. A margin of safety in buying shares works just like that extra time. The example is imaginary.
Why it matters to an investor
- Estimates go wrong: value rests on guesses about the future, and guesses often miss.
- It protects against bad luck as well as your own errors: a pandemic, raw-material prices or the taka’s value can change suddenly.
- It lowers the chance of permanent loss: pay too much and you may not get your money back even if the business does well.
- It builds discipline: not buying unless the price falls below a level set in advance, and not buying on hype.
How big should the margin be?
There is no fixed percentage rule. How big a margin makes sense depends on how uncertain your estimate is. A business with years of steady earnings, little debt and a simple model is less likely to be badly misjudged. A new, cyclical or heavily indebted business, or one you do not know well, is more likely to be, so it needs more room.
- Compare the price with a cautious or pessimistic estimate, not your most optimistic one.
- The more assumptions the value rests on, the more room to think about leaving.
Margins of safety beyond price
- In the company’s balance sheet: low debt and cash in hand make bad years easier to survive. This is the business’s own margin.
- In business quality: a lasting competitive advantage makes a sudden collapse in earnings less likely.
- In your own life: keeping an emergency fund, not investing borrowed money, and not putting money you will soon need into shares, so you are never forced to sell cheap in a bad patch.
Its limits
A margin of safety is only as good as the estimate of value behind it. If the estimate itself is badly wrong, a “30% margin” on paper does not really exist. Often a price is low because the business really is getting worse; then what looks like a margin is actually a trap.
And waiting for a large margin often means the chance to buy never comes, or that after buying it takes many years for the price to approach value. It is a protection whose cost is patience.
Worked example
- Say you estimate the value of a share in an imaginary company, “Rupsha Foods Limited”, at Tk 60 in a pessimistic case, Tk 80 in a middle case and Tk 100 in an optimistic case.
- At a price of Tk 75, the margin against the middle estimate is only (80 − 75) ÷ 80 ≈ 6%. And if the pessimistic case comes true, you have paid Tk 15 more than the share is worth.
- At a price of Tk 55, the margin against the middle estimate is (80 − 55) ÷ 80 ≈ 31%, and even in the pessimistic case the value (Tk 60) is above the price.
Result: the real question a margin of safety asks is “what if the bad case comes true?” Comparing only with your best guess makes you look safer than you are. There is no guarantee when the price will move towards value. The numbers are only an example.
Common mistakes
- Applying one fixed rule, such as “always a 30% margin”, to every company.
- Making the margin look big by comparing the price with an optimistic estimate.
- Thinking there is a margin because the price is far below its old peak. A past price is no measure of value.
- Destroying the protection yourself by buying with borrowed money.
Assume your estimate of value will be wrong, and pay a price at which even the bad case does not hurt badly: that is the margin of safety.