How large should the margin of safety be?

Graham gave roughly one third as a rule of thumb. The more uncertain the value, the larger the buffer should be. There is no fixed number.

Why do you still use the Graham formula?

Because it is a simple, independent cross-check to the DCF. If both agree, the result looks more robust; if they differ, the assumptions deserve a look.

What is a value trap?

A stock that looks cheap and stays cheap because the business or governance is the problem. The discount never narrows, so the margin is worthless.

Knowledge · Value classic

Margin of safety according to Graham and Dodd

The margin of safety is the gap between the share price and the estimated value. Benjamin Graham only bought at a discount so that errors in the estimate would not immediately mean losses.

At a glance

The idea comes from “Security Analysis” (Graham/Dodd, 1934) and “The Intelligent Investor”.
The Graham formula estimates value from earnings and growth, adjusted for today’s interest rate level.
As a rule of thumb, one third discount applies.
For us it is the cross-check to the DCF, not the main model.

Formula

Value = EPS × (8.5 + 2g) × 4.4 ÷ Y
Margin = 1 − price ÷ value

8.5 = P/E with no growth · g = expected growth in %
4.4 = AAA yield back then · Y = current AAA yield

Worked example 1: Tesla (Graham formula)

EPS $1.08, Y = 6.22%, base growth g = 25%:
1.08 × (8.5 + 2 × 25) × 4.4 ÷ 6.22 = 44.7 ≈ $45.
The share price is $354.72. There is no margin of safety, the price is almost eight times the formula value.

Worked example 2: three stocks compared

Stock	Value	Price	Margin
Porsche SE (net NAV)	€42.90	€28.42	+33.8%
Merck (Graham base)	$99	$145.27	−47%
Tesla (Graham base)	$45	$354.72	−688%

Porsche SE: The margin sits almost exactly at Graham’s third. Still, the discount is historically normal and not a buy signal.
Merck: The price is 47% above the formula value, so the margin is negative.

Margin of safety: value versus price

What the number does

The margin is a buffer against your own mistakes, not a promise of return.
Porsche SE shows this well: the margin is large, but the discount is permanent.

Pitfalls

The formula reacts extremely to g and is useless with losses or collapsing earnings. We discarded it for Porsche SE for that reason: the range ran from €13.60 to €94.08.
A value trap (permanently cheap) is not a margin.
The value is itself an estimate. A large margin on a wrong value is not safety.

FAQ

How large should the margin of safety be? Graham gave roughly one third as a rule of thumb. The more uncertain the value, the larger the buffer should be. There is no fixed number.
Why do you still use the Graham formula? Because it is a simple, independent cross-check to the DCF. If both agree, the result looks more robust; if they differ, the assumptions deserve a look.
What is a value trap? A stock that looks cheap and stays cheap because the business or governance is the problem. The discount never narrows, so the margin is worthless.