Why margin of safety matters

Margin of safety is the oldest and most important idea in value investing. Coined by Benjamin Graham in the 1930s, it's simply this: always pay less than something is worth.

Valuation is uncertain

Every valuation model involves assumptions. We assume a growth rate. We assume a discount rate. We assume the business will keep performing. Any of these can be wrong — and often are. A 10-year projection compounds errors: if your growth rate assumption is off by 2 percentage points, the fair value estimate could be off by 30% or more.

Margin of safety is the practical answer to this uncertainty. By buying at a significant discount to your best estimate of fair value, you've built in room for your assumptions to be substantially wrong and still earn a satisfactory return.

How much discount is enough?

We use 50% — the Payback Price is always half the Sticker Price. This is on the conservative end. Some investors use 25–30% for very predictable, stable businesses. We use 50% as a default because it's difficult to know in advance which businesses will perform exactly as modelled — and the asymmetry of outcomes (limited upside from overpaying, substantial downside) argues for caution.

For dividend-paying stocks valued on their income stream, we use a 33% margin of safety rather than 50%, because the regular dividend payments provide an ongoing return regardless of where the share price goes.