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Buffett talks about **"circle of competence"** constantly. Most investors nod along and then ignore it completely, or apply it so loosely that it means nothing.
I want to try to make it actually useful.
**What the concept is supposed to do**
Circle of competence is a filter, not a trophy. Its job is to answer one question as quickly as possible: *Can I evaluate this business, or am I just guessing?*
The honest answer determines everything that follows. If you're inside your circle, your analysis is built on genuine understanding. If you're outside it, you're pattern-matching on surface details and calling it research.
The problem is that most investors define their circle too generously and never test whether the definition is real.
**The two-minute test**
Before spending any meaningful time on a company, I try to answer three questions out loud (not in writing, out loud):
1. How does this business make money?
2. Why will it still be making money in ten years?
3. What would have to be true for it to fail?
If I can answer all three clearly, without reaching for jargon, I'm probably inside my circle. If I'm hedging, vague, or using industry terms I haven't fully unpacked, I'm not.
This sounds simple. It isn't. Most investors who try this discover their circle is significantly smaller than they thought.
**What "inside your circle" actually requires**
It doesn't mean you've read about the industry. It means you understand the business model well enough to have an informed view on its durability.
For a bank, that means understanding how net interest margin moves across rate environments and what that does to earnings. For a software company, it means understanding the actual mechanics of why customers renew. For a retailer, it means understanding the unit economics of a new location, not just the expansion story.
Reading an annual report gives you the facts. Being inside your circle means you know what to do with them.
**The honest mapping exercise**
I find it useful to write this down rather than carry it as a vague feeling. Three columns: businesses I understand well enough to evaluate durability (clearly inside), areas where I follow the logic but would need significant work before having genuine conviction (edge), and everything else (outside).
Most investors who do this end up with a much shorter first column than they expected. That's the point. A small, honest circle beats a large, imaginary one every time.
**Why it matters more than people think**
The circle isn't just about avoiding bad investments. It's about knowing when to have conviction.
A thirty percent drawdown in a business you truly understand is an opportunity to think clearly. The same drawdown in a business you half-understand is a crisis, because you don't know whether the thesis is intact or broken.
The real cost of operating outside your circle is that you won't know *when* you're wrong, which makes it impossible to respond correctly.
Curious how others think about this in practice. Do you have a formal way of mapping your circle, or is it more of an intuitive feel? And has it ever stopped you from buying something you later realised you didn't actually understand?