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Finance & Investing

3-6-3 Rule

Model #0708Category: Finance & InvestingDepth to apply:

By Updated 3 sources

4 min read

Contents

  1. 1. Core Idea
  2. 2. How to See It
  3. 3. How to Use It
  4. 4. Founders & Leaders
  5. 5. Connected Models
  6. 6. One Key Quote
  7. 7. Summary & Further Reading
Finance & Investing
Section 1

Core Idea

The 3-6-3 Rule is the old banking joke: borrow at 3%, lend at 6%, and be on the golf course by 3pm. It describes the era when banking was a simple spread business — take deposits cheaply, lend at a markup, and the margin was guaranteed by regulation and limited competition. The model matters because it illustrates what happens when a business is protected from disruption: margins are fat, innovation is low, and incumbents optimise for comfort, not value. When deregulation, technology, or new entrants arrive, the 3-6-3 model breaks. For founders, the lesson is two-sided: fat margins in a protected industry signal opportunity for disruption; and if your own business operates on a "3-6-3" assumption, it's vulnerable to anyone willing to compress the spread.

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Section 2

How to See It

Banking
You're seeing the 3-6-3 Rule when financial institutions earn wide, stable spreads with minimal innovation. The business is a toll on capital flow, not a value-creation engine.
Industry
You're seeing it when any industry has wide, regulation-protected margins and incumbents show little urgency to improve. The "3-6-3" mentality — fat spread, low effort — signals disruption opportunity.
Fintech
You're seeing it break when new entrants (fintech, neobanks, alternative lenders) compress spreads by using technology to undercut the cost structure or offer better terms.
Section 3

How to Use It

Scan for industries or business models that operate on a 3-6-3 assumption — wide margins, low innovation, regulatory protection. Ask whether technology, deregulation, or new entrants could compress the spread. If you're in a 3-6-3 business, ask what happens when the spread narrows. If you're entering one, the spread is your disruption surface.
Decision filter
"Are the margins in this industry fat because of genuine value creation, or because of protection and inertia? If protection, what compresses the spread?"
As a founder
Look for industries where incumbents earn wide spreads without proportionate value creation — that gap is your entry point. Build a cost structure or technology advantage that lets you operate on thinner margins. Then ask the same question about your own business: are our margins earned or protected?
Section 5

Founders & Leaders

J.P. MorganFounder, J.P. Morgan & Co.
Morgan built the most powerful banking institution of his era precisely by going beyond the 3-6-3 model — structuring complex deals, reorganising railroads, and acting as lender of last resort during panics. He earned outsized returns not from a passive spread but from active value creation in finance. Founders can take the lesson: the 3-6-3 model is a commodity trap. Durable advantage in financial services — or any spread business — comes from doing what competitors can't or won't, not from sitting on a protected margin.
Section 7

Connected Models

Reinforces
Creative Destruction
The 3-6-3 era ended through creative destruction — deregulation and technology eroded the protected spread. The model illustrates what creative destruction targets: fat, unearned margins.
Reinforces
Regulatory Capture
The 3-6-3 model was sustained by regulation that limited competition and guaranteed the spread. Regulatory capture keeps the model alive; deregulation kills it.
Leads-to
Cost of Capital
When the 3-6-3 spread compresses, cost of capital rises for the lender and falls for the borrower. Understanding the spread helps you see how cost of capital shifts with competition.
Section 8

One Key Quote

"Borrow at 3, lend at 6, be on the golf course by 3. The joke stopped being funny when fintech showed up."
— Traditional banking adage
Section 11

Summary & Further Reading

The 3-6-3 Rule: protected spread businesses earn fat margins with low innovation. The model signals disruption opportunity when technology or deregulation can compress the spread. Apply it to any industry where margins are wide and effort is low.
01
The Bankers' New Clothes — Admati & Hellwig (2013)
Book
How banking margins and incentives create fragility.
02
Fragile by Design — Calomiris & Haber (2014)
Book
The political economy of banking regulation and protected margins.
03
Breaking Banks — Brett King (2014)
Book
How fintech disrupted the 3-6-3 model.

Why this matters next

mental modelsIncentives

3-6-3 Rule applied the Incentives mental model

mental modelsMargin of Safety

3-6-3 Rule applied the Margin of Safety mental model

mental modelsInertia

3-6-3 Rule applied the Inertia mental model

mental models3-6-3 Rule

3-6-3 Rule applied the 3-6-3 Rule mental model

mental modelsCost of Capital

3-6-3 Rule applied the Cost of Capital mental model

mental modelsCost

3-6-3 Rule applied the Cost mental model

Frequently asked questions

What is 3-6-3 Rule?

3-6-3 Rule is a mental model used for better thinking and decision-making.

How do you apply 3-6-3 Rule?

To apply 3-6-3 Rule, identify situations where this framework is relevant, then use it as a lens to evaluate your options and decisions. The model is most useful when combined with other complementary mental models.

What category does 3-6-3 Rule fall under?

3-6-3 Rule falls under the Finance & Investing category of mental models. Other models in this category can be found on the Finance & Investing hub page.

Why is 3-6-3 Rule important?

3-6-3 Rule is important because it provides a structured way to think about problems that would otherwise be approached with intuition alone. Understanding this model helps you avoid common reasoning errors and make better decisions.

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