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📚 Stock Market Glossary

Clear, beginner-friendly explanations, real-world analogies, and visual formulas for key stock market terminology.

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Rule of 20 (Inflation-Adjusted Valuation Metric)

Valuation
💡 Key Takeaway: A classic Wall Street heuristic stating that the market is fairly valued when the sum of the S&P 500 P/E ratio and the YoY CPI inflation rate equals 20.
Seesaw Analogy: Inflation and P/E multiples sit on opposite sides of a seesaw. When inflation gains weight, the P/E multiple must drop to maintain equilibrium at 20.
😎 10-Second Show-off Pro Tip for Friends!
☕ Show-off Tip: 'Wondering if a 22x P/E multiple is overstretched? Add the current CPI inflation rate using the Rule of 20. If inflation is 3%, fair value P/E is 17x—meaning current multiples are pricing in aggressive future disinflation.'

📖 Beginner-Friendly Explanation

STEP 1

Core Concept & Meaning

The Rule of 20 is a revered equity valuation rule of thumb which asserts that the fair value P/E multiple for the overall stock market equals 20 minus the year-over-year Consumer Price Index (CPI) inflation rate.

When inflation runs hot, future corporate cash flows are discounted at higher interest rates, requiring lower P/E multiples. When disinflation takes hold, equity markets can comfortably support richer valuation multiples.

STEP 2

Why It Matters & Mechanism

  • Sum > 20 (Overvalued Zone): When P/E + CPI exceeds 20, equities are priced at a premium relative to macro inflationary friction, increasing drawdown risk.
  • Sum < 20 (Undervalued Zone): When the sum dips below 20, earnings yields offer attractive risk-adjusted entry points for long-term allocators.
STEP 3

Practical Investment Tips & Pitfalls

If annual inflation is 3.0%, the implied benchmark fair-value multiple for the S&P 500 is 17x (20 - 3). If the actual trailing P/E sits at 23x (Sum = 26), equities face valuation compression unless corporate earnings surge significantly.

📊 Rule of 20 Formulation
Rule of 20 Index = Trailing P/E + YoY CPI Rate (%) = 20 (Fair Benchmark)
▶ Sum > 20: Equity market is historically overvalued. ▶ Sum = 20: Market trades at theoretical macroeconomic fair value. ▶ Sum < 20: Equity market offers attractive undervalued entry multiples.

⚖️ Key Comparison at a Glance

CPI Inflation RateRule of 20 Fair P/EValuation Regime
1.0% (Ultra-Low Inflation)19.0x (20 - 1.0)Favorable liquidity supporting rich tech growth multiples
2.5% (Target Level)17.5x (20 - 2.5)Balanced baseline aligned with historical medians
4.0% (Sticky High Inflation)16.0x (20 - 4.0)Multiple compression favoring defensive value equities
6.0% (Severe Inflation Shock)14.0x (20 - 6.0)Aggressive de-rating and broad valuation contraction
⚔️ Don't Mix These Up! (Head-to-Head Comparison)
VSRule of 40
View Rule→
💡 Crucial Difference: The Rule of 40 evaluates individual SaaS tech profitability vs. growth, whereas the Rule of 20 assesses macro market-wide P/E against CPI inflation.
VSPEG Ratio
View PEG→
💡 Crucial Difference: PEG divides P/E by company earnings growth rate, while the Rule of 20 adds consumer inflation to aggregate market P/E.

📌 Practical Market & Real-World Example

When annualized CPI inflation printed at 3.2%, institutional allocators used the Rule of 20 to adjust the implied fair-value multiple cap down to 16.8x.