Understand PEG ratio — Stock-Market notes

What It Is

WHY this metric exists: Investors realized that comparing a P/E of40 for a 40%-growth company with a P/E of 15 for a 5%-growth company is aples-to-oranges. Peter Lynch popularized PEG to adjust for this distortion.

Derivation from First Principles

Start with the question: What's a "fair" P/E for a given growth rate?

Step 1: Assume earnings grow at rate $g$ (as a percentage) annually. After $n$ years: $\text{EPS}_n = \text{EPS}_0 \times (1 + g/100)^n$

Step 2: A rational investor pays more today if future earnings are higher. In a simplified world where P/E should scale linearly with growth ( first-order approximation), we set: $\text{Fair P/E} \propto g$

Step 3: Define a benchmark: If growth = 1%, what P/E should we tolerate? Let's say a P/E of 1per 1% growth is "fair" (this is Lynch's heuristic). Then: $\text{Fair P/E} = g \times 1 = g$

Step 4: The actual P/E is what the market gives us. Compare actual to fair: $\text{PEG} = \frac{\text{Actual P/E}}{\text{Fair P/E}} = \frac{\text{P/E}}{g}$

WHY this step? We're creating a ratio of ratios—how much you're paying (P/E) per unit of what you're getting (growth). PEG = 1 means the market's valuation matches the simple linear model.

Worked Examples

Example 1: Tech Growth Stock

Given:

Stock A: Price = $150, EPS =$ 3 Expected growth = 30% per year

Step 1: Calculate P/E $\text{P/E} = \frac{150}{3} = 50$

Why this step? We need the "price" component before adjusting for growth.

Step 2: Calculate PEG $\text{PEG} = \frac{50}{30} = 1.67$

Interpretation: PEG > 1 suggests the market has priced in high growth expectations. You're paying $1.67 of P/E for every 1% of growth—potentially overvalued unless growth accelerates beyond 30%.

Example 2: Mature Dividend Stock

Given:

Stock B: Price = $40, EPS =$ 4, Expected growth = 5% per year

Step 1: P/E $\text{P/E} = \frac{40}{4} = 10$

Step 2: PEG $\text{PEG} = \frac{10}{5} = 2.0$

Why this high PEG? Even though P/E is low, growth is anemic. You're paying $2 of P/E per 1% growth—actually more expensive than Stock A on a growth-adjusted basis!

Example 3: Value Turnaround

Given:

Stock C: Price = $25, EPS =$ 2.50, Expected growth = 25%

Step 1: P/E $\text{P/E} = \frac{25}{2.50} = 10$

Step 2: PEG $\text{PEG} = \frac{10}{25} = 0.4$

Interpretation: PEG well below 1—potentially undervalued. Market may be skeptical of the 25% growth forecast, or this is a hidden gem.

Example 4: Comparing Two Stocks

| Stock | Price | EPS | Growth | P/E | PEG | |-------|-------|-----|--------|-----| | Tech Co | $200 |$ 4 | 40% | 50 | 1.25 | | Pharma Co | $80 |$ 5 | 20% | 16 | 0.80 |

Why this matters? Despite Tech Co having a much higher P/E (50 vs 16), Pharma Co is relatively more expensive when adjusted for growth (PEG 0.80 < 1.25 means Pharma is cheaper per unit of growth).

Common Mistakes

When PEG Breaks Down

Zero or negative growth: PEG = ∞ or undefined.
Lumpy earnings: One-time charges distort EPS; use normalized earnings.
Non-linear growth: PEG assumes 1:1 P/E-to-growth. In reality, 100% growth doesn't justify P/E of 100(unsustainable).
Different industries: Software (high margin, scalable) justifies higher PEG than retail (low margin).

Recall Explain to a 12-Year-Old

Imagine you collect trading cards. Card A costs $50 and its value goes up$ 10 every year (20% growth). Card B costs $20 but only goes up$ 1 per year (5% growth). Which is the better deal?

If you just look at price, Card B seems cheaper. But Card A is growing way faster. PEG ratio is like dividing the price by how fast the value grows. For Card A: 50 ÷ 20 = 2.5. For Card B: 20 ÷ 5 = 4. Card A has a lower PEG, so even though it costs more, you're getting more growth for your money!

In stocks, "price" is the P/E ratio, and "how fast it grows" is the earnings growth rate. PEG helps you compare a super-expensivebut-fast-growing tech company with a cheap-but-slow utility company fairly.

Active Recall Checkpoints

What does PEG < 1 suggest about a stock's valuation?
Why can't you use PEG for a company with negative earnings?
If a stock has P/E = 30 and growth = 15%, what's the PEG? Is it attractive?
What's the difference between forward and trailing PEG?

Connections

Price-to-Earnings Ratio (P/E) — The numerator of PEG; understand P/E first
Earnings Per Share (EPS) — The foundation of P/E calculation
Growth Investing — PEG is the central metric for growth investors
Value vs Growth Stocks — PEG helps bridge the two philosophies
Discounted Cash Flow (DCF) — More rigorous model; PEG is a heuristic shortcut
Peter Lynch's Investment Strategy — Lynch popularized PEG in "One Up on Wall Street"
Relative Valuation Methods — PEG is one tool in the relative valuation toolkit

#flashcards/stock-market

What does PEG stand for? :: Price-to-Earnings-to-Growth ratio

What is the formula for PEG ratio?

PEG = (P/E Ratio) / (Earnings Growth Rate in%)

What does a PEG ratio < 1 suggest?

The stock may be undervalued relative to its growth rate; you're paying less than $1 of P/E per 1% of growth

What does a PEG ratio > 1 indicate?

The stock may be overvalued; the market has priced in high growth expectations, or growth may not justify the valuation

Why is PEG better than P/E alone for comparing stocks?

P/E doesn't account for growth rate—PEG normalizes P/E by growth, allowing fair comparison between fast-growing and slow-growing companies

If a stock has P/E of 40 and expected growth of 20%, what is the PEG?

PEG = 40 / 20 = 2.0

What's the difference between forward and trailing PEG?

Forward PEG uses next year's estimated P/E and growth; trailing PEG uses last year's actual data

Why is PEG unreliable for companies with negative earnings?

Negative earnings make P/E undefined or negative, rendering PEG meaningless

When does PEG break down as a valuation tool?

(1) Zero/negative growth, (2) Lumpy/one-time earnings, (3) Unsustainable growth rates (>50%), (4) Cross-industry comparisons with different margin profiles

A stock has Price =

60, EPS =

3, growth = 25%. Calculate PEG.

P/E = 60/3 = 20; PEG = 20/25 = 0.8 (potentially undervalued)

Who popularized the PEG ratio?

Peter Lynch, legendary Fidelity manager, in his book "One Up on Wall Street"

Can you use revenue growth instead of EPS growth for PEG?

No—PEG specifically requires earnings (EPS) growth, not revenue growth, because revenue can grow while profits shrink

Concept Map

Hinglish (regional understanding)

Intuition Hinglish mein samjho

PEG ratio ek bahut powerful tool hai jo apko yeh samajhne mein madad karta hai ki aap company ki growth ke liye kitna paisa pay kar rahe ho. Dekho, agar ek company ka P/E ratio 50 hai toh yeh sune mein bahut zyada lagta hai, lekin agar woh company har saal 50% growth kar rahi hai toh yeh price justified ho sakta hai. Dusri taraf, ek company ka P/E sirf 15 hai but growth sirf 3% hai toh actually woh comparatively zyada expensive hai!

PEG ratio ka formula simple hai: P/E ko growth rate se divide karo. Agar answer 1 se kam aye toh stock undervalued ho sakta hai (matlab acha deal), agar exactly 1 hai toh fair value hai, aur agar 1 se zyada hai toh market ne already future growth ko price-in kar diya hai. Peter Lynch ne yeh concept popular kiya tha apni investing strategy mein.

Lekin dhyan rakho, PEG ratio ko blindly follow mat karo. Agar company ke earnings negative hain ya growth sustainable nahi hai toh PEG misleading ho sakta hai. Cyclical industries mein bhi PEG thek se kaam nahi karta kyunki unki growth har saal bahut change hoti hai. Hamesha cross-check karo ki company fundamentally strong hai ya nahi—debt, profit margins, competition sab dekho. PEG sirf ek tool hai, complete picture nahi.

Test yourself — Financial Ratios

Connections