Concept: Position Sizing
Position sizing determines how much of your portfolio to allocate to each investment. It's arguably more important than WHAT you buy — because even the best...
What it is
Position sizing determines how much of your portfolio to allocate to each investment. It's arguably more important than WHAT you buy — because even the best stock pick destroys wealth if sized wrong (too much in one that fails, too little in one that succeeds).
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"Position sizing matlab — ek stock mein kitna paisa lagana hai. Best idea mein zyada, average idea mein kam. Lekin kisi mein bhi itna nahi ki raat ko neend na aaye."
Why it matters more than stock picking
| Scenario | Stock A (50% of portfolio) | Stock B (5% of portfolio) |
|---|---|---|
| Stock drops 70% | Portfolio impact: -35% (devastating) | Portfolio impact: -3.5% (manageable) |
| Stock gains 200% | Portfolio impact: +100% (great) | Portfolio impact: +10% (meh) |
Key insight: The downside of over-sizing is worse than the upside of missing a big position.
"Even a 10-bagger only matters if you own enough of it. And even a 100% loss only matters if you own too much."
Position sizing frameworks
Framework 1: Conviction-based allocation
| Conviction level | Position size | Criteria |
|---|---|---|
| Highest (5 stars) | 8-12% | Deep understanding, clear moat, reasonable valuation, long thesis |
| High (4 stars) | 5-8% | Good business, fair price, solid research |
| Medium (3 stars) | 3-5% | Decent opportunity, some unknowns |
| Low / Tracking (2 stars) | 1-3% | Learning the business, testing thesis |
Framework 2: Kelly Criterion (mathematical)
$$f^* = \frac{bp - q}{b}$$
Where:
- $f^*$ = fraction of portfolio to bet
- $b$ = odds (expected gain if right / expected loss if wrong)
- $p$ = probability of being right
- $q$ = probability of being wrong (= 1 - p)
Example: You think there's 60% chance a stock rises 50% and 40% chance it falls 30%.
- Kelly: (0.5 × 0.6 - 0.4) / 0.5 = 0.2 / 0.5 = 40%
- Half-Kelly (practical): 20%
Always use Half-Kelly or less
Full Kelly assumes perfect probability estimates. Since we're always uncertain, use 1/2 or 1/3 Kelly to account for overconfidence.
Framework 3: Risk-adjusted sizing
| Factor | Increases size | Decreases size |
|---|---|---|
| Business quality | Strong moat, high ROCE | No moat, cyclical |
| Valuation | Significant margin of safety | Fair or expensive |
| Understanding | Deep knowledge (in circle) | Edge of competence |
| Liquidity | Large-cap, liquid | Small-cap, illiquid |
| Concentration already | Low sector exposure | Already 3 stocks in same sector |
Practical rules for Indian retail investors
Starting portfolio (₹10-50 lakh)
- 12-18 stocks total
- Top 3 positions: 8-10% each (highest conviction)
- Middle 5-7 positions: 5-7% each (strong ideas)
- Bottom 5-8 positions: 2-4% each (emerging ideas, tracking)
- No single stock > 12%. No single sector > 25%
Scaling into positions
Don't buy your full position at once:
Trimming rules
- Stock appreciates to become > 15% of portfolio → trim back to 10-12%
- Thesis partially weakened → trim by 30-50%
- One sector has become > 30% → trim the weakest name in that sector
Common mistakes
Mistakes to avoid
- Equal weighting — Giving 5% to your best idea AND 5% to your weakest idea makes no sense. Let conviction guide allocation
- Averaging down without limit — Buying more of a falling stock can turn a 5% position into 20%. Set a max
- Tip-based large positions — "I heard XYZ is going 5×" → putting 25% into it = gambling
- Ignoring portfolio-level risk — 5 stocks at 8% each from the same sector = 40% sector bet
- Not having dry powder — Keeping 5-15% cash gives flexibility to add on corrections
The anti-sizing disaster protection rule
Before buying, ask: "If this stock goes to zero tomorrow, can I recover?"
If the answer is "barely" — your position is too large. Cut it to where a total loss would hurt but not destroy your financial life.
Related concepts
- Diversification — Position sizing is diversification's mathematical implementation
- Margin of Safety — Larger margin of safety → justifies larger position
- Circle of Competence — Size up only within your circle