Chapter 14 taught the 2% rule — the retail floor. This chapter is the formal math. Kelly criterion, volatility-based sizing, and how professional traders size positions based on edge and risk. Not required, but the difference between good and great.
The mathematically optimal position size for a repeating bet with known edge and known payoff.
Kelly fraction = (bp − q) / b
Where: b = payoff ratio (win amount / loss amount) p = probability of winning q = probability of losing = 1 − p
Your iron condor:
Kelly fraction = (0.818 × 0.85 − 0.15) / 0.818 = (0.695 − 0.15) / 0.818 = 0.545 / 0.818 = 0.666
Kelly says: risk ~67% of your account per trade.
This is way too aggressive for real trading.
Kelly assumes:
Overestimate your win rate by 10%, and Kelly recommends 2-3× the correct amount. Result: blow-up.
Most professionals use quarter Kelly or half Kelly:
For the iron condor above:
Compare to the 2% rule: quarter Kelly is 8× larger. That’s the trade-off — higher returns but bigger swings.
Kelly-based sizing only works if your edge estimates are accurate. For 95%+ of retail traders, edge estimates are optimistic. Backtest results include survivorship bias, hindsight bias, and small samples.
Realistic recommendation:
Instead of fixed % per trade, scale size inversely to volatility:
Position size = (Target volatility × Account value) / (Position’s standard deviation)
Or, expressed as: Position size = Target risk in ₹ / Expected daily standard deviation of position
Account: ₹5,00,000. Target: 1% daily portfolio vol.
Iron condor’s expected daily P&L standard deviation (from backtest): ₹1,200 per lot.
Target position: (0.01 × 5,00,000) / 1,200 = 5,000 / 1,200 = ~4 lots
Compare to fixed 2% rule (2 lots max). Vol-scaling doubles size when the strategy is quiet.
Now say Nifty vol spikes and iron condor daily P&L std doubles to ₹2,400/lot: Target = 5,000 / 2,400 = ~2 lots
Automatic reduction when environment gets riskier.
Rule: as market volatility (VIX) rises, reduce position count. As it falls, increase.
Simple version:
This ensures your account never faces catastrophic vol at maximum leverage.
If you run 3 strategies simultaneously (iron condors, calendar spreads, cash-secured puts):
Rule: treat total portfolio exposure as one budget.
Correlations matter — all three strategies may go bad simultaneously if there’s a big Nifty move.
Beyond simple sizing, monitor:
Rebalance when any exposure exceeds your set limit.
Some professional traders deliberately size BELOW Kelly to maximize compounding rate:
“Safety-first Kelly” — never bet so much that a bad streak forces you out. Even if it means lower short-term returns.
Warren Buffett’s essential insight: “Never lose money” isn’t literal — it means “never lose so much that you can’t recover.”
Use in this order as sophistication grows:
Skip levels only when you have data to justify it. Most retail should stay at levels 2-3 forever.
No matter how sophisticated your sizing math, keep an absolute cap: never risk more than 20% of your account across ALL open positions at any time. This protects against correlated blow-ups.
Start with:
After 12 months of tracked results: