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Position Sizing

Why It Matters

Even with correct directional judgment, poor position sizing leads to poor outcomes. Betting too much on any single option creates fragility (one bad outcome causes catastrophic loss). Betting too little on high-conviction ideas wastes opportunities. This tool provides a systematic framework for translating belief strength into allocation size.

The Key Move

For each potential allocation, ask:

  1. What's my edge or conviction level?
  2. What's the downside risk if I'm wrong?
  3. What's my maximum acceptable loss on this position? Set position size so that even worst-case scenarios don't exceed your risk limits. Use formulas like Kelly Criterion for sizing: allocation = edge / odds. Never size so large that a single failure is catastrophic.

Classic Application

Trading position limits. If you believe a stock will rise but your conviction isn't perfect, the Kelly Criterion suggests betting a fraction of your capital proportional to your edge. If you have a 55% chance of winning and 1:1 payoff, Kelly says bet 10% of capital (55% - 45% = 10%). This maximizes long-term growth while preventing ruin.

Surprising Application

Time allocation for learning new skills. If you're transitioning careers, how much time should you invest in the new field versus maintaining current income? Position sizing logic: the new skill is high-conviction but uncertain, so allocate enough time for serious progress (not a token 5%) but not so much that failure to gain traction quickly leaves you financially vulnerable. Perhaps 30-40% of working hours—enough to make real progress, small enough to maintain stability.

Failure Modes

Kelly Criterion assumes perfect knowledge of edge and odds—overestimating edge leads to over-betting. Doesn't account for correlation—if all positions are similar, individual position limits don't prevent concentration risk. Ignores liquidity—some positions can't be sized precisely or exited quickly. Assumes unlimited divisibility—can't invest 13.7% of yourself in a job. Requires risk tolerance as input—different people should size the same opportunity differently.

Go Deeper

Thorp, "The Kelly Criterion in Blackjack, Sports Betting, and the Stock Market"; MacLean et al., The Kelly Capital Growth Investment Criterion