Position Sizing
Position sizing is the rule for how much capital to put at risk in a trade or investment. It is the bridge between having an idea and surviving the consequences of being wrong.
The beginner version is simple: define the maximum acceptable loss before entry, then choose size from that loss limit. Do not choose size from excitement, conviction, or how much money you want to make.
Why It Matters
Sizing is not an afterthought. In repeated-risk games, bad sizing can ruin a trader even when the underlying idea has positive expected value. This is the practical trading implication of ergodicity: one person's path through time matters more than the average outcome across all possible worlds.
The question is not only "Is this a good trade?" but:
- Can I survive being wrong?
- Can I survive being right too early?
- Can I survive a streak of losses?
- Can I keep learning after this trade?
If the answer is no, the size is too large.
The Basic Formula
position size = allowed loss / distance to invalidation
Example:
- Account: 10,000
- Max risk: 1% = 100
- Entry: 50
- Invalidation/stop: 45
- Risk per share: 5
- Position size: 100 / 5 = 20 shares
The important part is not the exact percentage. The important part is that loss is chosen before ego gets involved.
Beginner Sizing Rules
- Risk tiny while learning.
- Size down after rule-breaking losses.
- Never size up to "make it back."
- Never use leverage to compensate for weak edge.
- Increase size only after repeated evidence of process quality.
- Keep most capital outside the learning account.
- Treat every large emotional reaction as evidence the position is too big.
dealing-with-loss makes the psychological point: after a major loss, anchoring to the old high-water mark creates the urge to recover quickly. That urge is exactly how traders turn one loss into a second, larger loss.
Sizing vs Conviction
Conviction is not enough. A trader can be highly confident and still wrong, early, illiquid, or unable to withstand volatility.
Better sizing considers:
| Factor | Sizing Implication |
|---|---|
| Edge quality | Better edge can justify more size, but only after evidence. |
| Volatility | Higher volatility requires smaller size. |
| Liquidity | Harder exits require smaller size. |
| Leverage | Leverage demands stricter limits. |
| Correlation | Similar positions should be treated as one larger bet. |
| Emotional pressure | If it distorts behavior, size is too high. |
Taleb's Tail-Risk Warning
fooled-by-randomness sharpens the sizing rule: size must be designed around bad alternative-histories, not the path that happened recently. High confidence after a winning streak is especially dangerous if the strategy has negative skew: many small wins can train a trader to increase size right before the rare loss arrives.
Practical addition: before increasing size, ask whether recent performance proves edge or merely proves survival in a favorable sample.
Connection To Edge
Sizing should follow trading-edge, not replace it. A weak idea does not become good because the size is small; a good idea can become fatal because the size is too large.
The order should be:
- Identify the edge.
- Define invalidation.
- Estimate downside.
- Choose size.
- Journal the decision.