Portfolios and riskIntermediate

How to size a position

Size decides how much an idea affects the whole. A large oversized thesis can be a bad portfolio.

3 min read Practical guide

By the end

You will convert conviction and possible loss into a weight compatible with your limits.

Start with the damage

Estimates adverse loss, including the possibility of total loss. Decide what portfolio decline you would accept if it occurs. Approximate maximum size = allowed risk / percentage loss of the asset.

Example

You agree to lose 1% of your portfolio and the adverse scenario is −40%: maximum initial weight 1/40 = 2.5%. It is not a recommendation, but a limit consistent with that hypothesis.

Qualitative adjustments

Reduces due to debt, illiquidity, binarity, correlation and poor data. Increasing by conviction only makes sense if it comes from evidence and the added damage remains tolerable.

progressive entry

Splitting purchases reduces risk for the moment, but it does not fix a bad valuation. Defines what evidence unlocks each leg and a maximum total. Unlimited averaging turns a broken hypothesis into concentration.

Added risk

Add exposures to the same cause and consider effective. Try common scenarios. Review sizes after fundamental moves and changes, with rebalancing rules.

Adverse loss.

Allowed risk.

Correlation.

Liquidity.

Maximum and sections.

From data to a decision

How much damage would being wrong cause?

This guide cannot predict the next move on its own. It can build a conditional reading: what supports upside, what increases downside risk, and which evidence must appear before acting.

Favourable reading

Size reflects conviction, volatility, liquidity, and acceptable loss.

Adverse reading

The position is driven by excitement or an attempt to recover previous losses.

Required confirmation

Calculate portfolio loss under a downside case before buying.

Reasoned example

A 10% position that can fall 40% risks 4% of the portfolio; decide whether that damage is acceptable.

Applied workshop

Turn the explanation into a process

Follow these steps in order and keep the result, so you can repeat the analysis and identify what changed your decision.

1Define the maximum portfolio loss tolerable for a failed thesis.
2Estimate per-share loss at fundamental invalidation or a technical level, including gaps.
3Divide risk budget by unit loss and apply concentration and liquidity limits.
4Reduce size as uncertainty, correlation or valuation rise; never increase it merely because price fell.

Review questions

How much would you lose if it failed completely?
Does the size let you think clearly?
Can you exit without moving the market?

Worked case

From the adverse scenario to weight

Portfolio 50,000, maximum desired loss per idea 0.75%=375. Entry 30 and fundamental invalidation 22: risk 8 per share. Approximate size 46 shares, 1,380 or 2.76% of portfolio.

If the stock is illiquid or binary, reduce. Don't artificially move your stop closer just to buy more.

Decision rule

First limit damage and then calculate quantity; Don't start with how much you want to earn.

Put it into practice

Calculate three sizes using adverse losses of 25%, 50% and 100%, maintaining the same maximum portfolio risk.