Position sizing and the art of the stop loss

Two investors can buy exactly the same stock on exactly the same day and end up with completely different outcomes. Not because one of them was smarter about the company, but because one of them had thought carefully about how much to buy and where to get out, and the other had not.

Position sizing is the least glamorous part of investing and the part that does the most work. It decides how much a single bad outcome can cost you, how long you can stay in the game, and whether a good strategy survives a bad month. Stock selection determines whether you are right. Position sizing determines whether being wrong matters.

This guide covers a risk-based approach to sizing, how to place a stop that respects the character of the stock rather than a round number, and the arithmetic that explains why a disciplined investor can lose more often than they win and still finish comfortably ahead.

Why sizing matters more than picking

Imagine a portfolio of twenty holdings. Nineteen of them behave unremarkably and one of them falls by half after a profit downgrade. If that holding was two per cent of the portfolio, you have lost one per cent of your capital and the year is essentially unaffected. If it was twenty per cent of the portfolio because it was the idea you felt most strongly about, you have lost ten per cent and you now need a fourteen per cent gain on what remains simply to return to where you started.

Nothing about the analysis differed between those two investors. The company was the same, the downgrade was the same, the conviction was the same. The only difference was the size of the cheque, and it was the size of the cheque that determined whether this was an ordinary week or a genuinely damaging one.

This asymmetry is why professional money managers treat sizing as a formal process with rules, while most private investors treat it as a feeling. The feeling usually says buy more of what you like most, which concentrates risk in exactly the positions where you are least likely to admit you were wrong.

Stock selection decides whether you are right.
Position sizing decides whether being wrong matters.

Start with the risk, not the stock

The conventional order of operations is to find a stock, decide it looks good, and then work out how much to buy based on how much cash is sitting idle. This reverses the only question that can be answered with any confidence.

You cannot know what a stock will do. You can know exactly what you are prepared to lose on it. So that is where sizing should begin: with a fixed, deliberately unexciting number that represents the most a single position may cost you if the thesis is wrong.

A common convention among systematic investors is to risk between half a per cent and two per cent of portfolio capital on any one position. On a $50,000 portfolio, one per cent is $500. That is the amount you accept losing if the position moves against you and you exit at your predetermined level. It is not the amount you invest. The amount you invest is usually many times larger.

The distinction between capital committed and capital at risk is the single most useful idea in this guide, and it is the one most often missed. A $6,000 holding in which you have defined an exit two dollars below your entry is not a $6,000 risk. It is a $500 risk in a $6,000 wrapper.

Choosing your risk percentage

Lower is slower and more survivable. Higher is faster in both directions. A few practical considerations:

• Portfolio size. On smaller portfolios, brokerage becomes a meaningful drag if risk per position is tiny, because position sizes shrink and fixed costs do not.
• Number of positions. Twenty positions at two per cent risk each is a theoretical forty per cent of capital at risk. That is far too much for most people to hold through a correlated drawdown.
• Your own temperament. The correct figure is one you can hold to on the worst day, not the one that looks best on a spreadsheet.

The sizing formula

Once you have fixed the dollar risk, the number of shares follows mechanically. There is no judgement left in it, which is the point.

With $500 of accepted risk and $2.50 of risk per share, the position is 200 shares. At $32.00 that is a $6,400 holding, or 12.8 per cent of a $50,000 portfolio. A large-looking position carrying a small, known risk.

Now widen the stop to $26.00, perhaps because the stock is more volatile than you first thought and a closer exit would be triggered by ordinary noise. Risk per share becomes $6.00, so the position becomes 83 shares, or $2,656. Same dollar risk, less than half the capital committed.

This is the mechanism doing its job. The wider the sensible stop, the smaller the position. Volatility is handled by size rather than by hope.

A worked portfolio

The table below sizes three hypothetical positions with $500 of risk each, using large ASX names purely to make the price points familiar. These are illustrative figures, not recommendations, and the prices are round numbers chosen for clarity rather than current market quotes.

Notice that the position values land within a reasonably narrow band here, between $5,250 and $6,400. That is a coincidence of these particular stop distances, not a feature of the method. Change one stop and the corresponding position changes with it while the risk stays fixed.

Notice also that share price is irrelevant to the calculation. A $105 stock is not riskier than an $8 stock. What matters is the distance to your exit as a proportion of the price, and how many shares that distance allows you to hold.

The equal dollar trap

The most common alternative to risk-based sizing is to put roughly the same dollar amount into every holding. It feels balanced and it is simple to administer. It also quietly loads your largest risks into your most volatile positions.

Consider $6,000 committed to two very different stocks. The first is a large cap where a sensible stop sits nine per cent below the entry. The second is a speculative small cap that routinely swings twenty five per cent, so any stop closer than that would be triggered by noise.

Equal capital has produced triple the exposure on the position least able to support it. Run that across a portfolio and your realised losses will cluster in the speculative end of the book, which is precisely the outcome the equal weighting was supposed to prevent.

Where to put the stop

A stop is not a prediction. It is the price at which your reason for holding is no longer valid. The best stops are placed where the thesis breaks, not where the loss becomes uncomfortable.

Three common approaches

1. Percentage stops. A fixed distance such as ten per cent below entry. Simple and consistent, but blind to the character of the stock. Ten per cent is a long way for an infrastructure utility and a normal Tuesday for a junior miner.
2. Technical stops. Placed just beyond a level that matters on the chart, such as below a prior swing low or a moving average. These respect the structure of the price action, but cluster where everyone else has placed theirs.
3. Volatility stops. Set as a multiple of the stock’s own recent trading range, commonly using average true range. A stock that moves fifty cents a day gets a wider stop than one that moves five. This adapts automatically across very different securities.

Volatility-based placement is the most defensible starting point because it answers the right question: how far can this particular stock move without telling me anything? Whatever method you choose, decide the stop before you buy, while you are still capable of thinking clearly about the position.

Moving stops, and when not to

A stop should move in one direction only. As a position advances, raising the stop to protect accumulated gain is sound risk management, and a trailing stop formalises it. Lowering a stop to give a losing position more room is not risk management, it is the abandonment of it. The decision to widen a stop is almost always made for emotional reasons and almost always makes the eventual loss larger.

What a stop cannot do

A stop loss instruction becomes a market order once the trigger price trades. It does not reserve a price for you. If the stock opens well below your stop after an announcement, you are filled at whatever the market offers, not at the level you nominated.

Return to the earlier example: 200 shares bought at $32.00 with a stop at $29.50, risking $500. The company releases a downgrade overnight and the stock opens at $26.80. Your stop triggers immediately but fills near the open, so the realised loss is $5.20 a share, or $1,040. More than double what you accepted.

An investor who treats their stop as a guaranteed maximum loss is running more risk than they believe. An investor who treats it as a usually reliable discipline with occasional overruns has sized correctly.

Correlation and hidden concentration

Position-level risk control can still leave you with a badly concentrated portfolio if the positions move together. Four Australian bank holdings at one per cent risk each are not four independent one per cent bets. In a sector-wide repricing they behave close to a single four per cent bet on Australian banking.

The same applies to the iron ore complex, to lithium names, to anything that responds to one macro variable. Diversification is a function of correlation, not of the number of lines on your statement.

The maths of being wrong

Investors tend to judge themselves on how often they are right. The arithmetic cares about something else entirely. Expectancy combines how often you win with how much you win and lose, and it produces the only number that matters over a long series of decisions.

The calculation is straightforward:

Expectancy per position = (win rate × average win) − (loss rate × average loss)

The first approach is wrong six times out of ten and makes money. The second is right seven times out of ten and loses it. The difference is entirely in the relationship between the average win and the average loss, which is to say the difference is entirely in the exits.

The second row describes a very recognisable pattern. Small gains are taken quickly because they feel good and are easy to justify. Losses are held because selling would confirm the mistake, so they are allowed to grow. Every individual decision feels defensible. The aggregate is a negative expectancy that a high win rate conceals for a long time.

You do not need to be right often. You need your average win to be meaningfully larger than your average loss, and position sizing is what makes that possible.

Putting it into practice

None of this requires sophisticated tooling. It requires the calculation to happen before the purchase rather than after, and it requires the numbers to be written down where you cannot quietly revise them later.

1. Fix your risk per position as a percentage of total capital, and leave it alone. Changing it based on how confident you feel defeats its purpose.
2. Identify the stop before the entry. Ask where the reason for holding would be proven wrong, then place the exit just beyond it.
3. Calculate the share count from the risk and the stop distance. Accept the number the formula gives you, including when it is smaller than you would like.
4. Record entry, stop and target at the moment you buy. A written level is much harder to rationalise away than a remembered one.
5. Review total and sector risk weekly, not position by position but across the whole book.
6. Track average win against average loss, not just win rate. It is the ratio that reveals whether the discipline is holding.

The investors who last are rarely the ones with the best individual calls. They are the ones whose worst month was merely disappointing rather than structural, because the size of every position was decided by arithmetic before conviction had a chance to weigh in.

Every Investor Signals notification includes entry, target and stop levels for exactly this reason. The levels are there so the sizing decision can be made mechanically, before the position is opened and before emotion has anything to work with.