Position sizing: the skill that matters more than stock picking

Almost all the attention in investing goes to what to buy. Almost all the damage comes from how much.

You can be right about most of your holdings and still do badly, if the one you were wrong about was three times the size of the others. And you can be wrong more often than you are right and do perfectly well, if your losses are small and your winners are allowed to run.

Start from what you do not know

The right size for a position is a function of your uncertainty, not your enthusiasm. In practice the two feel identical: conviction is usually strongest where knowledge is shallowest, because complications you have not yet discovered cannot temper it.

A useful discipline: before sizing, write down the three things that would have to go wrong for this holding to lose half its value. If that list is hard to produce, you do not understand the business well enough to take a large position in it.

A practical framework

For a portfolio you are building yourself, without leverage:

• A standard position: roughly equal weight. Ten holdings, about 10% each.
• A maximum: no single holding above about 15% of the portfolio, however good it looks.
• A sector cap: no more than about 30% in any one sector, since sector risk is real risk however many companies you spread it across.
• A minimum: large enough that transaction costs do not consume a meaningful share, and large enough to matter if you are right.

These are not laws. They are defaults that prevent the specific failure that ruins beginners: one enormous position that determines the entire outcome.

The arithmetic of losses

Losses and gains are not symmetric, and the asymmetry gets worse the deeper the hole:

• Lose 10%, and you need about 11% to get back to even.
• Lose 25%, and you need about 33%.
• Lose 50%, and you need 100%. You must double your money simply to return to where you started.
• Lose 75%, and you need 300%.

This is why avoiding large losses matters more than capturing large gains. A portfolio that never suffers a catastrophic single-position loss does not need spectacular winners to do well.

Liquidity is part of size

A consideration that matters more on PSX than in deeper markets. Before sizing a position, look at the average daily traded volume and ask how many days it would take to sell your intended holding without moving the price against yourself.

If the answer is more than a few days, the position is too large, regardless of how attractive the company is. A holding you cannot exit is not really a position; it is a commitment.

Letting winners grow, within limits

If a holding does well, it becomes a larger share of the portfolio by simple arithmetic. Up to a point that is fine, and it is what you wanted to happen.

Past your maximum, trim it back. This feels wrong every single time, because you are selling the thing that is working. But a position that has grown to 30% of your portfolio now determines your outcome by itself, and the reasons you diversified in the first place have not stopped applying.

What to write down before buying

1. The target weight, and why this size rather than a larger one.
2. The three things that would have to go wrong for it to halve.
3. The level or event at which you would accept the thesis is broken.
4. The maximum weight past which you will trim.

Four lines, written before you have any money at stake, which is the only time you can think about the position clearly.