Position sizing and concentration

In simple terms

Position sizing is deciding how much of a portfolio to put in each holding. It matters as much as choosing what to own: a great idea in a tiny position barely helps, and a bad idea in a huge position can do lasting damage.

This lesson is about long-term portfolios. The risk/reward and position size lesson in the technical analysis path covers sizing for individual trades with stop-losses.

Two approaches

Equal weighting: every holding gets the same share. With 20 holdings, each is 5%. It is simple, avoids overconfidence in any one idea, and limits the damage from any single failure.

Conviction weighting: more money goes to the ideas you believe in most. It can raise returns if your judgement is good, but it concentrates the risk of being wrong exactly where your confidence is highest.

Many investors use a blend: a core of roughly equal positions, with some variation, within firm limits.

Maximum position limits

A maximum position limit caps any single holding, for example at a set share of the portfolio at the time of buying, and a higher ceiling at which it will be trimmed. The right numbers depend on the investor, but the principle is simple: no single mistake should be able to ruin the plan.

How winners become concentration

Positions do not stay the size you bought them at. A holding that triples while the rest of the portfolio is flat becomes a much larger share.

Trimming large winners back towards a limit is a form of rebalancing. It feels like selling success, but it controls the risk that one holding's reversal wipes out years of gains. Taxes and costs on selling are a trade-off to weigh, as that lesson explains.

The "best idea" trap

It is tempting to put the most money in your best idea. But your conviction is not evidence that you are right. Even professional investors are wrong about many of their highest-conviction ideas. A sensible approach is to size positions so that your best idea being wrong still leaves the portfolio intact.

Funds and position sizing

Index funds and diversified mutual funds handle position sizing for you, holding many companies in proportions set by the index or the fund's rules. For many investors, a core of diversified funds, with any individual shares kept to modest positions, is a simple way to avoid concentration.

Why it matters

Many of the worst investor outcomes come not from choosing a bad investment but from having too much money in it. Position sizing and limits are simple rules that cap the damage from mistakes, and from successes that grow too large.

Limitations

There is no single right position size; it depends on the investor's goals, knowledge and capacity for risk. Strict equal weighting ignores differences in risk between holdings. Trimming winners has tax and cost consequences and can mean selling an investment that keeps rising.

Common mistakes

• Putting the most money in the idea you feel surest about.
• Letting winners grow unchecked into a large share of the portfolio.
• Holding many tiny positions that barely matter, alongside one huge one.
• Ignoring concentration through funds that hold the same shares.

Try it yourself

1. List the holdings in a portfolio you know, with their current values.
2. Calculate each holding's share of the total.
3. For the largest, calculate what a 50% fall would do to the whole portfolio.
4. Write down a maximum position limit you would use, and which holdings exceed it.