How to build a small-cap portfolio that can actually survive

Building a portfolio of small-cap stocks is a genuinely different task from building one of large-cap holdings. The rules that apply to blue-chip investing, the comforting notion that a handful of well-chosen names will do the job, simply do not transfer. Small-cap portfolio construction requires its own logic, and getting that logic wrong is one of the most common reasons private investors leave this market with less than they arrived with.

Start with the question of how many holdings to carry. Among experienced small-cap investors, a portfolio of between 15 and 25 names is generally considered the working range. Go below that and you are taking on company-specific risk that no amount of analysis can fully offset. A single profit warning, a key executive departure, or a failed clinical trial can do significant damage to a concentrated position, and at the small-cap end of the market those events are far more common than investors tend to anticipate. Go above 25 and you run into a different problem: you almost certainly cannot follow that many companies closely enough to make informed decisions when news breaks, and the portfolio begins to resemble a passive fund at active-fund prices. The 15-to-25 range is not arbitrary; it reflects the practical limits of what a diligent private investor can actually monitor.

Diversification in small-cap portfolios needs to be thought about along several axes simultaneously, not just sector. Sector diversification matters, of course. Holding five mining juniors, two biotechs at the same clinical stage, and three early-stage fintech companies is not a diversified portfolio even if the individual names are different. Each of those clusters will tend to move together when sentiment in that corner of the market shifts, and the result is a portfolio that feels diversified but behaves as though it is not.

Geography is a second axis that investors often underweight. The AIM market is genuinely global in reach: companies with operations in West Africa, Central Asia, South America, and Southeast Asia are all accessible from a UK brokerage account. That reach creates opportunity but also concentrates risk in ways that are easy to overlook. Jurisdictional risk, the possibility that a government changes the terms under which a company operates, renegotiates a mining licence, or imposes capital controls, is not a theoretical concern in many of the regions where small-cap explorers and developers operate. Spreading holdings across more politically stable geographies alongside higher-risk frontier positions is a structural decision worth making deliberately rather than arriving at accidentally.

Stage diversification is the third axis, and it is the one most often ignored. A portfolio that holds exclusively early-stage exploration companies will swing violently, because those companies derive almost all of their value from future possibilities. A portfolio that holds exclusively profitable, cash-generating small-caps will be more stable but will miss the asymmetric upside that the asset class can deliver. The most resilient small-cap portfolios tend to hold a mix: some companies at the speculative end where the potential is large and the risk is commensurate, and some at a later stage with real revenues, genuine margins, and a degree of predictability that provides ballast when the more speculative positions underperform.

Position sizing is where most investors get into trouble, not because they fail to understand the concept but because they let conviction override discipline. The rule is straightforward: position size should be calibrated to liquidity, not to how strongly you believe in a particular investment case. A small-cap stock with a daily trading volume of 50,000 shares and a wide bid-ask spread cannot be sized the same way as a more liquid mid-cap holding. If you need to exit in a hurry and the market is moving against you, illiquidity will cost you far more than you modelled. The practical implication is that the more thinly traded a holding is, the smaller the initial position should be, regardless of how compelling the story looks on paper.

Adding to positions is a discipline in itself. The most common error is averaging down without a thesis update: buying more of something simply because it has fallen, rather than because new information confirms that the original investment case remains intact and the lower price represents genuine value. Before adding to any small-cap position, the question to ask is whether the reason the share price has fallen is company-specific and temporary, or whether it reflects something more fundamental that the original thesis did not account for. If the answer is unclear, patience is usually the correct response.

Knowing when to sell is, if anything, harder than knowing when to buy. The two clearest sell signals in small-cap investing are a broken thesis and a valuation that has run well ahead of fundamentals. A thesis is broken when the core assumptions on which you based your investment have been proved wrong: the resource estimate was revised sharply downwards, the partnership did not materialise, the management team that attracted you has departed, or the competitive dynamic shifted in a way the company has not addressed. Valuation running ahead is a more comfortable problem to have, but it is still a problem. Taking some profit into strength, particularly in highly illiquid names, is simply good portfolio hygiene, not a sign of insufficient conviction.

Monitoring a small-cap portfolio is a habit that requires conscious boundary-setting. The RNS feed, the bulletin boards, the broker notes, the daily price movements: all of it creates noise that can push investors towards unnecessary activity. A company that you have researched carefully and hold for the right reasons does not need to be checked multiple times a day. What it does need is a structured review when material news is released: regulatory announcements, trading updates, results, and board changes all warrant attention. Daily price movements in the absence of news warrant almost none. The investors who have done best in this asset class over time are overwhelmingly those who committed to a process and resisted the urge to let short-term volatility drive their decisions.

Small-cap portfolio construction is ultimately a long-term exercise. The companies that deliver the most compelling returns rarely do so quickly, and the path from overlooked to outstanding is rarely smooth. Building a portfolio that can survive the rough periods, through sensible diversification, disciplined position sizing, and a clear framework for adding, holding, and selling, is what gives you the staying power to be there when the returns eventually arrive.

This post is drawn from The Little Book of Small-Caps by Cameron Oliver. Republished with permission.

Small Caps is a series drawn from first-hand experience of UK and global small-cap markets, updated as each new chapter arrives.

Disclaimer: The value of investments can go down as well as up, and you may get back less than you invest. This article is for informational and educational purposes only and does not constitute financial advice. Always do your own research and consider seeking independent advice before making any investment decision.