It is entirely possible to hold five different funds and believe you are well diversified, only to discover that three of them own the same handful of large companies in roughly the same proportions. This is overlap risk — the hidden duplication that occurs when funds with different names, different managers, or different marketing labels end up holding much the same underlying exposure. For a UK investor building a portfolio from several funds, understanding and checking for overlap is one of the more overlooked steps in genuine diversification.
What overlap risk actually is
Overlap risk arises when two or more funds in a portfolio hold significant positions in the same underlying companies, sectors, or countries, so that adding the second fund does little to spread risk further and may simply concentrate it. This can happen even when the funds look quite different on the surface.
How it happens without anyone noticing
- Market-cap weighting concentrates naturally. Because the largest companies dominate most mainstream indices, almost any fund tracking a broad developed-market or global index will hold significant positions in the same small group of mega-cap companies.
- Thematic and sector funds often converge. A "technology" fund, an "innovation" fund, and a "global growth" fund may each independently arrive at holding many of the same large technology-related companies, simply because that is where growth-style managers tend to look.
- Active funds within one manager's range can share ideas. Different funds run by the same asset manager sometimes draw on shared research, leading to correlated top holdings even across ostensibly different mandates.
- Regional funds nest inside global funds. A global tracker already contains UK, US, European, and Asian companies in proportion to their market size — adding a separate regional fund on top increases the weighting to that region rather than adding something new.
Why overlap matters
The point of holding multiple funds is usually to diversify — to reduce the impact of any single company, sector, or region underperforming. When overlap is high, a portfolio that looks diversified on paper, by virtue of having several holdings, can behave almost identically to a much more concentrated one in the way it responds to market movements.
- Risk that was meant to be spread across five funds may in practice be concentrated in a small number of shared "star" holdings.
- An investor may unknowingly be paying multiple sets of fund charges for what is substantially the same underlying exposure.
- A downturn concentrated in one sector — such as the technology sector — could hit far more of the portfolio than the fund names alone would suggest.
How to check for overlap in practice
Reading fund factsheets
Every UK-authorised fund publishes a factsheet, usually updated monthly, listing its top ten (or sometimes top twenty) holdings by weight, along with sector and geographic breakdowns. Comparing the top-ten lists of each fund in a portfolio side by side is the simplest starting point — if the same five or six company names keep reappearing near the top of every fund, that is a strong signal of overlap.
Comparing sector and geographic breakdowns
Beyond individual holdings, factsheets typically break down a fund's exposure by sector (technology, financials, healthcare, and so on) and by country or region. Two funds with very different top-ten lists can still have similar sector concentrations, which is another form of overlap worth checking.
Using portfolio analysis tools
A number of platforms and independent tools allow an investor to input their full portfolio and see a consolidated "look-through" view — combining all underlying holdings across every fund into one aggregated list, weighted by how much of the portfolio each fund represents. This is generally the most reliable way to see true overlap, since it accounts for actual portfolio weightings rather than just comparing fund-level lists in isolation.
A simple manual check
- List the top ten holdings of each fund in the portfolio.
- Note any company name that appears in more than one list.
- Estimate the combined weighting of that company across the whole portfolio by multiplying its weight in each fund by that fund's share of the total portfolio, then summing.
- Ask whether that combined weighting is larger than would be comfortable in a single standalone holding.
A worked example
Suppose a hypothetical investor, Daniel, holds a portfolio split across three funds: 50% in a global tracker, 30% in a "global technology" fund, and 20% in a "US growth" fund. He notices that one large technology company appears in the top five holdings of all three funds — at roughly 4% of the global tracker, 12% of the technology fund, and 9% of the US growth fund.
His combined exposure to that single company works out to approximately (0.50 × 4%) + (0.30 × 12%) + (0.20 × 9%) = 2% + 3.6% + 1.8% = 7.4% of his entire portfolio in one company — a concentration he had not intended and would not have accepted had he been asked directly to put 7.4% of his savings into a single stock. This illustrates how overlap can quietly build a concentrated position through several "diversified" funds rather than through any single deliberate decision.
Reducing overlap sensibly
- Favour genuinely differentiated exposures. If a satellite fund is meant to add something new, check that its holdings, sector weighting, or geography meaningfully differ from the core before adding it.
- Be cautious with multiple thematic funds. Several thematic or sector funds bought over time — technology, innovation, disruptive growth — often converge on the same underlying names even when their names suggest different strategies.
- Reduce the fund count where duplication is high. Sometimes the simplest fix is consolidating two overlapping funds into one, reducing complexity and cost without materially changing overall exposure.
- Review periodically, not just at purchase. A fund's top holdings can change meaningfully over a year or two as its managers adjust positioning, so a portfolio that had low overlap when built can develop overlap later without any action from the investor.
Overlap versus deliberate concentration
It is worth distinguishing overlap risk — unintended duplication an investor did not realise was happening — from deliberate concentration, where an investor knowingly decides to increase exposure to a particular company, sector, or region because they have a specific view. The difference is not the exposure itself but whether it was chosen consciously. A portfolio can quite reasonably hold a higher-than-index weighting to a sector if that is a deliberate, sized, and monitored decision; the problem is when the same outcome arises by accident, spread invisibly across several fund names that all sounded different on the way in.
| Signal | What it suggests |
|---|---|
| Same company appears in top five of multiple funds | Potential direct holding overlap |
| Similar sector breakdown despite different fund names | Indirect thematic or style overlap |
| Combined single-company exposure exceeds comfort level | Unintended concentration risk |
| Regional fund added on top of a global tracker | Increased regional weighting, not new diversification |
Overlap across accounts, not just within one
Overlap checks are often done fund-by-fund within a single account, but many UK investors hold funds across an ISA, a workplace or personal pension (SIPP), and sometimes a General Investment Account (GIA). If the global tracker sitting in an ISA is very similar to the default growth fund automatically selected in a workplace pension, the two accounts combined may carry far more overlap than either looks like in isolation. Because the ISA annual allowance is £20,000 across all adult ISA types and the pension annual allowance is £60,000 (or 100% of earnings if lower) in the 2025/26 tax year, many investors end up contributing to more than one wrapper over time, and it is worth periodically reviewing holdings across all of them together rather than account by account.
Workplace pension defaults
Workplace pension default funds are often broad, multi-asset, or global equity funds chosen for a wide range of employees. An investor who separately builds a personal ISA portfolio around a similar global tracker may not realise how much their total retirement savings are concentrated in effectively the same underlying exposure, simply spread across two account names.
Consolidation as a partial solution
Where old workplace pensions from previous employers sit alongside current savings, consolidating them (where suitable and after checking for any valuable guarantees or exit penalties that could be lost) can make an overlap review considerably simpler, since it reduces the number of separate fund line-ups being tracked.
The limits of overlap tools
While portfolio look-through tools are useful, they are not perfect. Fund holdings disclosed in factsheets are typically a snapshot from the previous month-end rather than real time, meaning a fund's true current overlap could differ slightly from what is shown. Actively managed funds can also change their holdings between disclosure dates more than passive trackers do, so an overlap check on an active fund is more of a periodic health check than a permanent answer. Treating an overlap review as something done once a year, alongside a broader portfolio review, is usually more realistic than expecting to monitor it continuously.
Key takeaways
- Overlap risk occurs when funds that look different actually hold many of the same underlying companies, sectors, or regions.
- Market-cap weighting, thematic fund proliferation, and adding regional funds on top of global funds are common causes of hidden overlap.
- Comparing top-ten holdings and sector breakdowns across factsheets, or using a portfolio look-through tool, are practical ways to check.
- Overlap can quietly build a concentrated position in a single company or sector without any single deliberate decision by the investor.
- The goal is not zero overlap but conscious exposure — knowing what concentration exists and confirming it reflects an intended choice.
- Always check current fund factsheets and platform tools, since holdings and allowances referenced can change over time.