Two index funds can both claim to track "the UK market" or "the US market" and still hold their underlying companies in very different proportions. The weighting methodology behind an index — the rule determining how much of each constituent company the fund holds — shapes the fund's risk, its concentration, and ultimately its return pattern. Understanding the main approaches helps investors see past the label and understand what they actually own.
What "weighting" means in an index
An index is not simply a list of companies — it is a list of companies combined with a rule for how much of each one contributes to the overall index value, and therefore how much of each one a tracker fund holds. The most common approaches are market-capitalisation weighting, equal weighting, and a range of alternative or "smart beta" weighting schemes.
Market-capitalisation weighting
The dominant approach used by most well-known indices — including the FTSE 100, FTSE All-Share, S&P 500, and MSCI World — is market-capitalisation weighting, where each company's weight in the index reflects its total market value (share price multiplied by number of shares in issue, often adjusted for shares freely available to trade, known as the "free float"). Larger companies therefore make up a proportionally larger share of the index and the fund.
Advantages
- Reflects the actual size and economic weight of companies in the market.
- Naturally low turnover, since weights adjust automatically as prices move, without needing frequent rebalancing trades.
- Tends to have lower trading costs as a result, supporting low ongoing charges.
Drawbacks
- Can become concentrated in a small number of very large companies, particularly in indices where a handful of technology companies have grown to dominate total market value.
- By construction, more money flows towards companies that have already grown larger and more expensive, and proportionally less towards smaller, potentially undervalued companies.
Equal weighting
An equal-weighted index gives every constituent company the same weight, regardless of its size. A 500-company equal-weighted index, for example, would allocate roughly 0.2% to each company, whether it is among the largest or smallest in the index.
Advantages
- Reduces concentration risk in a small number of dominant companies.
- Provides more meaningful exposure to smaller companies within the index, which some investors believe may offer greater long-term growth potential, though this is not guaranteed.
Drawbacks
- Requires regular rebalancing (to bring weights back to equal as prices drift), generating more trading activity and typically higher ongoing charges than market-cap weighted equivalents.
- Can result in greater exposure to smaller, potentially more volatile companies than many investors expect from a fund tracking a well-known market.
Comparing the two main approaches
| Feature | Market-cap weighted | Equal weighted |
|---|---|---|
| Weight determined by | Company's total market value | Number of companies in the index (all equal) |
| Concentration in largest companies | Can be significant | Deliberately minimised |
| Rebalancing frequency needed | Low — adjusts automatically with prices | Higher — requires periodic rebalancing trades |
| Typical ongoing charges | Often very low | Often somewhat higher, reflecting turnover |
| Exposure to smaller companies within the index | Limited | Proportionally much greater |
Other weighting approaches worth knowing
Fundamental weighting
Weights companies according to fundamental measures such as revenue, earnings, or dividends, rather than market price, aiming to reduce the influence of market sentiment on weightings.
Factor-based or "smart beta" weighting
Tilts an index towards specific characteristics believed by some to be associated with different risk and return patterns over time — such as "value" (lower-priced relative to fundamentals), "quality" (financially robust companies), "momentum" (recent strong performers), or "low volatility" (historically more stable share prices). These approaches sit between fully passive market-cap tracking and fully active management, and typically carry ongoing charges somewhere between the two.
Price weighting
A less common approach (used, for example, by the US Dow Jones Industrial Average) where a company's weight depends on its share price rather than its total market value, which can produce results that are harder to interpret in relation to actual company size.
Why this matters for diversification
An investor holding a market-cap weighted global tracker may be surprised to learn just how concentrated the fund has become in a small number of very large companies, particularly in periods when a handful of businesses have grown to represent a large share of total global market value. This does not necessarily make market-cap weighting unsuitable — it simply means the fund's diversification benefit is more limited than the large headline number of underlying holdings might suggest.
A worked example
Suppose a hypothetical global index has 100 constituent companies, and the ten largest companies collectively represent 35% of the total market-cap-weighted index, while the remaining 90 companies share the other 65%. An investor holding £10,000 in a market-cap-weighted tracker of this index would have roughly £3,500 concentrated across just those ten largest companies. In an equal-weighted version of the same 100-company index, each company — large or small — would represent exactly 1% of the fund, meaning the same £10,000 investment would allocate just £100 to each of those ten largest companies (a combined £1,000, rather than £3,500), and correspondingly more to the remaining 90 smaller companies. This is a simplified, hypothetical illustration of the mechanical effect of different weighting schemes, not a description of any specific real-world index.
Practical considerations when choosing a weighting approach
- Check a fund's factsheet for its top ten holdings and their combined weight, to understand actual concentration, rather than assuming based on the number of underlying companies alone.
- Compare ongoing charges between market-cap, equal-weighted, and factor-based versions of similar exposure, since turnover and complexity affect cost.
- Consider that equal-weighted and factor-based funds have historically shown different return patterns and volatility compared with market-cap weighted equivalents over various periods, though past patterns are not a guarantee of future results.
How index reconstitution and rebalancing works
Even market-cap weighted indices are not entirely static — most are reviewed periodically (commonly quarterly or semi-annually) by the index provider, adding companies that have grown to meet inclusion criteria and removing those that no longer qualify, in a process known as "reconstitution". Separately, "rebalancing" adjusts the weight of existing constituents to reflect updated market values or, for non-market-cap approaches such as equal weighting, to bring weights back to their target levels.
Why this matters for a tracker fund
Each time an index is reconstituted or rebalanced, tracker funds following that index must buy and sell holdings to match the new composition, generating trading costs that are ultimately borne by fund investors, and which are one of the factors contributing to a fund's overall tracking difference relative to a theoretical index return that assumes no trading costs at all.
Free-float adjustment: a further refinement to market-cap weighting
Many market-cap weighted indices do not use a company's full market capitalisation, but instead a "free-float adjusted" figure, excluding shares held by governments, founding families, or other strategic holders who are considered unlikely to sell into the open market. This adjustment means the index (and the fund tracking it) more accurately reflects the shares genuinely available for ordinary investors to buy and sell, rather than a company's total theoretical market value, which can otherwise overstate the practically accessible portion of very large companies with significant concentrated ownership.
How weighting choice interacts with sector concentration
Weighting methodology and sector concentration are closely linked. In a market-cap weighted index, a sector that happens to contain several very large companies will naturally represent a larger proportion of the index than its number of constituent companies alone would suggest. An equal-weighted version of the same index would reduce this sector concentration somewhat, simply by giving every company, regardless of sector, the same starting weight — though sector concentration could still emerge if a particular sector simply contains a larger number of constituent companies overall.
A practical check
Reviewing a fund's sector breakdown, available on its factsheet, alongside its weighting methodology, gives a fuller picture of the specific risks being taken than looking at either factor in isolation.
Multi-factor approaches
Some more recent index designs combine several factor tilts simultaneously — for example, blending value, quality, and low-volatility characteristics into a single index, aiming to capture the potential benefits of multiple factors while smoothing out periods when any single factor underperforms. These multi-factor indices sit further along the spectrum from pure market-cap weighting, generally carrying higher ongoing charges than a simple tracker, reflecting their more complex construction methodology.
Weighting choice is disclosed, not hidden
None of this information is concealed from investors — a fund's stated benchmark index, its weighting methodology, and its factsheet holdings breakdown are all publicly available, generally within the fund's KIID/KID, factsheet, and prospectus. The purpose of understanding these mechanics is simply to read those documents with a clearer sense of what the numbers actually mean, rather than assuming every fund tracking "the market" delivers an identical, generically diversified outcome.
A note on bond index weighting
Weighting considerations are not unique to equity indices. Many broad bond indices are weighted by the total amount of debt outstanding from each issuer, meaning the most indebted governments or companies naturally represent the largest weights in the index — a characteristic some investors find counter-intuitive, since it means a bond tracker fund can end up with its largest exposure to the issuers carrying the most debt, rather than necessarily the issuers considered financially strongest. This is a similar conceptual issue to market-cap weighting in equities, applied to a different asset class.
Key takeaways
- Market-capitalisation weighting is the most common approach and reflects company size, but can lead to concentration in the largest constituents.
- Equal weighting spreads exposure evenly across all constituents, reducing concentration but typically increasing turnover and ongoing charges.
- Fundamental and factor-based ("smart beta") weighting schemes offer alternative approaches that sit between passive tracking and active management.
- Checking a fund's actual top holdings and their combined weight reveals more about real diversification than the total number of holdings alone.
- Different weighting methods carry different costs, concentration levels, and historical return patterns worth understanding before assuming all index funds tracking "the same market" are equivalent.