Key Takeaways
Direct answer: Index concentration measures how much of a market-cap-weighted index's total weight sits in its largest constituents. It's most often expressed as cumulative top-N weight — for example, "the top 10 stocks are 35% of the S&P 500's weight" — and less commonly as a weight-based Herfindahl-Hirschman Index (HHI), the sum of every constituent's squared weight.
- In a cap-weighted index, each constituent's weight = its market cap ÷ the total market cap of every constituent in the index.
- Cumulative top-N weight (top 5, top 10, top 25) is the simplest and most widely cited concentration measure.
- A weight-based HHI (sum of squared weights) is a less common but more granular measure that also captures how unevenly weight is spread across the whole index, not just how much the largest few names hold in total.
- This weight-based HHI is conceptually similar to, but a distinct application from, the industry-competitive HHI on the Market Concentration, HHI, and Consolidation guide — that page sums squared market shares of competing firms in an industry, not an index's constituent weights.
- Concentration and diversification move inversely within an index: the higher the top-N weight, the fewer names are effectively driving the index's overall return.
What Is Index Concentration?
A market-cap-weighted index — the construction method used by the S&P 500, the Nasdaq-100, and most broad equity benchmarks — assigns each constituent a weight proportional to its market capitalization relative to the total market capitalization of every constituent in the index:
Constituent weight = constituent market cap ÷ sum of all constituents' market caps
That formula alone doesn't tell you how the resulting weights are distributed. Two indexes with 500 constituents each could both be cap-weighted and still look completely different: in one, weight could be spread relatively evenly; in the other, a small handful of mega-cap names could hold a large share of the total. Index concentration is the term for measuring that distribution directly — how much of the index's total weight is concentrated in its largest few names versus spread across the rest.
Common mistake
The common mistake is assuming "cap-weighted" and "diversified" mean the same thing. Cap-weighting is a construction rule, not a diversification guarantee — it will concentrate weight in whichever constituents have grown largest, and that concentration can rise or fall over time as constituent market caps change relative to each other, independent of the number of names in the index.
How Is Index Concentration Measured?
Two measures are used in practice, and they answer slightly different questions.
Cumulative top-N weight
The most common measure sums the weights of the N largest constituents by weight:
Top-N weight = sum of the weights of the N largest constituents
This is the figure behind headline statistics like "the top 10 stocks are 35% of the S&P 500's weight." It's simple to calculate and easy to communicate, which is why it's the most commonly cited concentration measure. The sibling Top-5 and Top-10 Index Weight guide covers this specific measure in depth, including how to interpret a rising top-N weight over time.
Weight-based HHI
A less common but more granular measure applies a Herfindahl-Hirschman Index (HHI) calculation to the index's constituent weights instead of top-N alone:
Weight-based HHI = sum of (constituent weight, as a whole-number percentage)² across every constituent in the index
Squaring each weight before summing gives disproportionately more influence to larger weights, so this measure captures concentration across the entire index, not just within a fixed top-N cutoff. An HHI computed on index weights this way ranges toward 10,000 as the index approaches a single dominant constituent, and falls as weight is spread more evenly across more constituents — the same range convention used on the industry-competitive HHI, described next.
How is this different from the industry HHI on Market Concentration, HHI, and Consolidation?
Both calculations square a set of percentages and sum them, but the inputs measure entirely different things. The Market Concentration, HHI, and Consolidation guide applies HHI to competing firms' market shares within an industry, to measure competitive concentration and pricing power — a small number of firms dominating an industry's sales. Weight-based index HHI applies the identical math to an index fund's portfolio weights, to measure how concentrated the fund's holdings are — a small number of constituents dominating the index's return. A stock can have a small market share in its own industry (low industry HHI contribution) while still carrying a large weight in a cap-weighted index (large index HHI contribution) if its absolute market capitalization is large. The two measures should never be used interchangeably.
Worked Example: Top-5 Weight and Weight-Based HHI
Hypothetical, illustrative numbers — not a real index or live market data.
Assume a hypothetical cap-weighted index with 20 constituents. The 10 largest constituents, by weight, are listed individually below; the remaining 10 constituents are smaller and, for this illustration, are assumed to be equally weighted at 3.8% each.
| Constituent | Weight | Weight² (contribution to HHI) |
|---|---|---|
| 1 (largest) | 12.0% | 144.00 |
| 2 | 9.0% | 81.00 |
| 3 | 8.0% | 64.00 |
| 4 | 7.0% | 49.00 |
| 5 | 6.0% | 36.00 |
| 6 | 5.0% | 25.00 |
| 7 | 4.5% | 20.25 |
| 8 | 4.0% | 16.00 |
| 9 | 3.5% | 12.25 |
| 10 | 3.0% | 9.00 |
Step 1 — top-5 cumulative weight: sum the weights of constituents 1 through 5: 12.0 + 9.0 + 8.0 + 7.0 + 6.0 = 42.0%. Just five of the twenty constituents hold 42% of the index's total weight.
Step 2 — top-10 cumulative weight: sum constituents 1 through 10: 42.0 + 5.0 + 4.5 + 4.0 + 3.5 + 3.0 = 62.0%. The remaining 38.0% of the index's weight is split across the other 10 constituents.
Step 3 — weight-based HHI: sum every constituent's squared weight. The 10 named constituents' squared weights (right-hand column above) sum to 144.00 + 81.00 + 64.00 + 49.00 + 36.00 + 25.00 + 20.25 + 16.00 + 12.25 + 9.00 = 456.50. Each of the other 10 constituents holds 3.8%, so its contribution is 3.8² = 14.44; across 10 of them that's 10 × 14.44 = 144.40. Total weight-based HHI = 456.50 + 144.40 = 600.90.
Step 4 — compare to an equal-weight baseline: if this same 20-constituent index were equal-weighted instead of cap-weighted, every constituent would hold 100 ÷ 20 = 5.0%, giving an HHI of 20 × 5² = 20 × 25 = 500. The cap-weighted index's HHI of 600.90 is meaningfully above that 500 baseline, confirming numerically what the top-5 and top-10 weights already suggested: this hypothetical index carries more concentration than an equally-weighted version of the same 20 names would.
Common mistake
The common mistake is computing top-N weight and stopping there. Top-N weight only describes the largest N names; it says nothing about how weight is distributed among constituent 11 versus constituent 200. The weight-based HHI in step 3 uses every constituent, which is why it moved the analysis from "the top 10 hold 62%" to a single comparable number that can be benchmarked against the equal-weight case in step 4.
Misconceptions Versus Reality
| Misconception | Reality |
|---|---|
| A cap-weighted index is automatically diversified because it holds hundreds of names | Cap-weighting concentrates weight in whichever constituents have grown largest; holding many names doesn't prevent a small subset from carrying most of the weight |
| Index concentration and the industry-competitive HHI measure the same thing | Index concentration applies HHI-style math to an index's portfolio weights; the industry HHI on Market Concentration, HHI, and Consolidation applies the same math to competing firms' market shares within an industry — different inputs, different questions |
| Top-N weight alone fully describes an index's concentration | Top-N weight only covers the largest N names; a weight-based HHI computed across every constituent can reveal concentration that a fixed top-N cutoff misses |
| Rising index concentration is inherently bad for a passive investor | Rising concentration is a description of how return is distributed across constituents, not a standalone judgment — see the sibling Top-5 and Top-10 Index Weight guide for how to interpret a concentration trend without over-reading it |
Risks, Limitations, and Exceptions
- Weight-based HHI requires knowing every constituent's weight, not just the largest few; incomplete data (as simplified in this guide's worked example) understates precision versus a full constituent-level calculation.
- Neither top-N weight nor weight-based HHI says anything about correlation between the largest constituents — that limitation is covered in detail on the sibling Top-5 and Top-10 Index Weight guide.
- Index weights change continuously as constituent prices move, and change discretely at scheduled rebalances and index reconstitutions; a concentration figure is only accurate as of the date it was measured.
- Different index providers may use different float-adjustment or capping methodologies, which can produce different concentration figures for indexes that appear to track the same market.
- The worked example in this guide uses illustrative, hand-calculated numbers for a hypothetical 20-constituent index, not a real index or live market data.
Frequently Asked Questions
How is index concentration measured?
Index concentration is most commonly measured as cumulative top-N weight — the sum of the weights of the N largest constituents in a market-cap-weighted index, such as "the top 10 stocks are 35% of the S&P 500's weight." A less common but more granular measure applies a Herfindahl-Hirschman Index (HHI) calculation to the index's constituent weights instead of top-N alone, summing the square of every constituent's weight to capture how unevenly weight is spread across the whole index, not just within the largest names.
Is index concentration the same thing as the industry HHI on Market Concentration, HHI, and Consolidation?
No. Both use a Herfindahl-Hirschman Index calculation, but they are applied to different inputs and answer different questions. The industry HHI on the Market Concentration, HHI, and Consolidation guide sums the squared market shares of competing firms within an industry to measure competitive concentration and pricing power. Weight-based index HHI sums the squared portfolio weights of an index's constituents to measure how concentrated an index fund's holdings are, which has nothing to do with competition between the companies themselves.
What is a constituent's weight in a cap-weighted index?
In a market-cap-weighted index, each constituent's weight equals its market capitalization divided by the sum of the market capitalizations of every constituent in the index. A company with a larger market cap relative to the rest of the index gets a proportionally larger weight, and therefore a proportionally larger influence on the index's return, without any additional shares needing to be bought or sold to create that influence.
Sources and Methodology
Cap-weighted index construction and concentration reporting follow conventions publicly documented by index providers and used across market-structure research. Key reference sources include:
- S&P Dow Jones Indices — Index Mathematics Methodology: spglobal.com/spdji — the constituent-weighting formula this guide's definition is built on.
- U.S. Department of Justice & Federal Trade Commission — Horizontal Merger Guidelines: justice.gov/atr — the origin of the HHI calculation convention (sum of squared whole-number percentages) referenced for the weight-based HHI measure.
The worked example in this guide uses a clearly labeled, hand-calculated hypothetical dataset, not live index or market data. This content was reviewed by the Swoopr Editorial Team in August 2026.
Related Reading
- Market Concentration & Leadership — the parent hub for this content group.
- Top-5 and Top-10 Index Weight — the deeper treatment of the top-N weight measure introduced above, including how to read a rising concentration trend.
- Equal-Weight vs. Cap-Weight Breadth — how comparing an equal-weighted and cap-weighted version of the same index reveals whether a move is broad or concentrated.
- Market Concentration, HHI, and Consolidation — the industry-competitive HHI, a distinct application of the same HHI math to firms' market shares rather than index weights.