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Active Share Calculator

Calculate how much your fund's holdings diverge from its benchmark index. Active Share measures whether a portfolio manager is truly making active bets or quietly mimicking the index while charging active management fees.

Relative Fund Holdings

Asset Ticker
Port Wt (%)
Bench Wt (%)
Del
Total Port Wt
100.00%
Total Bench Wt
100.00%

Aggregated Active Share

16.50%
Absolute variance from benchmark.
Fund Classification CodeCloset Indexer

Asymmetry Logic

Sum of Absolute Differences:33.00%
Equation Division Basis:÷ 2.0 Base Divisor
Fee Warning: This fund is a Closet Indexer. It mathematically mimics its benchmark index while likely charging high active management fees. Avoid paying a premium for beta.
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What is Active Share: Cremers-Petajisto Closet Indexing Metric & Fee Justification Analysis?

Active Share, developed by Martijn Cremers and Antti Petajisto in their seminal 2009 Yale study 'How Active Is Your Fund Manager?', measures the percentage of a fund's portfolio holdings that differ from its benchmark index. It is calculated as one-half the sum of the absolute differences between each holding's portfolio weight and its benchmark weight. Active Share ranges from 0% (perfect index replication) to 100% (zero overlap with the benchmark). The metric was specifically designed to detect 'closet indexers' — fund managers who charge active management fees (typically 0.80-1.50% annually) while constructing portfolios that barely deviate from the benchmark, delivering index-like returns minus the fee drag.

Mathematical Foundation

Active Share (Cremers-Petajisto, 2009)
Active Share=12∑i=1N∣wp,i−wb,i∣\text{Active Share} = \frac{1}{2} \sum_{i=1}^{N} |w_{p,i} - w_{b,i}|
wp,iw_{p,i}= Weight of asset i in the portfolio (as a percentage of total portfolio value). All portfolio weights must sum to exactly 100%.
wb,iw_{b,i}= Weight of asset i in the benchmark index (e.g., S&P 500, Russell 2000). All benchmark weights must sum to exactly 100%.
NN= Total number of unique holdings across both the portfolio and benchmark combined. If the portfolio holds a stock not in the benchmark, the benchmark weight for that stock is 0%, and vice versa.
12\frac{1}{2}= The divisor of 2 corrects for double-counting: every overweight position in one stock is exactly offset by an underweight position in another, so the raw sum of absolute differences always equals twice the true active share.

Laws & Principles

  • The Closet Indexing Detection Rule: Cremers and Petajisto's research found that funds with Active Share below 20% are 'closet indexers' — they statistically cannot outperform the benchmark by enough to justify active management fees. Funds with Active Share above 60% are 'truly active' and have the statistical capacity (though not guarantee) to generate alpha. The 20-60% middle range represents 'moderate divergence' — the manager is making some active bets but not enough to overcome fee drag in most market environments. If your fund charges 1%+ and has Active Share below 30%, you are mathematically better off in a 0.03% index ETF.
  • Active Share Is Necessary but Not Sufficient: A high Active Share (80%+) means the manager is making big bets that differ from the benchmark — but those bets can be wrong. Active Share measures how different the portfolio is, not how good the differences are. It must be combined with tracking error (volatility of returns vs. benchmark) and alpha (excess return) to determine if the manager's active decisions are generating value. A fund with 90% Active Share and -5% annual alpha is making large, consistently wrong bets — the worst possible combination.

Step-by-Step Example Walkthrough

" A concentrated stock picker invests in 4 positions: 15% Apple, 10% Tesla, 5% Microsoft, and 70% cash/other. The S&P 500 benchmark holds: 7% Apple, 1.5% Tesla, 6.5% Microsoft, and 85% other. Calculate Active Share. "

  1. 1. AAPL: |15% - 7%| = 8.0% absolute difference.
  2. 2. TSLA: |10% - 1.5%| = 8.5% absolute difference.
  3. 3. MSFT: |5% - 6.5%| = 1.5% absolute difference.
  4. 4. Cash/Other: |70% - 85%| = 15.0% absolute difference.
  5. 5. Sum of absolute differences: 8.0 + 8.5 + 1.5 + 15.0 = 33.0%.
  6. 6. Active Share: 33.0% / 2 = 16.5%.
Final Result: Despite holding only 4 positions with seemingly concentrated bets (15% in Apple, 10% in Tesla), the Active Share is only 16.5% — firmly in 'Closet Indexer' territory. The massive 70% cash/other allocation closely mirrors the benchmark's 85%, and the net portfolio-level deviation is small. If this fund charges 1%+ in management fees, the investor is paying active fees for near-index exposure. The manager would need to dramatically increase concentration in their conviction positions (or eliminate the cash drag) to push Active Share above the 60% 'truly active' threshold.

Quick Answer: What is Active Share and what does it tell you about a fund?

Active Share measures the percentage of a fund's holdings that differ from its benchmark index. It's calculated as Active Share = ½ × Σ|w_portfolio − w_benchmark| for every holding. A fund with Active Share below 20% is a "closet indexer" — replicating its benchmark while charging active fees. Above 60% indicates a truly active manager making concentrated bets. The metric was developed by Cremers & Petajisto (2009, Yale) specifically to detect funds that charge 1%+ fees for index-like performance.

Active Share Formula

Cremers-Petajisto Active Share (2009)

Active Share = ½ × Σ |w_portfolio(i) − w_benchmark(i)| for all i

  • w_portfolio(i)— Weight of asset i in the fund's portfolio (must sum to 100%)
  • w_benchmark(i)— Weight of asset i in the benchmark index (S&P 500, Russell 2000, etc.)
  • ÷ 2— Dividing by 2 corrects for double-counting: every overweight is offset by an underweight elsewhere in the portfolio
  • Range— 0% = perfect index clone; 100% = zero benchmark overlap (holds nothing the index holds)

Real-World Active Share Examples

Truly Active Fund — Concentrated Value Picker

Portfolio: 20 stocks, top 5 = 40% of fund | Benchmark: S&P 500 (500 stocks)

  1. Holdings overlap: Only 15 of 20 stocks appear in the S&P 500
  2. Weight deviation: Top 5 holdings average 8% portfolio weight vs. 1-3% benchmark weight
  3. Active Share: 82% — truly active

→ 82% Active Share justifies 0.80-1.20% management fee — manager is making real conviction bets

Closet Indexer — Large Cap "Active" Fund

Portfolio: 480 stocks, top 10 weights within ±0.5% of S&P 500 | Fee: 1.10%/year

  1. Holdings overlap: 475 of 480 stocks are in the S&P 500
  2. Weight deviation: Average holding deviates by only 0.03% from benchmark weight
  3. Active Share: 12% — closet indexer

→ 12% Active Share = paying 1.10% for an index fund (VOO costs 0.03%). Fee drag: ~$10,700/year per $1M invested

Active Share Classification Thresholds

Active Share Classification
0% – 20% Closet Indexer
20% – 60% Moderate Divergence
60% – 100% Truly Active
💡 Source: Cremers & Petajisto (2009), "How Active Is Your Fund Manager?" Review of Financial Studies. Active Share alone does not predict performance — combine with tracking error and net-of-fee alpha.

Pro Tips & Common Active Share Mistakes

Do This

  • ✓Compare Active Share to the fund's expense ratio before investing. If a fund charges 1.00% annually but has Active Share of 15%, you are paying 33× more than a 0.03% index ETF for nearly identical market exposure. Over 20 years on a $500K portfolio, that fee difference compounds to approximately $150,000 in lost wealth — entirely due to closet indexing fees.
  • ✓Verify that both portfolio and benchmark weights sum to exactly 100%. The Active Share formula mathematically requires that both weight vectors sum to 100%. If the portfolio weights sum to 95% (missing a 5% cash allocation), the calculation artificially inflates Active Share because that missing 5% appears as a phantom deviation from benchmark.

Avoid This

  • ✗Don't assume high Active Share means high returns. Active Share measures how different a portfolio is from its benchmark — not how good those differences are. A fund with 90% Active Share that massively overweights the wrong sectors will dramatically underperform the index. High Active Share is a necessary condition for outperformance (you can't beat the index by replicating it), but it is not a sufficient condition. Always pair Active Share with the fund's net-of-fee alpha track record.
  • ✗Don't compare Active Share across different benchmarks. A small-cap fund benchmarked to the Russell 2000 will mechanically have different Active Share dynamics than a large-cap fund benchmarked to the S&P 500. The S&P 500 is heavily concentrated in mega-caps (top 10 = ~30%), so even modest weight changes produce lower Active Share than the same changes in a more equally-weighted index. Compare Active Share only among funds using the same benchmark.

Frequently Asked Questions

What is Active Share and what does it measure?

Active Share measures what percentage of a fund's portfolio holdings differ from its benchmark index. It was developed by Cremers and Petajisto at Yale in 2009 to quantify closet indexing — the practice of fund managers who charge active management fees (0.80-1.50% annually) while constructing portfolios that closely replicate their benchmark index. Active Share is calculated as one-half the sum of absolute weight differences between every holding in the portfolio and the benchmark. It ranges from 0% (perfect index replication) to 100% (zero overlap). A fund with Active Share below 20% is a closet indexer; above 60% is considered truly active.

What is a closet indexer and why does it matter for investors?

A closet indexer is a mutual fund or ETF that markets itself as "actively managed" but constructs a portfolio nearly identical to its benchmark index. The manager collects high active management fees (1%+ per year) while delivering index-like returns — meaning the investor underperforms the index by the fee difference every year. Cremers and Petajisto found that closet indexers (Active Share below 20%) statistically cannot generate enough alpha to overcome their fee drag. An investor in a closet indexer paying 1.10% would be better off in a passive S&P 500 ETF costing 0.03%, saving $10,700 per year on a $1 million portfolio — compounding to over $200,000 in lost wealth over 20 years.

Does high Active Share guarantee better fund performance?

No — high Active Share is a necessary but not sufficient condition for outperformance. It proves the manager is making active bets that differ from the benchmark, but those bets can be wrong. The original Cremers/Petajisto research found that on average, high Active Share funds outperformed after fees, but with enormous dispersion. The top quartile of high Active Share managers generated significant alpha, while the bottom quartile destroyed value. To evaluate a high Active Share fund, combine it with: (1) tracking error (volatility of deviation from benchmark), (2) net-of-fee alpha (excess return after fees), and (3) information ratio (alpha per unit of tracking error).

Why do portfolio and benchmark weights need to sum to 100%?

The Active Share formula requires both weight vectors to sum to exactly 100% because it measures the reallocation of capital from benchmark weights to portfolio weights. If weights don't sum correctly, phantom deviations appear: a portfolio summing to 95% creates an artificial 5% "underweight" that inflates Active Share. Mathematically, the ÷2 divisor works only because the constraint Σw_port = Σw_bench = 100% guarantees that the total overweights exactly equal the total underweights. Breaking this constraint produces a meaningless number. Always include cash, money market, and "other" allocations to ensure both columns reach 100%.

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