Active Share is the percentage of a mutual fund’s holdings that differ from its benchmark index, and it is calculated by summing the absolute differences between each security’s weight in the fund and its weight in the index, then dividing by two. The result falls between 0% and 100%. A fund at 0% is a perfect index replica; a fund at 100% has no overlap with the benchmark at all. The measure was introduced in a 2009 paper by Martijn Cremers and Antti Petajisto and has become one of the standard tools for separating genuinely active managers from those charging active fees for near-index portfolios.1Oxford Academic. How Active Is Your Fund Manager? A New Measure That Predicts Performance
The Formula
For every security held by either the fund or the benchmark, take the absolute difference between its weight in the fund and its weight in the index. Sum those differences. Divide by two.
The division by two exists because every dollar overweight in one stock must be offset by a dollar underweight somewhere else. Counting both sides of that trade would double the score, so you halve it.
A Worked Example
Say a fund holds Stock A at 10% of its portfolio while the benchmark weights Stock A at 5%. The absolute difference is 5 percentage points. The fund does not hold Stock B at all, but the benchmark weights it at 3%. That contributes another 3 percentage points. You repeat this comparison for every security that appears in either the fund or the index, add up all the differences, and divide by two.
Morningstar’s published methodology walks through a simplified case: a fund holding eight securities at weights ranging from roughly 3% to 29%, with the benchmark holding the same securities at weights ranging from about 8% to 25%. The absolute differences are summed across all positions and halved to produce the final score.2Morningstar. Active Share Methodology for Multiple Portfolio Attribution In practice, a large-cap U.S. fund benchmarked to the S&P 500 requires comparing weights across all 500 constituents plus any positions the fund holds outside the index.
Two mechanical points are worth noting. Stocks the fund does not own but the benchmark holds count; their weight in the fund is simply zero. And off-benchmark positions, meaning stocks the fund holds that are not in the index at all, count in full because their benchmark weight is zero.
Reading The Score
The Cremers and Petajisto research treated 60% as the working line between closet indexing and genuine active management. Funds scoring below 60% hold portfolios so close to the benchmark that investors are effectively buying index exposure at active-management prices.3EFAMA. Report on Closet Index Funds Funds above 80% sit at the highly active end, where the manager is making large, deliberate departures from the index.1Oxford Academic. How Active Is Your Fund Manager? A New Measure That Predicts Performance
Between 60% and 80% is the gray zone. A fund at 65% is clearly making some independent bets, but a substantial portion of its portfolio still tracks the index. Whether that level of activity is worth the fee depends on how much the fund charges and how the manager has performed.
The most practical use of the number is spotting closet indexers. Paying an expense ratio of 75 basis points or more for a fund running at 40% Active Share means subsidizing a strategy that behaves largely like the benchmark. A plain index fund tracking the same benchmark would cost a fraction of that and deliver comparable returns. Active Share turns that mismatch into a specific number you can weigh against the fee.
Why The Benchmark Choice Changes The Answer
Active Share only means anything relative to a specified index, and switching the index changes the score. The same U.S. large-cap fund might register 70% against the S&P 500 and a different figure against the Russell 1000, because the two indices do not hold identical stocks at identical weights. Research from the Brandes Institute noted that an Active Share of 60 could be considered low for a broad U.S. equity portfolio but high for a portfolio compared against a more concentrated benchmark.4Brandes Institute. Active Share: Finding Value-for-Money
The size of the benchmark matters too. A fund holding 40 stocks measured against a 500-stock index will produce a high Active Share almost mechanically, because it cannot possibly hold most of the index’s constituents. The same Brandes study reported an inverse correlation of -0.52 between the number of portfolio holdings and Active Share for S&P 500 funds: fewer holdings meant a higher score, regardless of stock-selection quality.4Brandes Institute. Active Share: Finding Value-for-Money
The consequence for anyone using the number: do not compare Active Share across funds that use different benchmarks or invest in different market-cap segments. A small-cap fund at 85% and a large-cap fund at 85% are not equally active in any meaningful sense. The comparison only holds when both funds are measured against the same index.
What The Number Does And Doesn’t Tell You
The original 2009 study found that funds with the highest Active Share significantly outperformed their benchmarks before and after fees and showed persistent performance, while the lowest-Active-Share funds underperformed.1Oxford Academic. How Active Is Your Fund Manager? A New Measure That Predicts Performance Petajisto’s 2013 follow-up added a wrinkle. Funds that achieved high Active Share through picking individual stocks beat their benchmarks by an average of 1.26% per year after fees and 2.61% before fees. Funds that produced high tracking error through sector or factor bets, however, destroyed value on average.5Antti Petajisto. Active Share and Mutual Fund Performance
Not all researchers accept the evidence as settled. The CIBC Mellon research team noted that some of the outperformance attributed to high-Active-Share managers could reflect luck in factor tilts rather than genuine stock-picking skill.6CIBC Mellon. Oversharing Active Share The honest reading is that Active Share is a necessary condition for outperformance, not a guarantee of it. A low score tells you the manager cannot beat the index by much because the portfolio is the index. A high score tells you the manager has taken enough independent risk to have a shot, but says nothing about whether the specific bets are good ones.
Finding Or Calculating It Yourself
Active Share does not appear in standard fund regulatory filings like the prospectus or annual report. Some fund companies publish it voluntarily on their websites or in marketing materials, particularly when the score is high. Morningstar calculates it for many funds through its institutional research platform. Cremers maintains a database at the University of Notre Dame covering U.S. equity mutual funds, though the data may lag by several months.
If a fund does not disclose its score, you can compute it from the fund’s most recent quarterly holdings report, filed with the SEC on Form N-PORT, together with the benchmark’s published constituent weights. The arithmetic is straightforward; the tedium comes from working through hundreds of positions. Portfolio analytics tools can automate the process when holdings data is available in a workable format.
One check is worth doing whenever a fund advertises a high Active Share: confirm the benchmark it is being measured against. A fund that looks strikingly active against a broad market index may look far less distinctive against a narrower style or sector benchmark that better fits its actual investment approach.