Bill Ackman's activist concentration model

Model the concentration rule Ackman's fund publishes: 8 to 12 core holdings, no issuer above 25% of gross assets, tested by moving one position through your own upside and downside scenarios.

Bill Ackman Reviewed Aug 6, 2026 Rule located in Pershing Square Holdings, Ltd., 2026

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Interactive model

Put your numbers through the rule

The source establishes the rule. The values below belong to you, and the output is JMM's deterministic calculation.

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Every field recalculates immediately. Changed values can be copied into a shareable URL.

Portfolio value

Position size

Position upside scenario

Position downside scenario

Rest of portfolio return

Portfolio return, upside case18.5%

Position dollars: $250,000
Upside portfolio value: $1,185,000
Downside portfolio value: $985,000
Downside portfolio return: -1.5%

Position up 50%

$1,185,000

Position down 30%

$985,000

One concentrated position moves the whole portfolio. The calculator isolates that arithmetic; it does not score the activist catalysts behind the position.

Inspect the calculationThe same result in a readable record view

Upside scenario

Position move: 50.0%
Rest of portfolio: 8.0%
Portfolio return: 18.5%

Downside scenario

Position move: -30.0%
Rest of portfolio: 8.0%
Portfolio return: -1.5%

Source and translation

What the source says, and what the calculator adds

Source-supported ruleThe fund Ackman manages states its own concentration rule in writing: the substantial majority of the portfolio typically sits in 8 to 12 core holdings of large-cap North American companies with limited downside and predictable, recurring cash flows, and no single issuer may exceed 25% of gross assets at the time of investment.

JMM calculationJMM isolates the arithmetic of concentration: one position receives its entered share, the rest compounds at the entered portfolio return, and the calculator reports both the upside and downside portfolio outcomes.

Formula and methodPortfolio result = rest of portfolio at its entered return plus the position at its entered move. Position dollars = portfolio value × entered share.

Pershing Square Holdings, Ltd. Feb 18, 2026Pershing Square Holdings, Ltd. 2025 Annual ReportPage 1, "Company Overview": "The substantial majority of the Company’s portfolio is typically allocated to 8 to 12 core holdings usually comprising liquid, listed, large capitalization North American companies. The Investment Manager seeks to invest in high-quality growth businesses, which it believes have limited downside and generate predictable, recurring cash flows." Page 34, "Investment Policy": single-issuer exposure "may not, in the aggregate, represent more than 25% of the Company’s gross assets, measured at the time the investment is made."

Decision notes

What changes the answer

Concentration converts volatility into moves

At the 25% ceiling the annual report names, a 50% position move changes the portfolio by 12.5 points before the rest of the book is considered. The calculator surfaces that leverage.

The downside is the discipline

The model shows the same arithmetic in the downside case, which is where the limited-downside, predictable-cash-flow filter in the stated policy earns its keep.

Concentration is not a strategy. It is a multiplier on whether you were right.

Pershing Square publishes its own ceiling, and the arithmetic on this page is what that ceiling means: at 25% of gross assets, a position that moves 50% moves the whole book by 12.5 points before anything else happens. That cuts identically in both directions, which is the part the fund letters emphasise less than the wins. What makes the approach defensible for Ackman is not the concentration, it is the two conditions attached to it — large, liquid, predictable-cash-flow businesses, and the ability to act on management when the thesis stalls. An individual copying the position size without either condition has kept the multiplier and thrown away the mitigation. JMM would not run 25% in one name without a written answer to what you do if it halves.

Written by the JMM editor. This is a view, not a recommendation for your circumstances. Who writes this

Not the calculator’s limits. The rule’s.

The activist half is not available to you
Concentration works for a fund that can force a board conversation. A retail holder of the same weight has the same exposure and no lever, which is a materially worse version of the same trade.
A ceiling at purchase is not a ceiling afterwards
The stated 25% is measured when the investment is made. A position that doubles becomes a larger share of the book without any new decision, and the policy does not require trimming it back.

Limits

What this model does not know

Activist outcomes depend on catalysts, financing, and holding period, none of which the model scores.
A single-position scenario is not a proxy for Pershing Square performance or holdings.
Fees, taxes, hedging, and position changes over time are excluded.
The 25% figure is the fund’s stated ceiling at the time of investment, not a target weight and not the size of any actual position.

Questions people ask

Before you use the result

Does Ackman run exactly this allocation?

No. The annual report states a rule — 8 to 12 core holdings, no issuer above 25% of gross assets at purchase. The weight and the scenarios here are your inputs.

Is this an activist trade simulator?

No. It models the concentration math only; the activist campaign itself is not scored.

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