Hull Formula

Model Diplomat Research Team · Last reviewed June 22, 2026

TL;DR

The customary international law standard requiring "prompt, adequate, and effective" compensation when a state expropriates foreign-owned property.
Is the Hull Formula binding customary international law?
How does the Hull Formula differ from the Calvo Doctrine?
Where does the Hull standard appear in modern treaties?

Updated June 22, 2026

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Example

When Mexico nationalized its oil industry in 1938, U.S. Secretary of State Cordell Hull demanded "prompt, adequate, and effective" compensation for expropriated American oil companies — the exchange that gave the formula its name.

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The Hull Formula is the classical Western standard for compensation owed to foreign investors when a host state nationalizes or expropriates their property. It demands that compensation be:

Prompt — paid without undue delay;
Adequate — reflecting the full or fair market value of the asset; and
Effective — paid in convertible currency or otherwise usable form.

The phrase originates in a 1938 diplomatic exchange between U.S. Secretary of State Cordell Hull and the Mexican government following Mexico's expropriation of American-owned agrarian land and, later that year, the oil industry under President Lázaro Cárdenas. Hull insisted that international law required "prompt, adequate and effective" payment. Mexico countered with what became the Calvo Doctrine position: foreign investors are entitled only to the same treatment as nationals under domestic law, with disputes settled in domestic courts.

The Hull Formula dominated capital-exporting state practice through the mid-twentieth century but was contested by newly independent and developing states during the era of decolonization. UN General Assembly Resolution 1803 (1962) on Permanent Sovereignty over Natural Resources referred more ambiguously to "appropriate compensation," and the 1974 Charter of Economic Rights and Duties of States (Resolution 3281) further weakened the Hull standard by emphasizing host-state law. Capital-exporting states, however, never accepted these resolutions as binding custom.

In contemporary practice the Hull Formula has been substantially restored through the proliferation of bilateral investment treaties (BITs) and multilateral instruments such as NAFTA Chapter 11 and its successor USMCA Chapter 14, the Energy Charter Treaty, and most model BITs, which typically codify "prompt, adequate, and effective" compensation or its functional equivalent ("fair market value"). Investor-state tribunals at ICSID and under UNCITRAL rules routinely apply this standard when assessing expropriation claims, making the Hull Formula the de facto baseline of modern investment law despite its contested customary status.

Frequently asked questions

Is the Hull Formula binding customary international law?

Its customary status is disputed. Capital-exporting states treat it as custom, while many developing states historically rejected it. In practice, treaty law — especially BITs — now provides its primary legal basis.

How does the Hull Formula differ from the Calvo Doctrine?

The Hull Formula applies an international minimum standard requiring full market-value compensation in usable form, whereas the Calvo Doctrine limits foreign investors to the protections available to nationals under host-state law and courts.

Where does the Hull standard appear in modern treaties?

Variants appear in most bilateral investment treaties, the Energy Charter Treaty, NAFTA Chapter 11, and USMCA Chapter 14, typically phrased as 'prompt, adequate, and effective' compensation equal to fair market value.