Hull Formula

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.