Free tool · Public International Law
What Is Customary International Law? A Checker for State Practice and Opinio Juris
Customary international law is the body of rules that arise from the general and consistent practice of states followed out of a sense of legal obligation (opinio juris). Both elements are needed: practice without a sense of obligation is mere habit or courtesy. The practice must be widespread and representative, including the states most affected, though it need not be universal or long. A state that objected clearly while a rule was forming, and keeps objecting, is not bound by it (the persistent objector), but no state can object to a peremptory norm (jus cogens) such as the prohibition of genocide. Custom can bind states that are not parties to a treaty that states the same rule.
Tool · Customary International Law Checker
A rule of customary international law very probably exists and binds the state.
State practice: The practice is widespread and representative.
Consistency: The practice is general and consistent. Minor inconsistencies are treated as breaches of the rule, not as signs that it does not exist, if states justify them by reference to the rule (Nicaragua v United States).
Specially affected states: The states most affected (for example maritime states for a rule of the sea) have taken part.
Opinio juris: Evidence shows that states act because they believe the rule is legally required.
Duration: The practice has run for a considerable time.
Treaty parallel: A widely ratified treaty can be evidence of custom, or can crystallise or generate it. A rule may bind non-parties as custom even if it also appears in the treaty.
- Article 38(1)(b) of the Statute of the International Court of Justice refers to ‘international custom, as evidence of a general practice accepted as law’. The two elements, general practice and acceptance as law (opinio juris), must both be shown (North Sea Continental Shelf cases; the ILC’s Conclusions on identification of customary international law, 2018).
- Evidence of practice includes what states do and say: diplomatic correspondence, legislation, court decisions, military manuals and votes. Evidence of opinio juris includes statements that a rule is legally required, resolutions and treaty language, read with care.
Tests whether a rule of customary international law exists, using the two elements the International Court and the International Law Commission describe: general state practice and acceptance of it as law (opinio juris). It also shows the effect of a persistent objector and of a peremptory norm. Invented facts and simplified doctrine; real identification of custom involves detailed evidence. A teaching aid, not legal advice. Runs in your browser; nothing is stored or sent.
How to use it
- Choose how widespread and consistent the practice is.
- Say how clear the evidence is that states act out of a sense of legal obligation.
- Note any treaty, any persistent objector and whether the rule is peremptory.
- Read whether a customary rule exists and whom it binds.
Common questions
What is opinio juris?
The belief of states that a practice is required by law. It is shown by statements, resolutions, treaty language and the way states justify their conduct.
Can a treaty and custom cover the same rule?
Yes. A treaty may codify, crystallise or generate custom, and the rule may then bind non-parties as custom even though it is also in the treaty.
Can a state opt out of custom?
Only as a persistent objector, and only if it objected while the rule was forming and consistently afterwards. It cannot opt out of peremptory norms.
Learn the law behind the tool
This tool is part of our Public International Law course, with lectures, concept maps, flashcards, interactive exercises and a capstone case. Public International Law: Sources and Doctrines.
More free law tools
General information, simplified, mainly on American law, not legal advice. States and countries differ, and real cases turn on facts that a tool cannot capture.