How many countries in the world use common law?
List of national legal systems
Common law is currently in practice in Ireland, most of the United Kingdom (England and Wales and Northern Ireland), Australia, New Zealand, Bangladesh, India (excluding Goa), Pakistan, South Africa, Canada (excluding Quebec), Hong Kong, the United States (on a state level excluding Louisiana), and many other places. In addition to these countries, several others have adapted the common law system into a mixed system. For example, Nigeria operates largely on a common law system, but incorporates religious law.
Law of the United States
The United States and most Commonwealth countries are heirs to the common law legal tradition of English law. Certain practices traditionally allowed under English common law were expressly outlawed by the Constitution, such as bills of attainder and general search warrants.