Standard units may be understood in two senses.
Most countries in the world use standardized units from the metric system: The second meaning of standard unit refers to a unit of particular set of units of measurement called the standard system (versus the metric system).
The standard system includes the standard units of the foot, the pound (mass), and the gallon.
The United Kingdom and the United States both make, or have in the past made, use of the standard system.
However, each country defines them slightly differently: