Charlemagne (aka "Charles the Great") standardized the Imperial measuring system based on the length of his foot. Up until that time (~800AD), measuring systems were disjointed and it created problems for trade between countries and cultures.
Although many kings had tried to standardize a measurement system prior to Charlemagne, it was Charles the Great who was able to finally get it to stick. The system is used only by three countries today: Liberia, Myanmar, and of course the United States of America.
The world adopted a practical measuring system (the metric system) around 1799. You may ask, "So why does the US continue using the Imperial system?"
The world began integrating the metric system around the time of the Industrial Revolution in the US. Many companies were heavily invested in infrastructure that relied on the measurements of the Imperial system. Whenever the discussion of switching unit systems was proposed in Congress, the motion was always thwarted by lobbyists of big businesses.
Also, American citizens didn’t want to go through the time-consuming and expensive hassle of changing the country’s entire infrastructure.
It was also suggested that many citizens of the US believed they should keep their own system, which would set them apart from other countries and symbolize their status as a leader rather than a follower. To this day, the Imperial System requires countries interested in trading with the US to convert from their own measuring/weighing systems.
Footnote: Also included among Charlemagne's achievements is his unification of Eastern and Western Europe under his rule.
