Naturally there have been two systems of metrics developed by different nations and groups of people. One of them is Imperial system initially developed in Ancient Rome and adopted by British Empire. The other one is called US customary system, developed in that country after it gained independence.
However, there is the third international or IS (metric one), which is used for all the engineering and other calculations. USA has adopted this system for its engineering work, but it remains the only country in the world, where metric system did not attain official measuring nit status. Common people keep on using old US customary system instead.