Officially the United States is on neither the Imperial NOR the Metric System.
When this was being debated by our Founding Fathers after the Revolution but before the Constitution was written, Jefferson was pushing for the Metric System while Hamilton was against it as he felt that it would alienate the British who, despite our having recently defeated them, still ruled the seas and we would need to stay in their good graces if we wished to trade with other nations. Since neither side could win a consensus among the other players, they decided to take the politicians way out; they did nothing and left it up to society to choose whatever worked best. Of course, since most Americans at that time were already familiar with the British 'Imperial' system and the Metric system was still rather new, not even seeing wide use in France yet, where is was developed, until the their own Revolution a few years later, it wasn't a big surprise that the Imperial system remained the 'de facto standard' for America as well as most of the rest of the English speaking world.
But that didn't mean that there weren't 'skirmishes' still being fault across the country. It's like when I was in engineering school back in the 60's. There was a road down the middle of the campus which became a sort of 'icon' for this situation, with the science buildings on one side of the road and the engineering buildings on the other. The road became a border between the Metric domain (in the science classes like Physics and Chemistry) on one side and the Imperial domain (Mechanical and Civil Engineering) on the other.
Even today, while progress has been made in such areas as medical devices, precisions machinery, instrumentation and automotive in this country, the Aerospace and Defense segments are still the domain of the Imperial system. Working for a company that provides engineering software to all major engineering and manufacturing segments on a global basis we have to accommodate both worlds. A few years ago our A&D customers complained when we switched standards and made Metric units the out-of-the default for our software (all it takes is a single gesture to change the units but we had to be set it to something to start with) . This was actually based a recommendation that I personally made while working in Product Marketing when I was asked to do a study and determined that something over 70% of the software seats that we had installed and were currently servicing were in countries where Metric units were the standard. It was no-brainer as to what the default should be but that didn't stop the outcry from the American aerospace companies, many of whom felt that their's was the dominate industry and that their needs should be taken into consideration when we made our software design decisions and that the rest of the world needed to learn to live with that situation.