In spite of an attempt to convert the United States to the metric system of measurements in the 1970s and 1980s, the nation is one of the few in the world that still have not adopted metric into everyday use. Many of the other English-speaking countries did a conversion around the same time or a few years earlier (e.g. Australia, Britain, Canada, New Zealand, etc.) and in most cases have been more successful. Some say that the U.S. culture is stubborn to change, but I have another possible reason: the generational constellation in place at the time of the conversion attempt.
As Strauss and Howe have observed, the generations in the U.S. run a few years ahead of most of the other countries mentioned (except for maybe Canada), and the American attempt at conversion was a few years behind. What significance does that have? In the U.S., if you find someone who is strongly anti-metric there's a good chance they're a Boomer. For some reason Idealist/Prophet generations (of which the Boomers are one) appear to me to be the most resistant for learning new ways. You may think it is simply a function of age, but when the initial push to convert the U.S. to metric in the mid-1970s was made most of the legislators and others in power were members of the G.I. and Silent generations. By that time there were many Boomers well into young adulthood, who probably contributed to the stalling of the metric conversion effort.
In the other countries I mentioned, the push to metric was made a few years earlier. That along with most of them being a little behind the Americans on their generations meant that a change-resistant (in this arena; they are certainly change-inducing culturally though) Prophet generation had not yet exerted as a major force which enabled metric conversion to progress further.
So when can we expect a realistic chance at the U.S. going metric? A good guess is once the Boomers lose their majority in government control and we have less change-resistant Xers, Millennials, and Homelanders to help push us in the metric direction.
Here is the Wikipedia article on metrication (and the source for the metrication facts mentioned).