Cost is one reason the U.S. has been slow to adopt the metric system. Converting technical drawings and operations manuals for complex equipment with many parts can take thousands of man-hours. NASA engineers, for example, recently reported that converting the space shuttle's relevant drawings, software and documentation to SI units would require $370 million -- about half the cost of a typical space shuttle launch [source: Marks].
Of course, cost alone can't explain America's reluctance to go metric. Certain psychological attributes also play a significant role. American stubbornness makes its citizens resistant to change, especially when that change is being driven by foreign governments. Perhaps citizens still harbor distrust and ill will for being snubbed by the French when the metric system had its coming-out party in 1798. Or, more likely, they simply like doing things a bit differently. Individualism has always been a defining characteristic of the American experience. The quality allowed pioneers to hack a nation from an immense wilderness. You could easily envision bumper stickers with a variation on the old National Rifle Association (NRA) slogan: "You can have my inch-pounds when you pry them from my cold, dead hands."
The most logical explanation, however, just may be the failure of Congress to make the metric system mandatory in all 50 states, the District of Columbia and its territories. By making conversion voluntary in all major legislation since 1866, the U.S. has failed to restrict the use of traditional units in transactions that touch the daily lives of ordinary citizens. Until that mandate comes -- and it will likely come soon if the U.S. is to remain competitive with growing economic powers, such as China and India -- many Americans will continue to think in terms of inches and pounds instead of meters and kilograms.