Why doesn't the US use the metric system?

The US was half-heartedly interested in conversion to the metric system during the 60's, and there were probably other periods of interest as well. It is something that the US should do; the system is as accurate as any other (since accuracy is more a function of equipment and methodology, not defined units) but there is so much industry here, and the cost in down-time alone to re-tool would be so great, that this has not been much of a priority. The "rest of the world" uses the metric system; the U.S. is like the "lone ranger" on this issue. It would be a benefit to everyone if we would just accept the inevitability of switching systems. The metric system is important, and it is here to stay.

Because they use the older Imperial system invented by the romans the British also use the same measurement system, but most other coutries have adopted the metric system.

[update] The British no longer use the Imperial system.

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Eventually the USA and several other obscure territories will have to introduce the metric system.

And the later it will be even more expensive and difficult. I am surprised to find that most people in USA do not learn or know the SI system.

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Americans are trying to learn the metric System and its SI derivative, judging by all the arithmetic-homework questions on this site! Americans in general might not use them for everyday purposes - good for them say I - but US scientists and engineers are using the SI units more and more. They have to do so when writing for international technical publications, let alone in physical collaborations.

Officially, Britain has changed to the metric system but the Imperial measurements hang on mainly by habit, and in a few specified fields. The Statute Mile and Yard are still the only legal units for road distances in the UK, for example, and pubs are allowed to dispense beer in pints and half-pints although wines and spirits are in metric measures. Britain's railways appear to use kilometres (note spelling - French word, and a "meter" is a device that measures!) in some places, but Miles and Chains elsewhere. (1 chain = 22 yards = 1/80 mile.)