This is a very common question, but nobody really knows the answer. The only way to determine accurately the number of people who speak Esperanto would be to conduct a world-wide census, and of course this has never been done.
However, Professor Sidney S. Culbert of the University of Washington, Seattle, USA, has done the most comprehensive survey on language use ever attempted. He has conducted interviews in dozens of countries around the world and tested for "professional proficiency", i.e. much more than just "hello, please, goodbye".
Based on this survey, Prof. Culbert concluded that Esperanto has about two million speakers worldwide. This puts it on a par with "minority" languages such as Lithuanian or Hebrew. More information on this survey is available, partly in Esperanto. Complete results for all languages with more than one million speakers are published in the World Almanac and Book of Facts.
[There's a lot of debate over how many people speak Esperanto. Sometimes there is a tendency to exaggerate the number of Esperanto speakers, or, on the contrary, to minimize it. I've seen numbers ranging from 100 000 to 8 million. Prof. Culbert's estimate has two advantages over any other I've seen:
1. The method is sound. Doing a world-wide survey is the only valid way to estimate the number of Esperanto speakers, but it's so difficult that Prof. Culbert is the only person who has ever attempted to do so, to my knowledge.
2. The study attempted to find out how many people speak all languages, not just Esperanto. We can see whether the results obtained for other languages make sense; if they do, then the result for Esperanto is probably as valid as any other.
In short, Prof. Culbert's estimate that two million people speak Esperanto around the world is the most accurate answer we're likely to get. -- Ed.]
Some parents teach Esperanto (along with the local language) to their children; it is estimated that perhaps a thousand people speak Esperanto as a first language.
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