The World Almanac and Book of Facts publishes a table of the number of speakers of various languages. For many years, this information was provided by Prof. Sidney Culbert, who included a figure of one million Esperanto speakers, later increased to two million.
In the 1980s, as a young Esperantist who had heard many figures for the number of Esperanto speakers (including some far higher than two million), I wrote to Prof. Culbert for information about his estimates. Apparently he frequently received such questions. Along with a personal note, he sent copies of two letters, from 1967 and 1969.
In 1989, having repeated his estimates in various places, I asked Prof. Culbert for more information on how he derived his estimates. His response is the third letter.
The letters as shown here are almost exactly as I received them. Minor typos have been cleaned up, for example “eaach” -> “each”. A very long paragraph in the “Methods” letter has been broken up for legibility. Some typos may remain from the scanning process. Other notes and my translations from Prof. Culbert’s Esperanto are marked by double-parentheses “((note))”. My thanks to Jim Henry for major cleanup work on my translations.
Prof. Culbert passed away in Seattle, Washington on October 28, 2003.