Vocabulary size can be a mystery to many people. One study (Diller, 1978) shows that an American high school graduate knows, on average, 216,000 words!
Don’t believe it? Well, the researcher used a most scientific way of investigation which is employed by all vocabulary-size calculation formulas: He sampled Webster’s Third New International Dictionary, which contains 450,000 words, and found that a high school student knows 48% of the sampled words, which amounts to a vocabulary size of 216,000.
An authority on English vocabulary (Nation etc., 1990), however, maintains that “well-educated adult native speakers of English have a vocabulary of around 17,000 base words.”
The huge discrepancy exists because rigorous or relaxing standards can be adopted in dealing with a lot of factors including but not limited to:
Shall proper nouns be included or excluded?
Shall all word forms (including inflections) or only base words be counted? (Shall love, lover, loving, lovingly be counted as one “base word”?)
Shall we pick words randomly, like sampling a dictionary, or shall we use the most frequent words?
Prof. Mark Davies from Brigham Young University built Corpus of Contemporary American English (COCA), which adds an annual input of 20 million-word text and speech to the database since 1990 and now contains 520 millions words. Bases on COCA, he provides “the most accuratefrequency data of English” . You can find the most frequent 60,000 lemmas.
The same words in different word classes (parts of speech) are different lemmas, so love as a noun is a different word than love as a verb. To combine the frequencies or lemmas with the same word form but different word classes brings us closer to our intuition of what a word is, and can serve as a basis for investigating our vocabulary size.
So here is the 26 words picked from the Table of Contents of Core Vocabulary of English: 4,000 Upper-intermediate Words (Step by Step Towards Core Vocabulary of English Book 3). Count how many words you know and multiply the number by 154, and you will get your vocabulary size in the Upper-intermediate level. (This is supposed to work for high school students and ESL learners).
Here is the book: