How Many Words Do We Know? Practical Estimates of Vocabulary Size Dependent on Word Definition, the Degree of Language Input and the Participant’s Age

The findings so far are summarized in Table 4 and translated into reasonable estimates of words known. They show that the estimates depend on (a) the definition of word known, (b) the age of person, and (c) the amount of language input sought by the person. For the number of alphabetical types encountered, the low end is defined as a person who only gets input from social interactions; the high end is a person who constantly reads at a pace of 300 words per minute. For the number of lemmas and base words known the low end is defined as percentile 5 of the sample we tested, the median as percentile 50, and the high end as percentile 95.

The number of lemmas known arguably is what most people spontaneously associate with the answer to the question ‘how many words are known.’ The number of base words (word families) mastered indicates that these lemmas come from a considerably smaller stock of building blocks that are used in productive ways. Notice that the average number of words known by a 22-year-old (17,200), as estimated by Goulden et al. (1990), lies in-between our estimated number of lemmas known (42,000) and our estimated number of base words known (11,100), in line with the observation that Goulden et al. (1990) used less invasive criteria to define their base words.

Multiplying the number of lemmas by 1.7 gives a rough estimate of the total number of word types people understand in American English when inflections are included. The difference between alphabetical types encountered and the number of inflected forms known gives an estimate of names and unknown words people see in their lives. As Nation (2006) argued, it is easier to understand how a 20-year-old could have learned 11,100 base words (which amounts to 1.7 new words per day if learning commences at the age of 2) than to understand how they could have acquired 1.7∗42,000 = 71,400 new word forms (inflections included), which amounts to nearly 11 new words per day (see Nagy and Anderson, 1984, for such an estimate).

The estimates of Table 4 are for receptive vocabulary (understand a word when it is presented to you). Productive word knowledge (being able to use the word yourself) is more limited and estimated to be less than half receptive knowledge. The difference between receptive and productive word knowledge increases as the words become less frequent/familiar (Laufer and Goldstein, 2004; Schmitt, 2008; Shin et al., 2011).