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

Although it is correct that a language can contain an unlimited number of alphabetical types, this does not address the question how many word types people are likely to know, or, in other words, how large their vocabulary is. For this, we need practical answers to the following two questions: ‘How many alphabetical types are people likely to have encountered in their life?’ and ‘How many of these alphabetical types do we consider as words of a language?’ We will start with the first question.

The number of alphabetical types that people can come across is limited by the speed with which they process language. As we will show, the existence of corpora of hundreds of billions of word tokens should not mislead us into thinking that such exposure is possible in a lifetime. In addition, it is important to consider individual differences: Not everyone consumes language at the same speed. We will distinguish between three theoretical cases: (a) a person who gets all language input from social interactions, (b) a person who gets all language input from television programs, and (c) a person who gets all language input from constant reading. As we will see, this distinction gives rise to major differences in the number of words encountered. In addition, we must consider age: All else equal, older people will have encountered more words.

Mehl et al. (2007) asked a group of male and female students to wear microphones during the day and recorded 30 s of sound of every 13 min (the duration was limited to assure the participants’ privacy). On the basis of the data gathered, Mehl et al. (2007) estimated that their participants spoke about 16,000 word tokens per day, with no significant difference between men and women. Assuming that participants listened to the same amount of word tokens, the total input from social interactions would be equal to 16,000 × 2 × 365.25 = 11.688 million word tokens per year. For a 20-year-old, this adds up to about 234 million word tokens (assuming full input from day one); for a person of 60 years, it grows to slightly more than 700 million word tokens. Applying Gerlach and Altmann (2013) equation, this would predict that a 20-year-old has encountered 84,000 alphabetical word types while a 60-year-old has encountered 157,000 alphabetical word types. In all likelihood, the corresponding vocabularies would be smaller than the number of encountered types. One reason is that word diversity is considerably larger in written materials (on which Gerlach and Altmann’s estimate is based) than in spoken language (Hayes, 1988; Cunningham and Stanovich, 2001; Kuperman and Van Dyke, 2013). Another reason is that one cannot assume all word types to be memorized at their first encounter.

Van Heuven et al. (2014) sampled all subtitles from BBC1, the British public broadcaster’s most popular TV channel. On the basis of this corpus, it can be estimated that the yearly input for someone who constantly watches the channel, is 27.26 million word tokens per year (i.e., more than twice the input from social interactions). This results in a total input of 545 million word tokens for a 20-year-old and 1.64 billion word tokens for a 60-year-old. The numbers are likely to be overestimates, as broadcasts go on for some 20 h per day.

Reading rate is estimated between 220 and 300 word tokens per minute, with large individual differences and differences due to text difficulty and reading purpose (Carver, 1989; Lewandowski et al., 2003). If we take the upper limit and assume that a record reader reads 16 h per day, this gives a total of 300 ∗ 60 ∗ 16 ∗ 365.25 = 105 million word tokens per year. For a 20-year-old this adds up to 2.1 billion word tokens, and a 60-year-old has seen 6.3 billion word tokens. Applying Gerlach and Altmann’s (2013) equation, this would imply that these two people have encountered 292,000 and 543,000 alphabetical word types, respectively.

In summary, based on our assumptions about the amount of tokens encountered in different modalities and on the relationship between word tokens and word types, we can roughly estimate the number of alphabetical word types one has likely encountered: A 20-year-old exposed exclusively to social interaction will have encountered around 81,000 alphabetical types, while a 20-year-old exposed non-stop to text will have encountered around 292,000 different alphabetical types. For a 60-year-old, the corresponding estimates are 157,000 and 543,000 alphabetical types, respectively. As we will see in the next sections, we would not ordinarily consider all these alphabetical types as words of a language.