How many words does a native English speaker know?

If you search for this, you'll find answers anywhere from 10,000 to over 100,000. They can't all be right, but most of them aren't really wrong either. They're answering slightly different questions.

What counts as a word?

Take "walk". There's also "walks", "walked" and "walking". Are those four words or one? Most researchers count them as one, because they're just different forms of the same word. That unit is called a lemma.

Now add "walker". It's related, but it means something different, so it's counted as a separate lemma. Some studies go a step further and group "walk", "walker" and everything else built on it into one word family, which brings the total down a lot.

So the same person can be said to know about 11,000 word families or about 42,000 lemmas, depending on how you count. Both numbers describe the same vocabulary. It also means you can't directly compare a figure from one test with a figure from another, including ours.

The best estimate we have

The most thorough work on this was published in 2016 by Marc Brysbaert and his colleagues at Ghent University. They combined earlier research with a large online experiment in which people marked which English words they knew.

Their conclusion was that an average 20-year-old native speaker of American English knows about 42,000 lemmas. Counted as word families, that's about 11,000.

The differences between people were large. The bottom 5% knew around 27,000 lemmas, and the top 5% around 52,000.

It doesn't stop at 20

Vocabulary keeps growing through adult life. According to the same study, people learn about 6,000 more lemmas between the ages of 20 and 60. That works out to roughly one new word every two days, picked up from reading, work, the news and conversation.

Children learn much faster. During their school years they add thousands of words a year, most of them from reading rather than from being taught.

Knowing a word isn't one thing

When studies ask whether someone knows a word, they usually mean recognising it and knowing roughly what it means. The words you actually use when you speak or write are a smaller set. Most people understand plenty of words they'd never think to use themselves.

For a sense of scale, the second edition of the Oxford English Dictionary has full entries for about 171,000 words in current use. Even a very well-read native speaker knows only a fraction of them, and nobody needs most of them.

If you're learning English

Don't be put off by 42,000. Native speakers take twenty years to get there, and many of those words are rare ones you'll hardly ever meet. The most common few thousand words do most of the work. With them you can follow everyday conversation and much of what you read, and every thousand after that fills in more of the gaps.

A more useful question than "how far am I from a native speaker?" is "how far have I come since last time?"

Source: Brysbaert, M., Stevens, M., Mandera, P., & Keuleers, E. (2016). 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. Frontiers in Psychology.