When you read English, you often meet new words. How many times do you need to meet a word to learn it? In a study in Japan, students read an easy book with some new words. Words they met fewer than eight times were mostly forgotten after three months.

Another study found that ten meetings helped a lot, but it still was not enough. So read a lot, and also study new words on purpose.

Learning words while reading or listening for enjoyment is called incidental vocabulary learning. But how many encounters does it take? In 2003, Waring and Takaki asked 15 Japanese learners to read a graded reader that included made-up words appearing from once up to 15 to 18 times. Words met more often were remembered better, but words met fewer than eight times had almost completely lost their meaning for the learners three months later.

In 2007, Webb compared one, three, seven and ten encounters and found that knowledge grew with each encounter, especially at ten. Even then, the learners did not fully know the words. A 2019 meta-analysis found a moderate relationship between the number of encounters and learning, so other factors also matter.

Picking up words while reading or listening for meaning, known as incidental vocabulary learning, is often presented as an effortless route to a large vocabulary. Research on repetition offers a more sobering view. In 2003, Waring and Takaki had 15 intermediate Japanese learners read a graded reader containing made-up words that appeared anywhere from once to 15 to 18 times.

Frequently encountered words were learned and retained better, but the meanings of words met fewer than eight times had been almost entirely forgotten three months later. Webb's 2007 experiment with Japanese learners compared one, three, seven and ten encounters and showed that several aspects of word knowledge, including spelling, meaning and grammatical use, grew together with repetition; yet even ten encounters did not produce full knowledge.

A 2019 meta-analysis of 26 studies with about 1,900 participants found a moderate correlation of .34 between encounters and learning, with age, prior vocabulary, visual support and spacing also playing roles. A 2023 meta-analysis concluded that meaning-focused reading or listening typically leads learners to acquire only about 6 to 18 percent of new words. There is no single magic number of encounters. Extensive input works best when combined with deliberate review.