A child starting school probably knows and uses an average of 1,500 words. Day-to-day English conversation generally requires knowing 3,000 words, more or less, and a common modern translation of the Bible uses a vocabulary of around 8,000 words. The works of Shakespeare can be written out using a vocabulary of about 35,000 words. So how do you know when your vocabulary is big enough?

The answer to that question might first require another question. How big do you need your vocabulary to be? If your active vocabulary (the words you both know and use) includes all the terms you need to do your job, interact with friends and family, read the books that interest you, and understand the news you hear on the television or read on the internet or in the newspaper, then you might be satisfied that you’ve got the vocabulary you need. But are you really taking full advantage of the richness of the English language? An educated adult who is a native English speaker should generally know around 20,000 word families to consider their vocabulary “big enough.”

A “word family” is a group of words formed by adding on to (also called “deriving” or “inflecting”) a base word form. For example, the words read, reads, reading, reader, and readable are all part of the same word family. Once you know the affixes (the additions to the base word), you can easily learn groups of words in the same family. According to many linguists, if a person knows approximately 1,500 word families, they will be able to read and understand around three-quarters of standard English texts; with 15,000 word families memorized, they’ll have a nearly 98% fluency. You can research word families by using this tool developed by an educational organization in the UK, the British Council.

Paul Nation, of the Victoria University of Wellington (New Zealand) School of Linguistics and Applied Language Studies, has made a study of English vocabulary and how many words a person really needs to know in order to be able to speak English with a degree of fluency. His research has involved studying how people acquire vocabulary through reading, listening, and writing, and he has written several books on methodologies for teaching English as a second language. He has developed an initial vocabulary size test designed for non-native speakers, but it’s interesting to use as a benchmark for native speakers as well. You can download the test (a PDF file) and check to see where you are in your English vocabulary study by clicking here.