How many words are in the English language?

Counting words is tricky because of plurals, compound words, slang, technical terms and words that have fallen out of use. The Oxford English Dictionary’s online edition includes roughly 500,000 or more entries and word forms, and it keeps adding new ones each year. A 2010 Harvard–Google study analysing millions of digitised books estimated English had about a million words. Most adult native speakers actively know around 20,000 to 35,000 words. Everyday conversation relies on a much smaller core, and roughly 3,000 common words cover most of what we read and hear.