Based on Paul Nation’s BNC/COCA word family lists, which rank the most frequent 25,000 English word families using data from the British National Corpus and the Corpus of Contemporary American English. This site uses all twenty-five 1,000-word bands, from the full published list (1k–25k).
Adapted from the Wordset dictionary, derived from Princeton University’s WordNet. Licensed under CC BY-SA 4.0.
The test is adaptive: it starts at the easiest band and climbs one band per correct answer, backing off on a miss, with the step size shrinking each time you reverse direction (a staircase procedure). This converges on your vocabulary “ceiling” in far fewer questions than testing every band directly. Bands you weren’t directly tested on are inferred: assumed fully known below your estimated threshold, unknown above it. This mirrors the basal/ceiling approach used by academic vocabulary size tests such as Nation & Beglar’s Vocabulary Size Test.