TestYourVocab.com is part of an independent American-Brazilian research project to measure vocabulary sizes according to age and education, and particularly to compare native learning rates with foreign language classroom learning rates.

It functions by means of a quick three-part test: the first part with a handful of words to determine the general vocabulary level, a second part with a larger but narrower selection of words to determine the vocabulary level with greater precision, and a final (optional) survey to collect statistical information.

The site provides accurate results for virtually everyone, from very small children (with answers input by parents) to professional linguists. It can calculate vocabulary sizes from less than 100 words to more than 40,000 words.

For those interested in exactly how it works, please see the nitty-gritty details page. Or visit the Brazilian version of the site.

(New: we've recently launched a related tool, HardestWords.com, that analyzes a text and produces a report listing the "hardest", or rarest, words in the text.)