Find out how many words you know. Tick the words you recognise and get an estimate of your vocabulary. Free, no account.
Words are grouped by how common they are: the first thousand most frequent, the second thousand, and so on up to rare words. A few words are shown from each group. If you know half the words shown from a group, you are assumed to know about half the words in that group. The groups are then added up.
It does not check whether you can use a word in speech — only whether you recognise it. Recognition is always wider than active vocabulary. And it is an estimate: the result moves between attempts, which is why the number is rounded to hundreds.