Prof Alexander Arguelles recommends this test, but notes that it’s only designed to be accurate up to about 14 000 words families (which a well-read native can easily exceed).
Paul Nation (2006) reckons that 6 - 7 thousand word families should be understood for speaking and listening, and 8 - 9 thousand are required for reading.
I got 26 000. Prof Arguelles got 28 000 (tchah!).
What about everyone else?
@Helen
can you please explain more?
Could you please propose a link to this “test”?
Aargh!
Sorry I just noticed the time and had to start a conversation!
The link is: http://my.vocabularysize.com/
This is the lecture where he talks about it: Extensive Reading and Vocabulary Range - YouTube
I’m keen to use it as a diagnostic tool on my Russian students.
Hmmm … I guess I’m not very well read, I got 15200.
I should note that it is biased toward British English (at least, that’s my excuse). 
…definitely not well read. 11,700?? 
I got 11 300 the first time, and 14 200 the second time (OK for a non-native I suppose).
11 to 15 000 for a non-native sounds good for me.
The professor noticed with surprise that the scores that his non-native students got were generally 7 000 to 11 000, which is what his little boy scored. I don’t find it surprising. Although very well read non-natives can score about 17 000 or maybe more, most language learners I talk to haven’t read the shelves of novels that it takes to gain that wide a vocabulary.
Professor Arguelles thinks it’s skewed towards British English too.
“You know at least 22,500 English word families!” That is what it said.
I left as unknown the following words.
hessian
ruck
alum
erythrocite
some of the definitions are arbitrary
some of the words are not very useful vocabulary items
the definitions are sufficiently different that if you have any sense of the word you get it right
the words should get more difficult as you progress, but they don’t
Anyway, interesting.
23,100 Dammit skyblue : /
Though I did press the wrong answer by accident on one…I know plankton are little animals, not poison plants.
ruck is used in rugby, so I guess all americans and canadians can be forgiven for not knowing it : )
25,000 - Interesting test. I took a stab at all 4 unknown words you mentioned only really knowing ruck, a rugby term.
You know at least 8,500 English word families!
That’s OK for me.
I got 15,700. I did guess on a few of the ones I didn’t know, though, so I might have been lucky on some of them… 
Vera and I reckon there are 2 major ways to build up a really huge vocabulary:
Has anyone read anything to support the theory that watching TV increases learner’s vocabulary?
I scored 21,900. I agree with aybee77 that some of the words were definitely UK/NZ(?) English.
@steve & mark
I’ve never encountered the word “hessian” before myself.
A little baffled that you didn’t know the word “erythrocyte.” You sure you didn’t mean something else?
Also agree with the statement that some of the definitions are ambiguous (or outright wrong, actually, as is the case with the one for “microphone”).
what the #@%$&^%$ is “erythrocyte” ? I have not come across it before and do not expect to in the future.
By the way, we all know over around 10,000 word families or more according to this. How many words have we ever looked up in a dictionary? A few hundred maybe?
I think it is useless to know words that we do not use or come across in our reading and listening. And if we do come across a rare word that we really want to know, we just look it up. But that does not happen too often.
I don’t think that watching TV provide a large amount of new words. Except maybe for the specific words used in the news. But in this case, I think that reading would be more efficient as well.
I got 11,500, thanks to all the words borrowed from French.
As I am unlikely to take the test, I’ll just admit that I do not know the meaning of 'erythrocyte. The other three in Steve’s list get a tick from me. I shall embroider ‘erythrocyte’ onto the next piece of hessian I come across!
I think TV can get you started but will not help build up a “huge” vocabulary. The vocabulary used on TV is quite limited, I think.