A different interaction design is used by https://testyourvocab.com : just a list of words with a checkbox for each. But it might encourage overconfidence. Before their acquisition by Preply, they also had an interesting blog with statistical analysis: https://web.archive.org/web/20210724115604/http://testyourvo...

The two tests give me widely different results, probably because the sampled words aren't perfectly representative and so the results should have huge error bars to account for this sampling error.

I (native American English speaker, college prep school educated) had 5 words that I thought I knew, but still got wrong:

obsequious

laconic

sanguine

quotidian

enervate

On the other hand, I was able to correctly guess these words that I'd never seen before:

omphaloskepsis

crepuscular

absquatulate

callipygian

houghmagandy

quire

And then there were these, which were just totally foreign to me:

hippopotomonstrosesquippedaliophobia

nudiustertian

ergophobia

tittynope

Final estimate: ~73000 words