How can you estimate how many people know a word? [closed]

There are already plenty of sites that tell you the relative frequency of a word in various corpuses, but how can you use that number to find an estimate of what % of native English speakers know that word?

I think I remember using a site that would give you a rough estimate, i.e. "most", "some", or "few people know this word". Does anybody know what that site is? Does anybody know of a site that is more precise?

I don't think that this measurement of proportion knowing word X can be done directly.

An alternative would be to use the Flesch-Kincaid index, which gives a readability by grade level. This tool takes into account the difficult of the vocabulary and length of the sentence, among others.

As your ultimate goal is unclear, perhaps these questions and answers at this site might get you started.

• Using the online tool to rate a difficult sentence from philosophy: https://english.stackexchange.com/a/364259/3306
• See the Pearson's Level 6 list in a comment at English books reading complexity ratings
• Using the Okapi tool: What are reading comprehesion tools that can generate or suggest plainer language?