How much extra storage does MySQL fulltext index consume?

I am looking for a way to estimate how much extra storage will be needed if a MySQL TEXT column gets a fulltext index.

Intuitively speaking the size of the fulltext index is depende开发者_StackOverflownt on the total length of text but what exactly is affecting it?

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"I have 200 MB of text data to index and the resulting index is 130 MB."

So it's comparable to the total size of data you index.