Apparently (I read somewhere) you don’t want a Fog Index score greater than 20 if you are giving a speech to a live audience. This is important to those of us that feel we should write as if we were going to read our writing to a live audience, because this implies our writing should also have a Fog Index score not greater than 20.
The Fog Index is a type of readability test for your writing that essentially indicates how long–on average–your sentences and words are. The longer the words and sentences, the more difficult it is for readers to read, and the higher the score will be.
If you want a quick tool that calculates multiple readability tests (including the Fog Index) for your writing, check out: The Readability Test Tool.