Why three more formulas, beyond Flesch
Flesch Reading Ease and Flesch-Kincaid Grade Level are the most widely cited readability formulas, but they're not the only ones in real use. This tool also calculates the Gunning Fog Index, the SMOG Index, and the Coleman-Liau Index - three other published, peer-reviewed formulas that weigh sentence length and word complexity slightly differently. Seeing several formulas agree (or disagree) on a piece of text gives a more reliable picture than relying on any single number, since each formula has its own blind spots.
How Gunning Fog, SMOG and Coleman-Liau differ from Flesch
Gunning Fog and SMOG both key off "complex words" (words of three or more syllables) rather than average syllables per word, so a text with a handful of long technical terms scattered through otherwise simple sentences can score harder on these than on Flesch. Coleman-Liau is unusual in that it doesn't use syllable counts at all - it estimates difficulty from average letters per word and average sentence length, which makes it more resistant to the syllable-counting approximation errors that affect the other formulas.
What the "hardest sentences" list is for
Beyond a single score for the whole text, this tool also surfaces the two or three individual sentences that are the longest and most syllable-dense - the sentences doing the most to drag the overall score down. That's usually more actionable than the score alone: splitting or simplifying those specific sentences often improves readability more than a general rewrite of the whole piece.
What these scores mean
The Flesch Reading Ease score runs from 0 to 100 - higher means easier to read, based on average sentence length and average syllables per word. The Flesch-Kincaid Grade Level translates the same inputs into a rough US school grade needed to understand the text (a score of 8 suggests an eighth-grader could follow it). Shorter sentences and simpler, shorter words push both scores toward "easier"; long sentences packed with multi-syllable words push them toward "harder".
Why both scores are shown, not just one
Reading Ease gives an intuitive 0-100 scale that's easy to compare across pieces, while Grade Level translates the same underlying math into a more concrete, familiar reference point (a school grade) - showing both covers people who find either framing more intuitive, since they're derived from the same sentence-length and syllable-count inputs.
Why average words per sentence matters on its own
Sentence length is one of the two direct inputs driving both scores, so this number alone explains a lot of why a piece scores the way it does - a text with a reasonable vocabulary but consistently long, multi-clause sentences will still score as harder to read than one using shorter sentences, even with similar word choices.
What target score actually fits your content
There's no single correct target - general web content and marketing copy often aims for a Reading Ease score of 60-70 (comfortably understood by a broad audience), while technical, academic, or legal writing naturally scores lower and isn't necessarily "wrong" for scoring that way, since its audience and purpose are different from general content.