Every readability formula ever published measures the same two things: how long your sentences are and how long your words are. None of them reads your prose. A passage can score beautifully while being confusing, and a difficult passage can score badly while being perfectly clear, because sentence length is a proxy for difficulty rather than difficulty itself.
Knowing that is what makes the number useful. This calculator runs six formulas at once — Flesch Reading Ease, Flesch-Kincaid, Gunning Fog, SMOG, Coleman-Liau and the Automated Readability Index — and averages the five that report a school grade. Seeing them together shows you where they disagree, and disagreement is usually more informative than any single figure.
It also counts syllables by looking for vowel groups rather than consulting a dictionary, which is fast and approximate. Invented names, unusual spellings and heavy dialogue all push that estimate around, so treat the output as a measurement of your sentence and word lengths rather than a verdict on your writing.
The averaged grade level, as a single summary, and then the spread between the individual formulas as a second signal. They were each calibrated on different material — technical manuals, school textbooks, health leaflets — so they weight word length and sentence length differently. Where they agree you can be reasonably confident; where they diverge, something specific is happening in the text.
It is an approximation. The counter looks for groups of vowels in each word rather than checking a pronunciation dictionary, which handles ordinary English well and struggles with invented names, unusual spellings and words where vowels do not map to spoken syllables. For comparing two drafts of the same book the approximation is consistent enough to be useful.
That depends entirely on who you are writing for, and this tool deliberately does not prescribe a target. Published guidance on ideal scores by genre is mostly repeated assertion rather than measurement. The more reliable use is comparative: score a book your readers already enjoy, then see where yours sits relative to it.