The two scores
Reading ease is Rudolf Flesch's 1948 formula: 206.835, less 1.015 times the average words per sentence, less 84.6 times the average syllables per word. Higher is easier; most newspapers land between 50 and 70. Grade level is the Flesch-Kincaid formula worked out for the US Navy in 1975, 0.39 times words per sentence plus 11.8 times syllables per word, less 15.59. A grade of 8 means a typical US eighth grader, about 13 years old, can follow it.
What gets marked
Each sentence of 8 words or more gets its own grade. From grade 10 to 13 it is marked hard to read, and from 14 up very hard; splitting it in two usually fixes both. Passive voice is a form of "to be" followed by a past participle, as in "the report was written", including irregular ones like "was taken" and "were made". Adverbs are words ending in -ly, leaving out ones that are not adverbs, such as "only", "family" and "likely". Simpler words are about 60 phrases with a shorter swap, like "in order to" for "to" and "utilize" for "use"; the list under the marked-up copy says what to use instead.
How syllables are counted
There is no dictionary in the page, so syllables are counted by rule: each group of vowels is one, a silent e at the end does not count ("make" is one), "-le" after a consonant does ("ta-ble" is two), and every word has at least one. The rule gets a few words wrong, such as "everybody", so scores can differ from other checkers by a point or so. The marks are a guide too: a passive sentence or an adverb is sometimes the right choice.
Questions
What grade level should I aim for?
For anything the general public reads, such as a website, a letter or instructions, aim for grade 8 or below; around half of US adults read most comfortably at that level. School essays and reports can sit higher, but a grade above 12 usually means the sentences are longer than they need to be.
Why does a short text give an odd score?
Both formulas are averages over sentences and words, so a few sentences can swing them a long way. A one-line text with a single long word can score below zero or above 100. Paste a paragraph or more for a number worth comparing.
Is my writing sent anywhere?
No. The scores and the marks are worked out in this page as you type. Nothing is uploaded or saved, and closing the tab clears it.