How the sentence count actually works
A period, question mark, or exclamation point almost always ends a sentence, but not quite always: "Dr. Chen arrived at 3 p.m." has two periods that stay inside one sentence, and "The total came to 3.14 dollars" has a period that's just a decimal point. This tool scans for runs of . ! and ?, then holds back on splitting when a single period sits between two digits, follows a word from a short list of common abbreviations (Dr., Mr., Mrs., Inc., approx., etc., and similar), follows a two-letter or longer dotted abbreviation like "p.m." or "U.S.", or follows a capital letter standing alone as an initial ("J. K. Rowling"). Everything else, including a run like "?!" or an ellipsis, counts as one sentence boundary.
That rule set has a known blind spot: it always treats an abbreviation's period as continuing the sentence, so a sentence that actually ends right after "Inc." or "etc." gets folded into the one after it. There's no punctuation-only rule that resolves every case correctly, since English itself doesn't mark the difference; a period after "St." means one thing in "St. Louis" and another at the end of "Turn left on Main St." with nothing else to go on. Lightweight sentence counters generally accept this trade-off rather than trying to parse full grammar, because it gets the overwhelming majority of ordinary prose right.
Where the readability formulas come from
The Flesch Reading Ease score runs from roughly 0 to 100, with higher numbers meaning easier text: 206.835 minus 1.015 times the average number of words per sentence, minus 84.6 times the average number of syllables per word. Rudolf Flesch published that formula in 1948. The Flesch-Kincaid Grade Level rescales the same two inputs onto a US school-grade scale: 0.39 times average words per sentence, plus 11.8 times average syllables per word, minus 15.59. J. P. Kincaid, R. P. Fishburne, R. L. Rogers, and B. S. Chissom derived that second formula in a 1975 report for the US Navy, testing it against reading scores from enlisted sailors and building it from the same word and syllable counts Flesch's formula uses.
Both formulas need a syllable count, and there's no way to get an exact one without a pronunciation dictionary. This tool estimates syllables by counting groups of vowel letters in each word, which handles most everyday words correctly but can miss by a syllable or two on an irregular word (silent letters, unusual endings, extra vowel clusters). That kind of miss moves the Reading Ease score by a few points at most, not enough to jump a paragraph from one reading-level band to another.
How reading time is estimated
Reading-time estimates on this tool use 238 words per minute, the average adult silent reading rate for non-fiction English text found by Marc Brysbaert's 2019 meta-analysis of 190 studies covering more than 18,500 participants. The same analysis found fiction reads faster on average, at 260 words per minute, and reading aloud is slower, at 183 words per minute; this tool defaults to the non-fiction figure since most pasted text (emails, articles, reports, essays) fits that category better than fiction does. Any single reader can run faster or slower than the average, so treat the estimate as a rough planning number, not a stopwatch result.
Two worked examples, plain versus dense
"Read the instructions first. Then fill in each blank. Check your answers before you submit the form." is 3 sentences and 17 words, averaging 5.7 words per sentence and 1.3 syllables per word. That lands a Flesch Reading Ease score of 91.6, in the "Very easy" band, and a Flesch-Kincaid grade level of 1.9, meaning a strong first- or second-grade reader could follow it.
"Notwithstanding the aforementioned stipulations, applicants seeking supplementary compensation must, prior to formal adjudication, substantiate their eligibility through comprehensive documentation verifying continuous employment throughout the preceding fiscal quarter." is one 27-word sentence averaging 2.9 syllables per word. The formula scores it at -65.0, below the bottom of the 0-100 scale and squarely in the "Very confusing" band, with a Flesch-Kincaid grade level over 29, past any real school grade. Same idea, expressed in long words and one long sentence instead of three short ones, and the readability score moves by more than 150 points.