A readability score is a number that estimates how easy a piece of writing is to read. It does not know what the text means; it measures how long the sentences are and how long the words are, on the sensible basis that short sentences and short words are easier. That is a crude measurement, but it is a useful one, and it is how newspapers, insurers and government agencies check whether the text they publish is within reach of ordinary readers.
Paste at least a few sentences. The tool gives you an overall reading level, then a table of six scores.
The scores
Flesch Reading Ease gives a number, usually between 0 and 100, where higher is easier. 60 to 70 is “plain English” and the target for most public-facing writing; 30 or below is difficult, academic prose; 90 or above is children’s writing.
Flesch-Kincaid Grade converts the same ingredients to a US school grade. A score of 8 means an eighth grader should follow it. It is the figure built into some word processors.
Gunning Fog estimates the years of formal schooling a reader needs. Anything over 12 is hard going for a general audience.
SMOG looks at the number of long words (three or more syllables) and is popular in healthcare for patient information. It needs at least three sentences and works properly on 30 or more.
Coleman-Liau and the Automated Readability Index use letters per word instead of syllables, which makes them easier for a computer to get right. They are cross-checks, not the headline.
When the scores disagree by a grade or two, that is normal. Treat them as a range.
What these scores cannot judge
They know nothing about content. A confusing paragraph made of short, simple words can score as “easy”. A clear explanation of a technical subject with necessary long words can score as “hard”. Syllables are estimated using English spelling rules, so unusual words, names and foreign words can be off by one. And all of the formulas were designed for passages of 100 words or more; on a single short sentence the result is a rough guess.
Using the result
Look at what moves the score. Long sentences and long words both raise the level, and the table shows the average words per sentence and syllables per word so you can see which one is responsible. Splitting one 40-word sentence into two is nearly always better writing and always a better score. Replacing “utilise” with “use” does the same for words.
Do not chase a perfect score; a score of 60 to 70 for web copy is plenty. The syllable counter shows how syllables are counted word by word, and the word counter gives the sentence and word totals. For how long a piece takes to read, use the reading time calculator.