How to use the readability checker
- Paste a full paragraph or more. Scores based on one or two sentences swing widely.
- Read the Flesch Reading Ease score and its label. For most web pages, aim for 60 or higher.
- Check the grade level against your audience: around grade 8 suits the general public.
- Look at the highlighted sentences on the right and split or simplify the amber and red ones.
- Edit in the box and watch the scores change as you go.
The two Flesch formulas
Rudolf Flesch published the Reading Ease formula in 1948. In 1975, J. Peter Kincaid and colleagues adapted it for the US Navy to produce a grade level. Both use only two measures of difficulty: how long the sentences are, and how long the words are, measured in syllables.
Syllables are counted with rules that look at vowel groups and silent endings, the method readability software has always used. It is right for most words but can be off by one on unusual ones, which barely moves the score on a full paragraph.
Worked example
The sample paragraph about readability formulas has 47 words in 4 sentences and about 76 syllables. That is 11.75 words per sentence and 1.62 syllables per word, giving a Reading Ease of 58.1 and a grade level of 8.1. Its last sentence has 23 words, so it is the 1 sentence highlighted. Splitting it into two sentences, “…push the grade level up quickly. That can make a page harder…”, raises the score to 60.5.
What the scores cannot tell you
The formulas only count. They do not know whether your ideas are in a logical order, whether a term needs explaining, or whether a short word like “tort” is familiar to your readers. A page of short, simple sentences can still be confusing. Use the score to find sentences worth a second look, then judge them as a reader would.
Other readability formulas
Several later formulas follow the same idea with different inputs. The Gunning Fog index weighs the share of words with three or more syllables. SMOG, often used for health materials, counts those complex words across a sample of sentences. Coleman–Liau and the Automated Readability Index count letters instead of syllables, which is easier for software. Scores from different formulas rarely match exactly, so pick one and track it over time.
Key features
- Flesch Reading Ease and Flesch–Kincaid Grade Level, updated live.
- Long and very long sentences highlighted in place.
- Average sentence length, syllables per word and share of complex words.
- Handles abbreviations such as “Dr.” and decimals without breaking sentences.
- Private: drafts are analysed in your browser.
Practical use cases
Web and marketing copy. Most readers skim online, so short sentences help both people and search engines understand the page.
Public information. US federal plain language guidelines and the UK’s GOV.UK style guide both ask writers to use short sentences and common words for public-facing text.
Health and finance writing. Patient leaflets and consumer finance pages are often checked against a target grade level. Confirm the target with your organisation’s style guide.