The Sentence Length Heatmap paints each sentence in your text green, amber, or red by its word count so readability hotspots are obvious at a glance. Long sentences are one of the strongest predictors of low readability: plain-language standards recommend an average of 15-20 words, and sentences past 25 words force readers to hold too much in working memory. Instead of counting by hand, this tool shows you exactly which sentences to split.
How it works
- Sentence splitting. The text is divided on sentence-ending punctuation —
.,?,!— with guards so abbreviations (e.g.,Mr.,Dr.), decimals (9.99), and ellipses do not trigger false breaks. - Word counting. Each sentence’s words are counted as whitespace-separated tokens containing at least one letter or digit, so prices like
£9.99count as one word and stray punctuation is ignored. - Colour banding. Each sentence is shaded by the band it falls into:
| Band | Word count | Colour |
|---|---|---|
| Easy | under 15 | green |
| Moderate | 15 to 25 | amber |
| Hard | over 25 | red |
The tool also reports the total sentence count, the average length, and the longest single sentence.
Why sentence length matters
Reading is a working-memory task. Short-term memory can comfortably hold around 7 items at once. A long sentence strings together many clauses, qualifications, and sub-thoughts that the reader must hold in parallel while processing the surrounding text — and the load compounds when those clauses are nested or when the subject and verb are far apart. By the time the reader reaches the full stop, the start of the sentence may already be fading.
Plain-English guidance from the UK Government Digital Service, the Plain Language Action and Information Network (PLAIN), and Flesch-Kincaid research all converge on the same finding: an average sentence length under 20 words correlates with better comprehension across most audiences, and sentences above 30 words substantially reduce it.
What to look for in the heatmap
Clusters of amber and red: a block of amber sentences followed by a red one suggests a section that is generally dense. The solution is usually not just to split one sentence but to restructure the whole paragraph.
A single isolated red sentence: often a sentence that grew by accretion — someone added a qualification, a caveat, and then another — without checking the result. Find the main clause and make it the sentence; move the qualifications to a new sentence.
Low average but a very long outlier: the average can look fine while a single 35-word sentence hides inside the document. The longest-sentence statistic surfaces this — fix the longest red sentences first.
Practical techniques for splitting sentences
- Find the first
andorbutthat joins two complete thoughts and replace it with a full stop and a capital letter. - Move any
whichorthatclause into its own sentence: “The report, which was submitted last week and which covers Q1 data, …” becomes two sentences. - Move parenthetical qualifications to the next sentence rather than inserting them mid-clause.
- If a list appears embedded in a sentence with commas, turn it into a bullet list.
Tips
- A low average can still hide one runaway sentence; always check the longest figure and fix the reds first.
- Aim to keep most sentences green, allow occasional amber for rhythm, and eliminate red entirely from instructions, warnings, and notices where comprehension matters most.
- Legal and technical writing legitimately runs longer than marketing or UX copy — calibrate your expectations for the document type.