What sentence length is considered readable?

Plain-language guidance recommends an average sentence length of 15-20 words. Sentences under 15 words read easily, 15-25 are acceptable in moderation, and sentences over 25 words are hard to follow and should usually be split.

How does it decide where a sentence ends?

It splits on sentence-ending punctuation — full stops, question marks, and exclamation marks — while guarding against common abbreviations like 'e.g.', 'Mr.', and decimals so they do not cause false splits.

What's the difference between average and longest?

Average sentence length shows the overall reading load, while the longest sentence is often the single worst offender. A low average can still hide one very long sentence, so the tool reports both.

Does word count include numbers and symbols?

It counts whitespace-separated tokens that contain at least one letter or digit, so '£9.99' counts as one word and standalone punctuation is ignored. This matches how readability formulas treat words.

No. Splitting, counting, and colouring all happen locally in your browser. The text never leaves your device, so you can check unpublished or confidential drafts safely.

Sentence Length Heatmap

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 £9.99 count 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.

Tips and notes

A low average can still hide one runaway sentence, so always check the longest figure and fix the reds first.
The quickest fix is to split a long sentence at its first conjunction (and, but, which) into two shorter statements.
Aim to keep most sentences green, allow occasional amber for rhythm, and eliminate red entirely from instructions and notices where comprehension matters most.