WordToolboxReadability formulas differ in their arithmetic but agree on two inputs: sentence length and word length. Neither measures whether writing is good — they measure how much effort a reader spends decoding structure before they reach meaning.
Why long sentences cost the reader
A reader holds a sentence's grammatical structure in working memory until it resolves. Every extra clause postpones resolution. Past roughly 30 words, comprehension drops sharply not because the ideas are harder but because the scaffolding is.
Target averages by content type
| Content type | Average words per sentence | Notes |
|---|---|---|
| Landing page and ad copy | 8–14 | Skimmed, often on mobile |
| Blog posts and marketing articles | 14–20 | The general web default |
| News writing | 15–20 | Front-loaded facts |
| Technical documentation | 15–22 | Short sentences, precise nouns |
| Academic writing | 20–28 | Longer clauses are conventional |
| Scripts read aloud | 10–16 | Must fit one breath |
Variation matters more than the average
Ten consecutive 18-word sentences average well and read like a metronome. Mix a 6-word sentence against a 28-word one: the short sentence lands the point, the long one carries the reasoning. Averages are a diagnostic, not a target to flatten toward.
How to edit with the numbers
- Paste a draft section into the counter and read the average words per sentence.
- If the average exceeds your target by more than about 5 words, find the longest sentences and split them at their conjunctions.
- If the average is far below target, look for choppy sequences that should be joined with because, so, or which.
- Check average word length: above roughly 5.5 characters usually signals abstract or jargon-heavy vocabulary.
- Read the section aloud against the speaking-time estimate; anywhere you run out of breath is a sentence to split.
Things sentence length will not fix
- A structure that buries the conclusion. Short sentences in the wrong order are still confusing.
- Undefined jargon. Splitting a sentence does not explain the term inside it.
- Passive constructions that hide who acted.
- Paragraphs longer than about five sentences on mobile, where the block itself deters reading.
Treat readability numbers as an editing prompt. They tell you where to look; you decide what the sentence should say.