Average sentence length analyser

By Jack Stovell · published 2026-09-24

Average sentence length (avg_sentence_length) is the number of words per sentence, averaged across a piece of writing, and one of the simplest voice signals to check by hand. Across ScriptGrain's reference corpus of 299 published pieces, the median is 16.8 words per sentence, and the middle half of pieces fall between 14 and 20.

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At a glance

What average sentence length measures

Average sentence length measures rhythm at its most basic level: how many words you typically string together before a full stop. It sits in the Syntax group of a ScriptGrain voice profile, alongside things like sentence complexity and clause ordering. The definition the app uses is plain: "the average number of words in a sentence."

Here's the thing though: a number on its own tells you very little. A 17-word average could come from a writer who barely varies, every sentence landing between 15 and 19 words, or from someone who alternates a 4-word fragment with a 30-word sentence and lands on the same mean. That's why average sentence length is usually read next to sentence length variance. One tells you the centre, the other tells you the spread.

The value stored in your profile is Claude Sonnet 5's reading of your writing samples, produced in two passes: one per sample, then a synthesis across all of them. It is an informed estimate. Code counts sentences only when a draft is scored and in the free tools.

How average sentence length is scored

Average sentence length is counted in code from a draft when ScriptGrain scores it, then compared with your profile's stored figure at a voice-match weight of 2. The comparison uses a tolerance: whichever is larger, 8 words or 70% of your profile's average. Similarity is 1 minus the gap divided by that tolerance, floored at 0, so a huge miss can't push the score negative.

This is one signal among many. Voice match pulls together sentence, punctuation, pronoun, article and word-length features counted in code, plus formality, humour, structure and rhythm judged by Claude Haiku, all weighted and calibrated. Scores of 90%+ are "Excellent," 75 to 89% "Good," 60 to 74% "Fair," under 60% "Low." Full detail on how the pieces combine lives on the voice measurement framework page, and voice match has its own glossary entry if you want the short version.

How to analyse average sentence length in your own writing

Average sentence length is easy to calculate by hand if you're patient. Here's a method that mirrors how code counts it:

  1. Take a sample of your writing, at least a few hundred words.
  2. Split the text into sentences wherever a full stop, question mark or exclamation mark is followed by a space, or wherever a line breaks.
  3. Drop any fragment under two words; it's noise, not a sentence.
  4. Count the words in each remaining sentence.
  5. Add up the total word count and divide by the number of sentences.

That's your average. Do it across a few samples and you'll start to see your own baseline; do it across samples from different projects and you'll see how much your rhythm shifts by context. ScriptGrain's free voice profile reads average sentence length from a writing sample among all 45 attributes, and the free writing style analysis counts it with its own sentence splitter.

Typical average sentence length in published writing

Typical average sentence length in published writing is about 17 words. ScriptGrain's reference corpus, 299 recent public pieces from 47 sources (blogs, newsletters and essay sites) built on 20 September 2026 and measured with the product's own code, gives these percentiles: p10 is 11.6 words, p25 is 14, the median is 16.8, p75 is 20, and p90 is 23.4.

So what counts as short or long? By the homepage's own thresholds, an average under 13 words reads "short," under 21 "mid-length," and 21 or more "long." The habits report uses the corpus directly: at or above the 75th percentile (20 words) you get "Long sentences"; at or below the 25th (14) it's "Short sentences." Context still matters (a technical explainer and a punchy opinion piece won't share a target), but these numbers give you something to measure against instead of guessing.

How to make AI write longer sentences

Average sentence length rises when you stop cutting related ideas apart with full stops. Try this:

  1. Let clauses join with "and," "which" or "because" instead of ending the sentence early.
  2. Switch to a formal register if your tool supports one; ScriptGrain's Formal register specifically asks for longer, more structured sentences.
  3. Keep a handful of short sentences in the mix. Raising the average shouldn't mean flattening the variation; that just produces a wall of uniform, dense prose.

When ScriptGrain writes from a profile, the system prompt sends your stored average under "Treat every attribute as a hard constraint, not a suggestion", with the rule "Match sentence rhythm, length variance, and complexity exactly". The same profile reaches ChatGPT, Claude and other tools through the API and MCP server.

How to make AI write shorter sentences

Average sentence length drops when you give the model a number to aim for and a rule for splitting.

  1. Run a counter on your own writing and hand the model your actual average.
  2. Split any sentence carrying two ideas into two separate sentences.
  3. Cut throat-clearing openers like "it is important to note that"; they add length without adding meaning.

A ScriptGrain profile sends your average as a hard constraint on every draft, and the same profile reaches ChatGPT, Claude and other tools through the API and MCP server. In ScriptGrain's SGR-001 study, on a challenger-bank brief, the profiled draft averaged roughly 11 words a sentence against roughly 17 for the bare model, and sentences under eight words doubled from 24% to 48%.

Questions

How do you calculate average sentence length?

Split your text into sentences at full stops, question marks or exclamation marks followed by a space, or at line breaks. Drop fragments under two words. Count the words in each remaining sentence, add them up, divide by the number of sentences. That's the average. It's mechanical, which is exactly why it's useful: two people counting the same text with the same rules should land on the same number.

What is a good average sentence length?

There's no single correct number, but the reference corpus gives useful markers: median 16.8 words, with the middle half of the 299 pieces falling between 14 and 20. ScriptGrain's homepage calls an average under 13 short and 21 or more long. Genre and audience matter more than any fixed rule: technical writing and conversational blog posts sit in different places, and both can be "good" for their purpose.

How is sentence length scored in a voice profile?

In voice match, it's counted directly from your draft by code and compared against your profile's figure (the extraction model's reading of your samples), carrying a weight of 2. Similarity equals 1 minus the gap divided by a tolerance (the larger of 8 words or 70% of your profile's average), floored at 0. It's one of 28 attributes (out of 45) that can move your score.

How do I make ChatGPT write longer sentences?

Ask it to join related clauses with "and," "which" or "because" rather than splitting them into separate sentences, and request a more formal, structured tone. Keep a few short sentences in the mix so the writing doesn't turn dense and uniform. In ScriptGrain, the Formal register asks for longer, more structured sentences.

How do I make AI write shorter sentences?

Give it a target: your own average sentence length from a counter, and ask it to write close to that figure. Ask it to split any sentence holding two ideas into two sentences, and to cut throat-clearing openers such as "it is important to note that." Then run the same counter on the draft to see whether the average moved.

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