How long should a sentence be? Average length, and the unsourced 8-word claim

The short answer: an average, and a point to check

Plain English guidance sets an average of 15 to 20 words a sentence, and GOV.UK asks you to check any sentence over 25. Those are two different kinds of limit. Martin Cutts, author of the Oxford Guide to Plain English, advises: "Over the whole document, make the average sentence length 15-20 words" (as quoted by Siteimprove). The GOV.UK style guide says: "Do not use long sentences. Check sentences with more than 25 words to see if you can split them to make them clearer."

Where "8 words, 100% understood" comes from

The figure has no source that anyone shows. Writing guides repeat that readers understand 100% at an average of 8 words or fewer, 90% at 14 words and under 10% at 43. Ann Wylie's How long should a sentence be? credits the American Press Institute and "studies of 410 newspapers", and Siteimprove's page gives the same ladder. Neither names the study, gives a year, or links to anything but an image credit. A search for the study itself, on 2026-11-14, found only these repeats.

That does not prove the numbers are wrong. It means a reader cannot check them, and a number that cannot be checked should not be written as "research shows". Two details add to the doubt.

• It looks like an older table. The ladder on those pages runs from "very easy" at 8 words or fewer to "very difficult" at 29 or more. Rudolf Flesch's sentence-length scale in The Art of Readable Writing (1949) has the same shape, and published reproductions of it give 29 or more as very difficult and 25 as difficult. The 8-word end of that table was not checked against the book, so this is a likeness, not a proven origin.
• It sets a score for a story, from an average. "Readers understood 100% of the story" at an average of 8 words says nothing about a text with one long sentence in it, which is the next point.

An average hides the long sentence

Two texts can have the same average sentence length and read very differently. The average is words divided by sentences, so one 45-word sentence among short ones is averaged away. Both of these 60-word recipe texts have 5 sentences and an average of 12.0 words, counted with the word counter's own sentence rule.

Text B opens "Preheat the oven. Grease the tin. Mix the butter and sugar. Fold in the flour." and ends with a single 45-word sentence that packs the heating time, the greasing, the lining and the beating into one run. A reader meets four easy sentences and then one that is far past the GOV.UK check point.

So an average is the right target for a whole document and the wrong test for a single paragraph. Use it to set the general level, then look at the longest sentence.

What a readability formula does with sentence length

The Flesch formulas use only the average sentence length, so they cannot see the 45-word sentence. Per Wikipedia's summary of the Flesch–Kincaid tests, the two formulas are:

• Flesch Reading Ease: 206.835 − 1.015 × (words ÷ sentences) − 84.6 × (syllables ÷ words). Higher is easier.
• Flesch–Kincaid grade level: 0.39 × (words ÷ sentences) + 11.8 × (syllables ÷ words) − 15.59.

The sentence term is the same size for Text A and Text B: 1.015 × 12 = 12.18 points off the Reading Ease score, and 0.39 × 12 = 4.68 toward the grade level. The scores can still differ through the syllable term, but nothing in either formula sees that one sentence is 45 words long.

Going from an average of 12 to the top of the plain English range, 20, therefore costs about 8 points of Reading Ease and 3 grades, before syllables are counted. The word counter shows both scores with this arithmetic.

How to check your own text

Paste prose only, read the average, then read the longest sentence. The word counter's average sentence length is words divided by sentences, and its sentence rule is strict about what ends one: a full stop, exclamation mark or question mark followed by a space or the end of the text, unless the full stop ends a known abbreviation or a single capital letter. It also treats the end of every line that has text as the end of a sentence.

That last rule matters. A heading, a list item or a table row with no full stop counts as a sentence of its own, and short ones pull the average down. Paste paragraphs without the headings, then:

1. Read Average sentence length. Between 15 and 20 matches the plain English range.
2. Read Longest sentence. If it is past 25 words, find it and ask whether it carries two ideas.
3. If the average is above 20, shorten a few of the longest sentences, not all of them.
4. If it is far below 15, look for runs of very short sentences that could be joined where one idea leads to the next.

The guide why word counts differ explains why the word total behind the average can differ between tools by a few in a hundred, which moves the average by a fraction of a word and does not change the advice.

Short is not always better

The aim is a readable rhythm, not the lowest number. Cutts's advice is an average over the whole document, which leaves room for short and long sentences together. A text of identical 15-word sentences has the right average and still reads as mechanical, and a text of nothing but 5-word sentences can hide how one idea leads to the next.

Two ideas in one sentence is a better reason to split than a word count. A 30-word sentence with one clear clause order can read better than a 20-word one that crams in two points. Treat the 25-word point as a prompt to look, as GOV.UK does, and the 15 to 20 average as the level to hold across the page.