Average sentence length: what it tells you and what to aim for

Average sentence length is total words divided by total sentences. A 300-word passage with 15 sentences averages 20 words per sentence. It is the simplest measure of how hard a text is to read, and it sits inside most readability formulas, including Flesch Reading Ease and the Flesch–Kincaid Grade Level.

Why sentence length matters

A reader has to hold the start of a sentence in mind until the end, where the meaning finally settles. The longer the sentence, the more they have to hold. Long sentences also tend to stack clauses, qualifications, and asides, so the reader has to work out how the pieces fit together.

Short sentences are not automatically clear. But long sentences are harder to get right, and most unclear writing has too many of them.

What to aim for

Plain-language guidance often suggests an average of about 15 to 20 words per sentence for general readers. Treat that as a target range, not a rule. A few points help put it in context:

• Audience matters. Instructions, public notices, and web pages read by a broad public benefit from a lower average. Academic writing for specialists often runs longer, and readers expect it.
• Medium matters. On a phone screen, a 30-word sentence fills several lines. Shorter sentences are easier to follow when the reader can see only a few lines at a time.
• Purpose matters. Step-by-step instructions work best with one action per sentence. An argument may need a longer sentence to keep a cause and its effect together.

In Op-Ed Insight’s editor, a sentence of 25 words or more is flagged as long, and 35 or more as very long. Those are prompts to look, not errors. Some long sentences are fine.

The average hides the spread

Two passages can both average 18 words per sentence and read very differently. One might have sentences of 16, 18, and 20 words. The other might have sentences of 5, 6, and 43 words. The second contains one sentence most readers will struggle with, and the average gives no hint of it.

That is why the spread matters as much as the middle. Look for:

• Outliers. Any sentence over about 35 words deserves a second look. Can it be split where an “and”, “but”, “which”, or semicolon joins two ideas?
• Runs of the same length. Five medium sentences in a row can feel flat, even if each one is fine on its own.

Variety and rhythm

Good prose varies its sentence length. A short sentence after a long one lands with force. A longer sentence after several short ones lets the reader relax into an explanation. Here is the same point written two ways.

Before: The new schedule starts on Monday. The office opens at eight. The front desk closes at five. Calls after five go to voicemail.

23 words · 4 sentences · grade 2.6

After: The new schedule starts on Monday, when the office will open at eight and the front desk will close at five. After five, calls go to voicemail.

27 words · 2 sentences · grade 4.5

The first version is not wrong, but every sentence has the same weight, and it reads like a list. The second groups the hours into one sentence and lets the exception stand out on its own.

Variety works in the other direction too. If every sentence is short, the text feels choppy. If every sentence is long, it feels like wading.

How to measure your own

Counting sentences by hand is tedious and easy to get wrong. Our sentence-length checker splits your text into sentences, gives the average, and shows a distribution chart of sentence lengths. The chart is the useful part: you can see at a glance whether your sentences cluster in a comfortable range or whether a few outliers are dragging the reader through 40-word stretches.

Tip: Start with the longest sentence in the text. Splitting it usually helps more than trimming three medium ones. Our guide to writing shorter sentences shows how.

Sentence detection is not perfect. Abbreviations with periods, decimal numbers, and lists without end punctuation can confuse any automatic splitter, so glance at the result before you trust the average.