Of all the statistics you can gather about a piece of writing, average sentence length (ASL) is one of the most actionable. It is easy to measure, strongly correlated with readability, and directly within a writer's control. Whether you are writing for a general audience, an academic committee, or a ten-year-old, knowing your average sentence length is a fast, reliable way to calibrate the difficulty of your prose.
The Formula
If a 500-word text contains 25 sentences, the average sentence length is 20 words. That is all there is to it mathematically — though the interpretation of that number requires more context.
How Sentences Are Counted
The tricky part is deciding what counts as a sentence. TextAnalyzer detects sentence boundaries using sentence-ending characters — by default, the full stop (.), exclamation mark (!), and question mark (?). You can customise this list in the settings, which is useful for text that uses ellipses, em-dashes, or non-standard punctuation.
Edge cases — abbreviations, decimal numbers, section headers — can cause overcounting, so no automated counter is perfectly accurate. But for the purposes of style analysis, the estimate is close enough to be genuinely useful.
What Readability Research Says
Sentence length is a component of several major readability formulas:
- Flesch Reading Ease (Flesch, 1948) uses average sentence length and average syllables per word. Longer sentences reduce the score (harder to read).
- Flesch–Kincaid Grade Level uses the same inputs to estimate the US school grade level required to understand the text.
- Gunning Fog Index uses ASL and the percentage of complex words to estimate reading grade level.
Across these formulas, the pattern is consistent: shorter sentences are associated with easier, more accessible text.
What Is a Good Average Sentence Length?
| Audience / Context | Recommended ASL |
|---|---|
| Children's reading (ages 6–10) | 6–10 words |
| General audience (newspapers, blogs) | 14–20 words |
| Business and professional writing | 15–22 words |
| Academic writing | 20–30 words |
| Legal documents | Often 30–50 words (not recommended) |
The Importance of Variety
Average sentence length is just that — an average. A text where every sentence is exactly 20 words is monotonous. Good writers deliberately vary their sentence lengths: short sentences for emphasis and impact, longer sentences for explanation and nuance.
Consider these two passages, both with an ASL of roughly 15 words:
The first passage uses variety — a 5-word sentence punches hard after a longer one. The second rolls everything into one long clause that buries the key information.
How TextAnalyzer Calculates Average Sentence Length
TextAnalyzer counts the total number of words and the total number of detected sentences, then divides one by the other. It also displays the longest sentence and shortest sentence in your text, which helps you see the range — not just the average.
Tips for Improving Your Sentence Length
- If your ASL is above 25 words for a general audience, look for long sentences that can be split at a conjunction or semicolon.
- If your ASL is below 10 words, try combining related short sentences to create more nuanced, flowing prose.
- Read your text aloud. If you run out of breath mid-sentence, it is probably too long.
- Aim for a mix: roughly one short sentence (under 10 words) for every 2–3 medium sentences, with long sentences used sparingly for extended explanation.
