How long should a sentence or paragraph actually be?

There is no correct length, but there is useful guidance — and the most important principle is variation, not any particular target.

Sentences. Plain-language guidance commonly suggests an average around 15 to 20 words, with the emphasis on average. Readability research consistently finds comprehension declines as sentence length rises, particularly past about 25 words, because working memory has to hold the structure open until it resolves.

But writing every sentence at 18 words is monotonous and exhausting in a different way. Good prose alternates: a long sentence developing an idea, then a short one landing it. Short sentences carry emphasis precisely because they contrast with what surrounds them.

The better test than word count is whether a sentence contains one idea. A 40-word sentence expressing a single connected thought reads more easily than a 20-word sentence containing two unrelated ones bolted together.

Paragraphs. Traditionally, a paragraph is one unit of thought, ending when the topic shifts. Practically, medium matters enormously:

Print and academic writing tolerate longer paragraphs — often 100 to 200 words.

Online reading favours much shorter. Screens, scrolling and scanning behaviour make dense blocks likely to be skipped entirely. Three to five sentences is a common online maximum, and single-sentence paragraphs are legitimate for emphasis.

Journalism uses very short paragraphs — often one or two sentences — partly for narrow columns and partly for scanning.

What actually determines readability more than either measure: familiar vocabulary, concrete rather than abstract nouns, a clear actor performing a verb, and logical order.

On readability scores such as Flesch-Kincaid: they count syllables and sentence length only. They cannot detect whether the writing makes sense, so treat them as a rough flag rather than a target to optimise.

Read it aloud. Where you run out of breath, the sentence is too long.