Marketing & GrowthFree
Long Sentence Checker and Highlighter
Paste your writing and see every sentence over your word limit marked with its count and a suggested split, with the rest left as written.
Paste a blog post, email, report or web page, and River marks every sentence longer than your chosen limit, 25 words unless you pick another. Each marked sentence shows its word count and one suggested place to split it, usually where a second idea begins. Nothing else in your text is rewritten. At the top you get a short summary: how many sentences you wrote, your average length, and how many went over the limit, so you can see at a glance whether this is a habit or a few slips.
Grammar checkers bury long sentences among dozens of other suggestions, and rewriting tools change your voice while they shorten things. This does one job. It shows you where sentences run long and leaves the decision to you, because some long sentences are fine. A list of items or a carefully built argument can run past the limit and still read well. Seeing the count beside each one helps you judge which to split and which to keep.
It is for anyone writing for busy or broad audiences: marketers, small business owners, support teams, people writing policies or instructions, and anyone writing in a second language. Use it as a final pass before you publish, once the content is settled. It is an editor's aid, not a score, so it counts words sensibly around abbreviations, numbers and lists, and tells you when a sentence boundary is ambiguous rather than guessing silently.
Why sentence length matters
Long sentences make readers hold several ideas in their head before they reach the point. GOV.UK, which writes for the whole UK public, sets 25 words as its limit and asks writers to check anything longer to see if it can be split. Its own style guide repeats the advice. That is a sensible default for most public writing, and a tighter limit of about 20 words suits instructions and anything read on a phone.
Here is an illustrative line from a returns policy. “Our new returns policy, which we introduced in March after listening to feedback from hundreds of customers who told us the old process was confusing and slow, now lets you return any item within 60 days without a receipt.” That is 39 words, and the point arrives at the end. Split it: You can now return any item within 60 days without a receipt. We changed the policy in March after customers told us the old process was confusing.
Not every long sentence needs splitting. A sentence that lists several short items, or one whose clauses follow a clear order, can read easily at 30 words. What matters is whether a reader can follow it in one pass. Variety also helps: a page of sentences that are all exactly 12 words feels choppy. Use the flags as prompts to reread, then split where the sentence carries two ideas or buries its main point.
How it works
Paste your writing
Paste any text and pick a word limit, or keep the default of 25 words.
River counts sentences
It finds each sentence boundary, counts the words and marks every sentence over the limit.
Review the flags
Each long sentence shows its count and a suggested split, with the rest left untouched.
Split in chat
Ask River to split any flagged sentence for you, keeping your words and meaning.
What you get
- Every sentence over your limit marked in your text, with its exact word count beside it
- One suggested split point for each long sentence, usually where a second idea begins
- Your words left exactly as written, with no rewriting of anything you did not ask about
- A short summary of total sentences, average length and how many went over the limit
- Sensible counting around abbreviations, numbers, lists and quotes, with unclear cases noted
- A choice of limit, from 20 words for instructions to 40 for expert readers
Common questions
How long is too long for a sentence?
For most public writing, sentences over about 25 words deserve a second look, which is the limit GOV.UK uses. Instructions and mobile reading benefit from shorter sentences, around 20 words. Expert readers handle longer ones. The right limit depends on your audience, so pick the one that fits and treat the flags as prompts, not rules.
Will it rewrite my text?
No. It only marks sentences over your limit and suggests where each could be split. Your words stay as you wrote them. If you want a flagged sentence split for you, ask in the chat and River will rewrite only that sentence, keeping your meaning and voice. Everything else stays untouched.
How does it count words in tricky cases?
Numbers, abbreviations such as e.g. and Dr., and hyphenated words each count as one word, and a full stop inside an abbreviation does not end a sentence. Bulleted lists are treated as separate items. Where it is unclear whether a sentence has ended, River notes it rather than guessing. Counts may differ slightly from other tools.
Is this the same as a readability score?
No. Readability scores combine sentence length and word length into a single number, which tells you a page is hard but not where. This points at the exact sentences. Long sentences are one of the most common and easiest problems to fix, so it is a practical place to start before worrying about a score.
Does it work in languages other than English?
It counts words in most languages that separate words with spaces, such as Spanish, French or German, and marks long sentences the same way. Suggested split points work best in English. Typical sentence lengths differ between languages, so you may want a higher limit for languages with longer average sentences.
Built by
Chandler Supple
Co-founder & CEO, River
Chandler has spent the last six years working in AI, building tools for millions of users and Fortune 100 companies. In 2025, he co-founded River to build a new kind of operating system for AI agents.
Long Sentence Checker and Highlighter
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