Best grammar checker extensions

LanguageTool is a multilingual grammar and style checker that covers more than 30 languages. It began as an open-source grammar checker and is still best known for its rule-based engine, published under the LGPL on GitHub. The browser extension is a separate, closed-source product.

It checks spelling, grammar and style, and the free version works in most web text fields. Premium adds extra checks for several major languages, texts of up to 150,000 characters per field, AI paraphrasing and a custom style guide. The extension is made by LanguageTooler GmbH, a German company that Learneo bought in April 2023.

Know who owns it. Learneo is a US company that also owns QuillBot and Course Hero. Checking happens on LanguageTool's servers, so your text leaves your device. Its policy, updated 17 August 2026, says it does not use input from LanguageTool users to train its models, with no opt-out needed. That promise is specific to LanguageTool, and the same policy says its extensions may collect website content and metadata while you browse. The Chrome rating is a low 3.3. It suits multilingual writers, but turn it off on sites with sensitive text.

Grammarly is the most complete writing assistant, and the one most people picture when they say grammar checker. It checks spelling and grammar, suggests tone and clarity changes, and includes AI features for drafting and rewriting. It works on Chrome, Firefox and Edge, and on Safari through a Mac app.

The company renamed itself Superhuman in October 2025, and the legal entity in the current privacy policy is Superhuman Platform Inc., but the product is still called Grammarly. The extension runs your text through Grammarly's servers and asks to read and change data on all websites, which is how it works in any text field.

Open the privacy settings on day one. Grammarly's support page says the Product Improvement and Training setting is on by default for free, Premium and single-user Pro accounts, which lets it use your content to train its models. You can switch it off at account.grammarly.com/security/privacy. The privacy policy, effective 6 July 2026, says it does not sell user content and earns money from paid plans, but that identifiers and browsing data are shared with advertising partners. Ratings are mixed, at 3.9 on Chrome and Edge. It is polished and widely used, but also the pick that asks for the most trust.

Harper is a free, open source grammar and spell checker that runs entirely on your device. It is written in Rust and compiled to WebAssembly so it can run inside the browser. It was started by Elijah Potter, who now works at Automattic, and the code is released under the Apache-2.0 licence.

Its site says every check happens locally, with no cloud round trips, no telemetry and no AI model in the loop. It supports English and its main dialects, and it offers no tone rewriting, paraphrasing or AI drafting, so it is a proofreader, not a writing assistant. It runs in Chrome, Firefox and Edge, and the project also ships editor plugins, an Obsidian plugin and a macOS desktop app.

Those privacy points are the developers' claims, and we did not audit the extension's network traffic. Its manifest allows connections only to itself and to writewithharper.com, and we could not find out what the latter is for. It is much smaller than the commercial options, with about 30,000 Chrome users, so ratings rest on few reviews. If you write in English and want the basics without an account, it is a good fit. It replaces the basics of Grammarly, not the extras.

ProWritingAid is aimed at novelists and long-form writers more than at a quick email check. Made by Orpheus Technology Ltd, it started as a style analysis tool for authors. Besides grammar and spelling it offers reports on style, readability and structure, and paid plans add manuscript-level feedback.

The browser extension brings the checker to web text fields on Chrome, Edge and Firefox. The free plan limits each report to two runs a day on 500 words of text, with 10 rephrases a day. Paid plans remove that limit, and ProWritingAid sells both subscriptions and a lifetime option, which is unusual in this category.

Its help centre says most reports use assistive AI, only Sparks and Rephrase use generative AI, and your writing is never used to train its AI models or algorithms. That is the company's own statement. It does not name its model providers or state a retention period, and it points to its privacy policy and trust centre instead. As with every checker here, your text goes to its servers. Firefox ratings are weak, at 3.1 from 28, and we could not confirm a Safari extension. Choose it for long manuscripts.

QuillBot began as a paraphrasing tool and is now an all-round AI writing assistant, with a paraphraser, grammar checker, AI detector and humaniser. It is made by QuillBot, a Learneo business, and Learneo also owns Course Hero and LanguageTool. With about 6 million Chrome users and a 4.7 rating, it is far larger than most picks here.

In a browser it works inside text fields on most sites, with grammar suggestions, paraphrasing and chat, and it asks for access to all sites. The free plan gives two paraphrase modes and Premium adds more. It is listed on Chrome and Edge, while Mozilla shows its Firefox listing as disabled by the developer.

Read the data terms before installing. QuillBot's help centre says that since a privacy policy update on 8 November 2025 it stores text inputs from browser extension users by default, with an opt-out. A separate opt-out for AI training exists, but only for account holders, and Team Plan inputs are excluded from training by default. The default for the training control is not clear from the page we read, so check the Data and Privacy menu in the extension settings. Use it if paraphrasing is the job, but not for sensitive text.