LanguageTool API Review

See how this open source grammar checker scored in our review.

LanguageTool is a free open source grammar and spelling checker that includes an API. It operates similarly to the Office checker but is more advance, due to use of rules and statistical analyses. If you’re a writer and struggle with the complexities of grammar rules, then LanguageTool will be very useful for you; however, this review is geared towards developers as the focus is on the API.


  • Primarily rule-based checker for Grammar, Spelling, and Style
  • Offers many languages
  • High false positive rate (low precision)
  • High false negative rate (low recall)
  • More expensive than comparable alternatives
  • Easy to use

Full Review

LanguageTool is an open source grammar checker that provides a platform on which to build grammar and spelling checkers. In addition to the open source offering, the company behind it sells access to their own API that includes some additional modules for greater accuracy.


The API is priced by volume with the following packages:

  • $29/mo = 250 calls/day
  • $39/mo = 500 calls/day
  • $59/mo = 1000 calls/day
  • $99/mo = 2500 calls/day

While this pricing is not the most expensive in the industry, it’s approximately 2X the cost of GrammarBot’s API.


The front-page of the LanguageTool website is pre-populated with some text with errors flagged. This demonstrates that LT is indeed capable of detecting many types of errors.

However, as we enter in different text examples, it becomes apparent that LT’s accuracy is sub par. Consider the following text that was pasted in:

John and Bob eats burgers, fries, and dessert every Friday night. Zach would love to hang out and eat apples pie with his friends, but he as to work a double shift tonight. It was four year ago that he began working there. He decided to assembly the best group of cool miners Appalachia had to offer. But he works long hours and would rather be at the beech.

This text contains more than a half dozen errors, yet the response from LanguageTool detects only two:


While LanguageTool supports a wide array of integrations (Word, Docs, Firefox, etc.) and languages, it’s accuracy leaves much to be desired and this is what API customers are seeking. For an alternative with better accuracy and lower costs, we invite you to try our very own GrammarBot API.