What is the best free grammar checker?

Our research indicates that the best free grammar checker available online is the Scribbr grammar checker.

We tested 10 of the most popular checkers with the same sample text (containing 20 grammatical errors) and found that Scribbr easily outperformed the competition, scoring 18 out of 20, a drastic improvement over the second-place score of 13 out of 20.

It even appeared to outperform the premium versions of other grammar checkers, despite being entirely free.