In general, no percentage of plagiarism is acceptable in a research paper. However, different academic institutions and publishers may have different guidelines and policies for what constitutes acceptable levels of similarity, but as a general rule, it is recommended to keep the similarity index below 10-15%. This includes both direct quotes and paraphrasing.
It’s important to note that self-plagiarism, which is the reuse of one’s own previously published work without proper citation, is also not acceptable. Overall, it’s always best to strive for originality and to be diligent in citing all sources of information to avoid any accusations of plagiarism.
At Engineer’s Planet, we use Turnitin for checking plagiarism. Turnitin is one of the mostly used tools and has a record accuracy level.