Here's how:
1. Go to the "Review" tab. This is usually located at the top of the Word window.
2. Click "Proofing". You'll find a group of options under this.
3. Select "Readability Statistics". A dialog box will appear.
4. View the Results. This will give you different readability scores, including the Flesch Reading Ease and the Flesch-Kincaid Grade Level, along with other metrics. The Flesch-Kincaid Grade Level is the one that estimates the US grade level needed to understand the text.
Remember that these are just estimates. The actual grade level needed to understand a document can vary based on the reader's background knowledge, vocabulary, and reading skills.