How to make div 100% of page, not view port

I haven’t been able to find any answers searching online to figure out how to make a div 100% of the page height.

To give clarity: I have a div that’s designed to look like a grid, and set it to absolute so that it sits behind all of the other content (like It’s set relative to the Body, but setting it to 100% height doesn’t set it to 100% of the page height. It looks like it’s just setting it to 100VH.

Any tips?