How much storage does iOS 12 require? I have a iPad air 1st gen 16GB (model A1474).

I want to install iOS 12 on it, but I am not sure if 16GB is sufficient storage. 

Right now I have iOS 9.3.5, and I can't see how much does the iOS itself takes, but I have enough storage for many apps and music.

Does anyone know how much storage does the iOS 12 takes on the iPad? I am afraid I will not have any storage left for apps and data.
After installing iOS 12, and then restoring my device to factory default I see that the iOS take 5.1 GB.