An iPad cannot fill itself up to 16 GBs baecuae a 16 GBs iPad has, after formatting, installation of iOS and apps, ONLY starts out with a free data storage amount of 12.7 to 13 GBs.
In addition, iOS, itself needs a storage buffer of approx. between 2-3 GBs of free data storage space ti function at a normal, nominal, acceptable performance level.
So, actual, usable storage in a 16 Gbs iPad is only about 9-11 GBs.
If your iPad has exceeded this 11 GBs storage limit, you need to free up free data storage space on an iPad/iDevice, you need to start by backing up your iPad to either Apple's iCloud or by connecting your iDevice to a computer running the latest iTunes.
Then delete any installed apps that you no longer use or use infrequently enough that they aren't missed (you can always reinstalled any purchased or free apps at some time later, as you need them).
Thin out/delete any music/videos/movies/TV shows/images/eBooks/eMagazine/iBooks/audio books/Documents/PDFs, etc.
Delete any old saved emails that maybe saved to your iPad.
To permanently delete photos/images off of your iDevice go into the Photos app and find the Recently Deleted folder/album and permanently delete any photos/images stored in that folder/album.
Also, you can check to see what stuff is eating up all your valuable data storage space by going into the Settings App,,General Settings Panel, in the right hand column, tap Storage & iCloud usage panel ( in iOS 11, it’s iPad Storage ), in the top Storage Panel, tap the Manage Storage panel, wait for a list of apps to generate that will tell you how much space each app is taking up on your iDevice.
Then, do a hard reset of your iPad by holding down both the Home and sleep/wake buttons simultaneously until your iPad goes to black and restarts with Apple logo, then release the buttons.
Good Luck to You!