10 GB of 16 GB used. iPad storage full? Why?

Your old 16 GBs iPad is full!

What year and model iPad and what iOS version is it currently running?

The amount of available, practical, usable storage is based not only on how much iOS/iPadOS takes up but on the actual size that 1.0 GB is equivalent to, which for Apple 1.0 GB ≈1.074 GBs

Also, iOS/iPadOS devices need to maintain a minimum constant of between 2-4 GBs to ensure nominal, expected performance and function of iOS/iPadOS.

Also, my “current” installation of iPadOS 14 takes up approx. 6 GBs of my iPad's internal storage space.

Your version of iOS/iPadOS maybe taking up the same or less internal data storage than my version!

So, with all of this stated,

A 16 GBs storage iPad starts out, after initial drive formatting, at approx. 14 GBs of free, internal storage.

After the installation of iPadOS, at nearly 6.0 GBs, leaves 8 GBs of of free, internal data storage space.

iOS/iPadOS needs a minimum of 2-4 GBs of constant remaining storage, AT ALL TIMES to insure/ensure nominal, acceptable operation of all iOS/iPadOS functionalities.

This leaves a 16 GBs iPad with ONLY a practical, usable storage space of between 4 and 6 GBs AND THAT IS IT!

This is virtually NO internal data storage space to do much of anything with that iPad!

This is why Apple quietly, silently did away with all 16 GBs iOS device back sometime in 2016.

A 16 GBs iPad with any recent versions of iOS/iPads are going to eat up a larger amount of internal storage space, just for the operating system, alone.

Your iPad model just does not have any sufficient data storage capacity with more modern iOS/iPadOS versions.

Sorry.

You may really need to seriously consider a new iPad model OR a much “newer” , used iPad model capable of running the latest iPadOS versions tbat have much larger internal data storage options, now.

The minimum recommend internal data storage amount I would recommend, now, in 2020 is either a 64 GBs model iPad or a 128 GBs iPad, if you decide to purchase a larger capacity iPad model that will have some longevity/”futireproofing”.

A 64 GBs storage iPad starts out with 60 GBs. Minus approx. 6.0 GBs for iOS leaves 54.GBs. Minus 2-4 GBs for iOS iPadOS system reserve equals approx. between 50- 52 GBs of usable data storage space.

A 128 GBs storage iPad starts out with 119 GBs. Minus 6 GBs for iOS leaves 113 GBs. Minus 2-4 GBs for iOS iPadOS system reserve equals approx. between 109-111 GBs of usable data storage space.

While the cheapest 2018 base model iPad comes in 32 GBs, a 32 GBs storage iPad starts out with approx 30 GBs. Minus 6.0 GBs for iOS/iPadOS leaves 24 GBs. Minus 2-4 GBs for iOS iPadOS system reserve equals approx. between 20.- 22 GBs of usable data storage space.

That is not a whole lot of data storage in this day and age, but, at least, that much data storage might get you by as this, at least, 4 times the storage amount of a 16 GBs iPad model.

Your current iPad Air model is either running iOS 12.4.9 or iPadOS 13/14.

iOS 12 uses, at least, over 4 GBs. iPadOS uses anywhere between 5-6 GBs.

These are much larger operating system files than what older iOS versions used to take up for data file sizes.

Sorry & Best of Luck to You!