Q: How much memory is user available on iPad 128GB?

I think you mean flash storage, not memory.

There is probably somewhat less than 115GB available.  You need an absolulte minimum of 10% of the available space available at all times.  That would drive you right down to 115GB immediately.  But in addition to that, when you plug your iPad into iTunes, it will show you less than 128GB available at the top of the iTunes display.

So I would say you have about 100-maybe 105GB available.

You don't need 10% free, that's a holdover from early computer days when drives were 100mb.

To the OP, 115 Gb sounds right.

The OS uses some, etc.

I respectfully disagree with your statement "You don't need 10% free."

I've seen posts on this forum of i-devices misbehaving when less than 10% of storage was available, and increasing the available storage remedied the issue.

The difference is due to the way manufacturers measure storage space, and how a hardware device does it.

From the article:

How OS X and iOS report storage capacity

http://support.apple.com/kb/TS2419

Storage device manufacturers measure capacity using the decimal system (base 10), so 1 gigabyte (GB) is calculated as exactly 1,000,000,000 bytes. The capacity of the storage media in your Mac (Mac OS X v10.5 or earlier), iPad, iPod, iPhone and other Apple hardware is measured using this decimal system. This is detailed on product packaging and online through the statement "1 GB = 1 billion bytes."

Storage space is measured in base 2, as in the binary system computers use. So a Kilobyte is not 1000 bytes, but 1024 bytes. a megabyte is then 1024 Kilobytes, and a Gigabyte is the 1024 Megabytes. This makes the discrepancy much larger as sizes increase.

So a 128GB iPad has 128,000,000,000 (64 billion) bytes of available storage space. In actual computer measurement it equals around 119GB.  From that you still have to substract the ammout of space used up by the OS and preinstalled  software, and you should be around the 117GB mark maybe a little less.

My wifi only iPad Air reports 115.12GB available in iTunes.