whats the usable memory of a 128gb ipad?

The operating system will only take a couple of GB of that storage, so you can expect somewhere around 126 GB storage space. By the way, that 128 GB is storage, not memory. The memory is 1 GB for the newer models.

I believe I remember reading that you will only have about 115GB or so.

In actual physical storage space you should have around 116 to 117 GB.

Memory which is different from storage space is 1 GB.

That is mainly because of how disk/storage space is advertised and how it is actually used. See:

How Mac OS X and iOS report storage capacity

The 128 GB iPad has 128 GB of storage but iOS (and OSX) uses

1 GB is calculated as 1,073,741,824 bytes.

As the GB of storage goes ups, the more "GB" are lost to the conversion.

I just got the new iPad Air 128 gb today and after going thru setup prompts there was 115 Useable Capacity - 13 gigs loss is disappointing but nothing to fret over I guess - 115 is still a lot of space

\\dev\\null wrote:

I just got the new iPad Air 128 gb today and after going thru setup prompts there was 115 Useable Capacity - 13 gigs loss is disappointing but nothing to fret over I guess - 115 is still a lot of space

But you aren't loosing 13GB. Read llaass' link about it. You'll see in reality you are only loosing abou 4GB to OS and preinstalled Apps.