1GB in decimal =.93GB in binary. Thus, 32GB native memory in decimal equals just 29GB in binary. It is the same size, just represented differently. 5GB is reserved for Windows recovery tools, which in case of a problem, you will be glad that it's there. Windows RT, Microsoft Office and other built in apps take 8GB and before you know it you have 16GB of binary disk space. A Microsoft Surface RT with a 64GB drive would have 46GB of binary disk space left after accounting for the pre-installed programs.
There are three ways that a Microsoft Surface RT owner can increase the storage space on his device. You can use cloud storage fromSkyCloud. A microSD, microSDHC, or microSDXC card can increase storage by up to 64GB, or you can buy a USB flash drive or hard drive. So don't worry about those pre-installed programs taking up your disk space as there are quite a few ways to have enough storage space to keep your Microsoft Surface RT tablet filled with your content.
source: Microsoft via WMPoweruser