Surface with Windows RT comes with either 32 GB or 64 GB of internal storage space (hard disk space). However, not all of this space is available for your music, photos, videos, and other content.
The actual amount of available storage in the $500 model (32GB) is closer to 16GB. The 64 GB version has approximately 46 GB free hard disk space.
According to Microsoft, the Windows RT operating system, the preview version of Microsoft Office, and other built-in apps take up about 8GB of storage. Another 5GB are reserved for Windows recovery tools.
The total disk size reported by Windows in the 32GB Surface tablet is 29GB. However,
the advertised local disk size is shown using the decimal system, while Windows displays the disk size using the binary system. As a result, 1 GB (in decimal) appears as about 0.93 GB (in binary). The storage capacity is the same, it's just shown differently depending on the how you measure a GB (decimal or binary).
The difference in advertised versus actual storage in the Surface with Windows RT is greater than what's typical for a tablet. Apple's 32GB iPad comes with roughly 28GB of available storage out of the box, and the 16GB Nexus 7 has a little more than 13GB available to users.