The ISE Design Suite installer/updater estimates the amount of space that ISE Design Suite uses to install the tools. This value only takes into consideration the disk space used by the installed tools and not the size required for the installer/updater files.
For the installer, if you are installing with the Single File Download, all installer files are already on your drive. Therefore, the space usage and availability are accurate.
For Local XilinxUpdate or Single File XilinxUpdate, the update must unarchive to a temporary location prior to updating the install. This will double the disk space required.
WebInstaller/WebXilinxUpdate requires downloading of installer files, for this reason, more disk space is required. The downloaded files are deleted at the end of installation as long as you do not choose to keep the files; therefore, the value presented is the final disk usage and not the memory requirement. For an approximate value of disk space required when installing/updating using WebInstaller/Web XilinxUpdate, add .8 times the total estimated space required.
XilinxUpdate also requires up to 2 GB of space to backup your update. In 11.x and earlier, this number must be accounted for during the update, but can then be freed once the update is installed. The files reside in a ".backup" folder that is stored within each Xilinx tool.