>>Is there an API function to report how much space is "really used" (round up to the nearest cluster) by a file? 'Cause I do get correct information when I check a file's properties in the Explorer.
>>
>No there isn't. If I recall correctly, information regarding the cluster size is stored by the ROM BIOS. Windows probably loads this information on startup. There's no way via the API to interface with the ROM BIOS interrupt functions to retrieve this information.

Stupid question George ... since, by whatever means, Windows "knows" this information, why wouldn't they provide a "hook" so programmers could get the information programmatically?