>>>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.
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>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?

It's reading the information from the BIOS (I think) on startup. These days the cluster size really isn't a big issue given the low cost of large hard drives. It's of realitively little importance. Further, cluster size is valid only on the local machine (if it's being reported correctly, which it isn't always). A network OS, such as Novell may report back that the cluster size is 32K, when in reality it's much smaller.