function GetFileSizeOnDisk(const FileName: TFileName): Cardinal;

var

spc,bps,nofc,tnoc : Cardinal;

ClusterSize,

ClustersCount,

FileSize : Cardinal;

begin

Result := 0;

if not FileExists(FileName) then

Exit;

//Call GetDiskFreeSpace to find out disk cluster size.

if not GetDiskFreeSpace(PAnsiChar(ExtractFileDrive(FileNa me)),spc,bps,nofc,tnoc) then

Exit;

//Cluster size = Bytes Per Sector * Sectors Per Cluster

ClusterSize := bps * spc;

//Get actual file size.

FileSize := GetCompressedFileSize(PAnsiChar(FileName),nil);

ClustersCount := FileSize div ClusterSize;

//Calculate file size on the disk.

Result := ClustersCount * ClusterSize;

//if the file occupies a cluster partially, add cluster size to file size, because

//a cluster is the smallest unit of disk which is accesible.

if FileSize > Result then

Inc(Result,ClusterSize);

end;

برگرفته شده از سایت دنیای برنامه نویسی دلفی(http://mt85.blogfa.com)

Speed test