function GetSpaceOnDisk(const Path: String; const InMegabytes: Boolean; var Free, Total: Cardinal): Boolean;

Returns the number of free and total bytes or megabytes on a drive. Path specifies a directory on the drive or UNC share to check; it can be either the root (e.g. C:\) or an existing subdirectory. The setting of the InMegabytes parameter determines whether it returns figures in bytes or in megabytes (2^20), rounded down. Returns True if successful, False otherwise.

The figures returned by this function are capped at 2147483647 (2^31-1). Therefore, if InMegaBytes is False, it will return no more than 2147483647 bytes. If InMegaBytes is True, it will return no more than 2147483647 megabytes.

var
  Path: String;
  FreeMB, TotalMB: Cardinal;
begin
  // Get and display free megabytes on the Program Files drive
  Path := ExpandConstant('{pf}');
  if GetSpaceOnDisk(Path, True, FreeMB, TotalMB) then
  begin
    MsgBox('There are ' + IntToStr(FreeMB) + ' megabytes free on ' +
      Path, mbInformation, MB_OK);
  end
  else begin
    // the function failed
  end;
end;

GetSpaceOnDisk64