what is the difference between size and size on disk(when you right click on something and go to properties)...and why is size on disk always bigger??

?? your not very clear in your Q, but if this is what i think you mean. if you have a HD that is 20GB according to a manufacturer, once formatted it may only give 18.5GB of storage space. this is dependant on being formatted as FAT/FAT32/NTFS/ or LINUX format's they all have different ways of clustering data. if you mean size on disk used and size of all files seen, note you may not be counting hidden files such as OS stsyem files. mick

You are confusing MB, and mb. A mb is not 1000000 bytes, but 1048676 (1024x1024) bytes. Manufactures rate drives in MB simply because this gives a larger number and is more impressive. Every million bytes is a MB, but it takes 1048676 bytes to make a mb which is how your computer displays size. Al

Actually a million bytes (8 bits per byte) is truly 1,000,000 bytes on disk - those same bytes in memory are the number you refer to. IBM, Microsoft and others refer to a megabyte as 1024 bytes for consistency. So the OEM refers to the size correctly, and we have to compensate for it when looking at the values reported by the O/S.

I think Dan is referring to the actual size of a file, and the minimum size that same file takes up on your hard drive. E.g., if you have a 187 bite file, on a 20GB FAT 32 hard drive, it will take up 16mb of space, since all files take up a minimum (and a multiple of) 16 mb.

Size on disk=file could be fragmented or grabbing up extra space on the disk for future writing if they differ. File size=what the file would be if you closed it. I think thats what it means.:)

Formula for PC (file size + allocation unit -1) / allocation unit now take the integer part of your answer and multiply by the allocation unit. This gives you the "size on disk"