Get file size on disk from file size

Question

I have a file on Windows machine with this size and i need to caclulate file size on disk from file size:

Size 3,06 MB (3.216.171 bytes)

Size on disk 3,07 MB (3.219.456 bytes)

I have 512 bytes/sector file system

How to calculate how many sectors I need to use, to store the file from file size?

I understand 3219456 / 512 = 6288, but how to calculate size on disk from file size?

Thare is a way to get size on disc from file size?

I miss something?

It's not the sector size that is important, but the cluster size. — Ben Voigt, Jun 26 '20 at 20:57

score 3 · Accepted Answer · answered Jun 26 '20 at 21:00

Your file length is 0x31132B.

The required storage (rounded up to the nearest cluster) is 0x312000. Your clusters are either 4kB (0x1000) or 8kB (0x2000).

This can be computed as:

clusterSize * ceil(fileSize * 1.0 / clusterSize)

(The 1.0 prevents integer division.) In integer math, it is:

clusterSize * (1 + (fileSize - 1) / clusterSize)

You get the cluster size from GetDiskFreeSpace, which you'll need to call anyway to figure out if your file will fit. See this existing answer:

Getting the cluster size of a hard drive (through code)

Of course, other things can affect the true storage used by storing a file... if a directory doesn't have enough space in its cluster for the new entry, if you are storing metadata with it that doesn't fit in the directory, if you have compression enabled. But for an "ordinary" file system, the above calculations will be correct.

Hello Ben, works very well your solution, thank you for your time. — user1504222, Jun 27 '20 at 02:30
This also won't work for sparse files. You should call GetCompressedFileSize to get size on disk. — Raymond Chen, Jun 27 '20 at 13:05

Get file size on disk from file size

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