disk space overhead in ext4

I'd like to know if there's some rule (or formula) I can apply to find out how much of disk space will be used by the filesystem in an ext4 partition. for example, in a partition of 100 GB, how much can I actually use? does it depend on other parameters like inode size, etc?

I don't know if it applies to you, but just in case: most of the time the huge majority of the apparent overhead of ext2/3/4 will be due to the default significant amount of reserved blocks for root. Make sure you specify -m 0 (or 1) to mkfs.ext4 when creating the filesystem or to tune2fs if adjusting it afterwards. The default is 5, which stands for 5% of total blocks and can be excessive in most cases.

The most significant overhead comes from the inode tables, so it obviously depends on how many inodes you allocate. With the default options, every 128mb of disk gets 2 mb ( 8192 x 256 bytes each ) of inodes, or 1.6% overhead.

Difficult question to answer this as the amount of DATA space would vary with the number of files, their size, ACLs, Root reserved space, blocksize etc. There is not really a easily fixed answer to this question.