lvm2 size of logical volume after lvcreate --size 348g after mkfs.ext3 -m 0 is only 343 gigabytes?

[Mitchell Laks <mlaks@post.harvard.edu>]

Hi I am trying to backup a volume which has size 347G. So I thought to use lvm2.
So I created a volumeby

lvcreate -n test --size 348G my-volume

I thought I would be giving myself an extra Gigabyte there :).

then i did
mkfs.ext3 -m0 /dev/my-volume/test

then when I mounted the device
mount /dev/my-volume/test /mnt

then df -h
only shows 343G instead of 348G.

Note that tune2fs shows that I have indeed have 0 reserved blocks 
(ie mkfs.ext3 -mo /dev/my-volume/test is equivalent to tune2fs -m0 /dev/my-volume/test

So my question is where have those extra 5G gone to?

I need to back up a 347.

So I had tp set it up with 353G to have the correct size.

Can anyone explain this to me? 
What kind of formula to use to calculate how much space I will need in 
advance for creating the ext3 or in the future ext4 file system? 5G seems
like a lot.

Thanks,
Mitchell Laks