In Ubuntu/Debian by default while creating ext2/ext3/ext4 file system 5% of disk space is reserved for super user across each partition.
Except for root partition 5% reserved space is not required for other partitions. So we reuse that reserved space.
We can reuse this reserved space with tune2fs command.
First get file system device path for which you want to reduce reserved space. Use df -h command to get the details:
$ df -h
Filesystem Size Used Avail Use% Mounted on
/dev/sda5 28G 4.7G 22G 18% /
udev 2.8G 4.0K 2.8G 1% /dev
tmpfs 1.2G 880K 1.2G 1% /run
none 5.0M 0 5.0M 0% /run/lock
none 2.9G 2.2M 2.9G 1% /run/shm
/dev/sda7 311G 166G 131G 56% /home
To make only 2% space as reserved on /home use following command:
$ sudo tune2fs -m 2 /dev/sda7
In above command /dev/sda7 my file system device path for /home
Now check your space details with df -h and observe the size of /home partition it should be increased.