Use the command,
df -h
Filesystem Size Used Avail Use% Mounted on
/dev/vda1 40G 17G 21G 45% /
none 4.0K 0 4.0K 0% /sys/fs/cgroup
udev 991M 4.0K 991M 1% /dev
tmpfs 201M 348K 200M 1% /run
none 5.0M 0 5.0M 0% /run/lock
none 1001M 0 1001M 0% /run/shm
none 100M 0 100M 0% /run/user