Corncob dictionary (58k english words):

Passphrase entropy:

Comparison with random passwords, lower/upper case + digits:

pwgen -sn 6    27.1 bits
pwgen -sn 8    36.6 bits
pwgen -sn 10   48.5 bits
pwgen -sn 12   58.8 bits
pwgen -sn 14   72.2 bits
pwgen -sn 15   75.2 bits
pwgen -sn 16   83.5 bits
pwgen -sn 18   95.2 bits
pwgen -sn 20   104.1 bits
pwgen -sn 25   130.0 bits