cubage unit

NOUN
11 LETTERS · STARTS WITH C

cubage unit is a valid 11-letter word worth 15 points in Scrabble and 21 points in Words with Friends.

SCRABBLE WORDS WITH FRIENDS
C3U1B3A1G2E1U1N1I1T1
3 + 1 + 3 + 1 + 2 + 1 + 0 + 1 + 1 + 1 + 1 =15 PTS
DOUBLE WORD → 30TRIPLE WORD → 45
MEANING

What does cubage unit mean?