From tip to tip it's hard to say but at least 8 or 9 inches maybe ten, when he reaches for food his tentacles go pretty far, I just took this photo and this tank is a 25 year old perfecto 29 gal with the standard trim, you can judge a little. His mantle has quadrupled in size, it is quite large and I still don't have a top on my tank so he can shoot water half way across the room at me and hit me every time if I don't dodge him.