According to the product page it is - 15.75 x 12 x 5.5", but apparently it can fit a Brain Cell - size 6T, which in turn has an external dimension of 13.0 × 12.3 × 2.4". In other words 0.3" higher than the bag that fits it.

This question is very relevant to a potential Id bag purchase, since I have an 9-Cell (protruding) battery on my Lenovo T60, which makes the 6T Braincell short by 0.8 inches (battery part). If I cannot adjust the internal padding enough, then I may need to have the Braincell in the Id with an open lid and the laptop protruding, further increasing the need for interior height.