How many jam tins can fit into this box if they have a diameter of 8cm and are 11cm high?

I have worked out the volume of the box to be $\displaystyle 21120\,\textrm{cm}^2$ and the volume of the tin to be $\displaystyle \pi \cdot 4^2 \cdot 11 = 552.92\,\textrm{cm}^2$.

When I divide the box by the cans, I get $\displaystyle \frac{21120}{552.92} = 38.20$ cans.

But the answer should be 30 cans. What have I done wrong?