I'm assuming you want percent error. The formula for this is given by taking the absolute value of the difference between your actual and expected, and then dividing that by the expected to get a relative approximation. Therefore, your formula is
$$\frac{\left|\mathrm{Expected}-\mathrm{Actual}\right|}{\mathrm{Actual}}$$
or
$$\frac{\left|11-16\right|}{11}=\frac{5}{11}=45\% \text{ error}$$
You can then specify that your answer was above the expected result