The simplified ratio of $8$ to $16$ is $1:2$.

To simplify a ratio, you first need to identify the greatest common divisor (GCD) of the two numbers. The GCD is the largest integer that divides both numbers without leaving a remainder. In the case of $8$ and $16$, the GCD is $8$.

Next, you divide both numbers in the ratio by this GCD. For the ratio $8:16$, this process involves dividing both $8$ and $16$ by $8$, which results in the following calculation:

Thus, the ratio $8:16$ can be simplified to $1:2$. This indicates that for every $1$ part of the first quantity, there are $2$ parts of the second quantity. Simplifying ratios not only makes them easier to interpret but also facilitates comparison.

Mastering the simplification of ratios is an essential skill in GCSE Maths, as it aids in solving problems related to proportions, scaling, and practical applications such as adjusting recipes or interpreting maps. Remember to always seek the GCD to simplify ratios effectively.