To find the length of each side of Tommy's square sandbox, we can start by using the formula for the perimeter of a square, which is:
\[ \text{Perimeter} = 4 \times \text{side length} \]
Given that the perimeter is 28 feet, we can set up the equation:
\[ 28 = 4 \times \text{side length} \]
To find the side length, we can divide both sides of the equation by 4:
\[ \text{side length} = \frac{28}{4} = 7 \text{ feet} \]
Therefore, each side of the sandbox is 7 feet long.