The volume \( V \) of a cube can be calculated using the formula:
\[ V = s^3 \]
where \( s \) is the length of an edge of the cube. In this case, the edge length is \( s = 2.5 \) ft.
Now, we will calculate the volume:
\[ V = (2.5)^3 \]
Calculating \( (2.5)^3 \):
\[ (2.5)^3 = 2.5 \times 2.5 \times 2.5 \]
First, calculate \( 2.5 \times 2.5 \):
\[ 2.5 \times 2.5 = 6.25 \]
Now multiply that result by \( 2.5 \):
\[ 6.25 \times 2.5 = 15.625 \]
Thus, the volume of the cube is:
\[ V = 15.625 \text{ ft}^3 \]
Finally, we round \( 15.625 \) to the nearest thousandth, which is still \( 15.625 \).
So, the volume of the cube is:
\[ \boxed{15.625} \text{ ft}^3 \]