The volume \( V \) of a cube can be calculated using the formula:
\[ V = s^3 \]
where \( s \) is the length of the edge of the cube.
Given that the edge length \( s = 2.5 \) ft, we can calculate the volume:
\[ V = (2.5)^3 \]
Calculating \( (2.5)^3 \):
\[ (2.5)^3 = 2.5 \times 2.5 \times 2.5 \]
Breaking it down:
\[ 2.5 \times 2.5 = 6.25 \] \[ 6.25 \times 2.5 = 15.625 \]
Thus, the volume of the cube is:
\[ V = 15.625 \text{ ft}^3 \]
Rounding 15.625 to the nearest thousandth gives:
\[ 15.625 \text{ ft}^3 \]
So, the final answer is:
\[ \boxed{15.625} \text{ ft}^3 \]