The volume \( V \) of a cube is calculated using the formula:
\[ V = s^3 \]
where \( s \) is the length of an edge of the cube.
For a cube with an edge length \( s = 2.5 \) ft, the volume can be calculated as follows:
\[ V = (2.5)^3 \]
Calculating \( (2.5)^3 \):
\[ 2.5 \times 2.5 = 6.25 \] \[ 6.25 \times 2.5 = 15.625 \]
Thus, the volume of the cube is:
\[ V = 15.625 , \text{ft}^3 \]
Rounding to the nearest thousandth, the answer remains:
\[ \boxed{15.625} , \text{ft}^3 \]