The formula for the volume \( V \) of a cube is given by:
\[ V = s^3 \]
where \( s \) is the length of one edge of the cube.
For a cube with an edge length of \( 2.5 \) ft, the volume can be calculated as follows:
\[ V = (2.5 , \text{ft})^3 \]
Calculating \( (2.5)^3 \):
\[ (2.5)^3 = 2.5 \times 2.5 \times 2.5 \]
First, calculate \( 2.5 \times 2.5 \):
\[ 2.5 \times 2.5 = 6.25 \]
Next, multiply by \( 2.5 \) again:
\[ 6.25 \times 2.5 = 15.625 \]
Thus, the volume of the cube is:
\[ V = 15.625 , \text{ft}^3 \]
Rounding to the nearest thousandth:
\[ V \approx 15.625 , \text{ft}^3 \]
Therefore, the volume of the cube is:
\[ \boxed{15.625} , \text{ft}^3 \]