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 \( s \) is 2.5 ft.
Now, we can calculate the volume:
\[ V = (2.5)^3 = 2.5 \times 2.5 \times 2.5 \]
Calculating step by step:
- \( 2.5 \times 2.5 = 6.25 \)
- \( 6.25 \times 2.5 = 15.625 \)
Thus, the volume of the cube is:
\[ V = 15.625 \text{ ft}^3 \]
Rounded to the nearest thousandth, the volume is:
\[ \boxed{15.625} \]