The volume \( V \) of a cube can be calculated using the formula:
\[ V = a^3 \]
where \( a \) is the edge length of the cube.
Given that the edge length \( a \) is 2.5 ft, we can calculate the volume as follows:
\[ V = (2.5)^3 = 2.5 \times 2.5 \times 2.5 \]
Calculating that step-by-step:
- \( 2.5 \times 2.5 = 6.25 \)
- \( 6.25 \times 2.5 = 15.625 \)
So, the volume of the cube is:
\[ V = 15.625 , \text{ft}^3 \]
Rounded to the nearest thousandth, the volume is:
\[ \boxed{15.625 , \text{ft}^3} \]