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