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.
Given that the edge length \( s \) is 2.5 ft, we can substitute this value into the formula:
\[ V = (2.5)^3 \]
Calculating \( (2.5)^3 \):
\[ (2.5)^3 = 2.5 \times 2.5 \times 2.5 \]
First, calculate \( 2.5 \times 2.5 \):
\[ 2.5 \times 2.5 = 6.25 \]
Now, multiply \( 6.25 \) by \( 2.5 \):
\[ 6.25 \times 2.5 = 15.625 \]
Thus, the volume of the cube is:
\[ V = 15.625 \text{ ft}^3 \]
Now, rounding to the nearest thousandth does not change the value since it's already in the required precision:
\[ V = 15.625 \text{ ft}^3 \]
Therefore, the volume of the cube is \( \boxed{15.625} \) ft³.