To find the surface area of a sphere, we use the formula:
\[ \text{Surface Area} = 4\pi r^2 \]
where \( r \) is the radius of the sphere.
Given that the diameter of the sphere is 40, we first find the radius by dividing the diameter by 2:
\[ r = \frac{40}{2} = 20 \]
Now, we can plug the radius into the surface area formula:
\[ \text{Surface Area} = 4\pi (20)^2 \]
Calculating \( (20)^2 \):
\[ (20)^2 = 400 \]
Now substituting back into the formula for surface area:
\[ \text{Surface Area} = 4\pi \times 400 = 1600\pi \]
So the surface area of the sphere is:
\[ \boxed{1600\pi} \]