What is the diameter of a cylinder with a diameter of 8 cm and height of 5 inches?

Of course the diameter of a cylinder with a diameter of 8 cm is 8 cm, but I suspect you were asking about volume or surface area.

First, the measurements should be converted to the same units. Since there are 2.54 centimeters per inch, the cylinder has a diameter of 8 cm and a height of 12.7 centimeters.

The volume of a cylinder is pi * (1/2 d)^2 * h, so the volume of cylinder with diameter 8 cm and height 5 inches is pi * 16 * 12.7 = 997.46 cubic centimeters.
The surface area of a cylinder is (2*pi*r*h) + (2*pi*r^2) = (in our case) 556.06 square centimeters.