Halve the diameter to find the radius. For example, if you want a circle with a diameter of 4 inches, take half of 4, or 2 inches.
Draw a line that is the length of the radius. In the example, use the ruler to draw a line that is 2 inches long.
Place the point of the compass on one of the ends of the line.
Rotate the compass with the writing end touching the paper to draw a circle with the desired diameter.