When you need to draw a small circle, up to a foot in diameter, a compass is the perfect tool. But if you need a bigger circle, or if you don't have a compass, you can make your own, custom-tailored to the job.
First, drill a small hole in the end of a strip of wood. Next, drive a finish nail into the center of the stock out of which I want to cut the circle. Nip the head off the tip of the nail.
Insert the nail into the hole you drilled in the wood.
Using a tape measure, make a mark equal to the radius of the circle you want to draw.
Remove the board from the workpiece and drill a second hole at the location you just marked.
To draw a circle, insert the nail in the first hole, pass the tip of a pencil through the second hole and draw your circle.
With this technique, you can make a circle of any size. If you want a circle eight feet in diameter, all you would need is a longer piece of wood.