Circles are flattering in the landscape, lending rounded lines that soften and offset the hard angles of square houses and outbuildings. Creating a circle with concrete blocks is a straightforward task, but trying to "eyeball" the circle may lead to a lopsided shape. For a uniform circle you'll need a few supplies that you probably have around the house.
Choose a flat spot of ground. Determine the diameter of the circle, and divide that number in half. For instance, if you want a 10-foot circle, divide 10 in half and you would have 5 feet.
Measure a lightweight piece of wood or a broomstick from one end to the length of your divided number. In the example, that would be 5 feet. Make a mark on the stick at that point, but don't cut it.
Hammer the 4-inch nail through the center of the stick at the mark, so it protrudes on the other side.
Place a concrete block on the ground in the center of the proposed circle and use your hammer to gently tap the nail in the stick into the center of the concrete block. Tap gently and the nail will slip right in.
Turn the stick. You've just created a "trammel," a rotating device used to measure round shapes. By moving the stick, the outside end will rotate in a perfect circle that you will use to guide your block placement.
Position the concrete blocks in the circle, using the trammel as a guide. Check each one as you go by rotating your trammel. The end of the stick should come very close, approximately 1/8 inch from each block, without touching the blocks.
Adjust the blocks as needed to form an exact circle.