# The Q&A Archives: Building with blocks

Question: I am wanting to build a fire pit (or raised garden) with the semi-curved retaining wall type block and was looking for a formula to make it 5' in diameter. I have looked online and am unable to locate such a formula. Thanks in advance.

Answer: To build a round wall, you first must find the center of the circle, even if you plan to build only a portion of a circle or arch. From the center point, a string is pulled, which, when moved around creates a circle. This represents the radius of the circle and is used to place each stone at precisely the same distance from the center point to create a nice, even circle. So, drive a stake into the ground in the center of the proposed area and attach a string to the stake, then make a mark on the ground in the shape of your half circle. Measure the half circle. Then measure the length of one of your interlocking blocks. Multiply its length by the number of inches/feet in your semi circle.

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