Math CentralQuandaries & Queries


Question from eric:

what is the square footage of the inside wall of a fire pit 2 feet deep and five feet in diameter? I need to estimate both the floor and wall square footage to order bricks.


The area of a circle is π times the radius squared where π is approximately 3.1416. The circle that forms the floor has a radius of 2.5 feet and hence its area is π × 2.52 = 19.6 square feet.

To find the area of the wall imagine that it is lined with a large sheet of paper. Take the sheet of paper out and roll it flat. It will measure 2 feet high and the length is the circumference of the circular pit. The circumference of a circle is π times the diameter and hence the rectangular sheet of paper measures 2 feet by π × 5 = 15.7 feet. Thus the area of the wall is 2 × 15.7 = 31.4 square feet.



Wall area = pi*d*h = 31.4 square feet. Each course will be about 15' 6" long. You may want to fine-tune the radius to get an exact number of bricks with the mortar width you want rather than cutting bricks.

For the floor, you will need to cut bricks, have gaps, or have bricks extending beneath the wall, so the theoretical area of 19.6 square feet may not be a very good guide to brick use.


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