How to Calculate Gravel Needed to Fill a Fire Pit

Multiply the depth of the gravel layer by 3.14 (pi or π). In this example, we'll say the depth of the gravel is 6 inches. Convert the 6-inch depth into feet, so 6 inches equals .5 feet. Multiply .5 by 3.14 to get a depth of 1.57 feet.

Square the radius of the fire pit. The radius is the distance from the center of the circle to the edge, or half the diameter of the circle. In this example, the radius is 3 feet, so 3 times 3 equals 9 feet.

Multiply the products from steps 1 and 2 together, so 1.57 times 9 equals 14.13. You need 14.13 cubic feet of gravel for the fire pit.

Add 10 percent to the total to give yourself a little extra gravel in case you need it. Ten percent of 14.13 cubic feet is an additional 1.4 feet, bringing the total amount of gravel you need to 15.5 cubic feet.