# How to Calculate Gravel Needed to Fill a Fire Pit Gathering around a fire pit at the end of a summer barbecue or on a cool, crisp autumn night is something many people enjoy doing. One of the first steps to building a fire pit is gathering the proper amount of materials. Gravel provides a stable base for the stones and drainage from the center of the pit, so you don't have to build a fire on wet or muddy ground. Determine the amount of gravel you'll need by computing the volume of a cylinder. Gravel provides a solid base for a fire pit.
1. 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.

2. 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.

3. 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.

4. 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.