How Much Red Lava Rock for Landscaping

Measure the Site

• Before you can calculate the amount of red lava rock needed, you must measure the area of the space you are covering. The simplest way to do this is to measure the length and width with a tape measure, but landscaping features are rarely shaped as perfect rectangles. If you're covering an L shaped garden or curved plant bed, break it into smaller rectangles and calculate the area separately, then add them together. Multiply length by width and round this number up.

Determine the Depth

• In addition to area, the other figure you must know is the minimum depth of the layer of lava rocks. If you're using the rock for mulch or a rock garden, a 2- to 4-inch thick layer is thick enough to provide even coverage while suppressing weeds. For a walkway or other surface intended for foot traffic, you'll need a depth of at least 6 inches. Add an additional 2 inches if the area has poor drainage. Lava rocks absorb heat during the day and release it once the sun sets, which reduces underground moisture.

Calculate Cubic Yards

• Multiply the square footage by the depth to find the amount of lava rocks you need in cubic feet. For example, to lay a 4-inch thick layer over 100 square feet, you need 400 cubic feet of red lava rock. Many loose landscaping materials are sold in cubic yards, so you must then convert your cubic feet into cubic yards by dividing the cubic feet by 27. In the example, 400 cubic feet divided by 27 equals 14.8 cubic yards.

Order Extra

• It's advantageous to purchase 5 percent more material than you expect to use. Buying extra ensures you'll have enough lava rocks to complete your project even if there's some spillage. It also accounts for the fact that the rocks pack down under foot pressure. Save any extra and plan to top off your layer once the rocks settle into place over a few weeks.