The measurements required for a retaining wall design will vary depending on the type of wall built and its intended function. For example, a wall that will be exposed on both sides will require more material than one that is exposed on one side. You also must calculate how much backfill, if any, will be needed. Completing the calculations are not difficult, but you must know the dimensions of the stone or paver that are planned for use.
Measure the length of the area in inches where you plan to build the wall. Account for any curvature of the landscape.
Multiply the length by the wall's desired height. This will give you the total square inches of area. For example, a wall that is 180 inches long by 48 inches high leads to an area measuring 8,640 total square inches.
Multiply the length and width of one paver or stone to determine the area on the stone's face. For example, a paver stone that is 6 inches long by 2 inches high has an area of 12 square inches.
Divide the area calculated in Step 2 by the area of the paver calculated in Step 3 to determine the minimum number of stones required for a wall that is a single stone thick wall. Using our example, 8,640 divided by 12 is 720.
Multiply the number of required stones by 1.2 to account for breaks and for the portion of stones that will be underground to support the wall. Using our example, 720 multiplied by 1.2 is a total of 864 stones.
Increase the number of stones by the desired thickness. For example, if you want a wall that is three pavers deep, multiply the stone count by three for the total number of stones required for the wall.
Measure the distance between where the ground will hit your wall and the top of the wall. For example, if the ground measures 18 inches high from the base of your wall design and your wall will measure 4 feet high, the distance is 30 inches.
Square this measurement. For example, 30 inches times 30 inches is 900 square inches.
Multiply the result of Step 2 by the length of the wall in inches. In the example, 900 square inches multiplied by 180 inches is 162,000 cubic inches. However, you must divide this number in half to account for the area behind the wall that is already filled. So to completely fill in the area behind this example, about 81,000 cubic inches of backfill will be required.
If your retaining wall will measure more than 28 inches high, the first 6 inches of backfill behind the wall should be gravel to help facilitate drainage.