A retaining wall should have the power to hold back the weight of the soil, plants and at times small structures behind it. The wall gets its strength from a sturdy base consisting of gravel — about 5 inches deep for short walls and about 7 inches for tall walls — which can help avoid sifting and allows water to drain under the wall. A 1-inch layer of coarse sand on top of the gravel will help with leveling. The first one or two tiers of stone are also buried to present a hardy, well-anchored base. Doing some basic calculations for the base and the wall makes it possible to determine the right amount of stone and gravel necessary for building a durable retaining wall.
Gravel Needs
Divide the base thickness of 5 to 7 inches — depending on the height of this wall — by 1,944 to determine the square yardage of this thickness of the gravel base under the wall.
Assess the width and length of the wall. Double the wall width, and use the actual length measurement when determining gravel needs.
Multiply the width and length measurements to determine the square footage of this wall. Multiply this number by the square yardage of this gravel thickness to determine the total amount of base gravel required for the retaining wall.
Multiply the length of the wall by the desired height, then divide this number by 81 to determine the amount of backfill gravel required. Insert the backfill amount to this base amount to determine the total cubic yards of gravel needed for finishing the wall.
Stone Needs
Assess the length in feet of the desired wall. Determine the desired height in feet and then add .5 for this measurement. The .5 accounts for its 6 inches of rock footers that sit underground after retaining wall installation. If you’re constructing a wall with deeper footers, adjust this measurement to correctly reflect the desired thickness of the wall footers.
Multiply the length and adjusted height measurement to determine the square footage of this retaining wall. Multiply the square footage number by 144 to find the size of this wall in square inches.
Measure the average width and length of the stone blocks in inches. Multiply both of these numbers to determine the normal measurement of each block in square inches.
Divide the square inch width of the wall by the square inch width of the average block. The resulting number is a strong estimate of the amount of blocks necessary to complete the retaining wall.