How to Calculate the Amount of Materials Needed to Build a Retaining Wall

Retaining Wall Calculator

When building a retaining wall, you need to estimate the number of stone blocks, capstones, and cubic yards of gravel needed to complete the project. Getting an accurate landscaping estimate will help you plan for the cost of materials and labor to build a retaining wall. You can figure the amount of materials following the steps below, or by using the convenient retaining wall calculator on the left.

This article assumes that you build a retaining wall following this method: First dig a trench that is twice as wide as the wall, as long as the finished wall, and 8 inches deep. Then fill the base of the trench with coarse gravel, and build the wall so that about 6 inches (0.5 feet) of the wall rests below ground. Leave about 4-6 inches of space between the back of the retaining wall and the soil, then use more coarse gravel for backfill behind the wall.

Calculating Number of Stone Blocks

The number of stone blocks depends on the finished height and length of the retaining wall. Multiply the wall's length by its height plus 1/2 a foot to find the total square footage of the wall. That is, calculate

Wall Area = L(H + 0.5) sq. ft,

where L is the length of the wall in feet and H is the finished height of the wall in feet. Next find the face area of each stone block by multiplying its length and width in inches. That is, calculate

Stone Block Area = AB sq. inches,

where A is the length of the block in inches and B is the width of the block in inches. Now, compute the total number of stone blocks needed with the formula

Number of Blocks = 144L(H + 0.5)/(AB).

The factor of 144 is necessary to convert feet to inches. Example: Suppose a wall is to be 20 feet long and 3 feet height, built with stones that are 6 inches by 10 inches. Then the number of stones needed is

144(20)(3 + 0.5)/[(6)(10)] = 10080/60 = 168.

You should purchase stone blocks to spare in case of breakage.

Calculating the Number of Capstones

To figure the number of capstones needed, dividing the length of the wall in feet by the length of the capstones in inches, then multiply by 12. For example, if the wall is 20 feet long and the capstones are 8 inches wide, then the number of capstones needed is [20/8](12) = 30. You may need to buy a few more in case some of the capstones break.

Calculating the Amount of Gravel

First estimate the amount of gravel needed to pack the trench at the base of the retaining wall. If the wall is L feet long and T inches thick, then the trench needs to be L feet long, about 8 inches deep, and 2T inches wide. Assuming that about 5/8 of the trench's volume will be filled with gravel, the cubic yards of gravel need for the trench is given by the formula

Trench Gravel = (5/1944)LT cubic yards,

where L is in feet, T is in inches, and the number 5/1944 accounts for the 8 inch depth and conversion factors for feet to yards and inches to yards. Next, estimate the cubic yards of gravel needed for backfill behind the retaining wall. Assuming about 4 inches of space between the stones and the earth, the volume of gravel needed is given by the formula

Backfill Gravel = LH/81 cubic yards,

where L and H are the length and finished height of the wall in feet. Example: Suppose a retaining wall that is 20 feet long, 3 feet high, and 6 inches thick. Thus, L = 20, H = 3, and T = 6. The total amount of gravel needed is

(5/1944)LT + LH/81
= (5/1944)(20)(6) + (20)(3)/81
= 0.30864 + 0.74074
= 1.05 cubic yards of gravel.

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