When building a retaining wall, DIY landscapers need accurate estimates on the amount of stone blocks, cap stones, and gravel needed to complete the project. Getting a landscaping estimate will help you plan for the cost of materials and labor to build the retaining wall, which will help you save time and money.
The number of stone blocks and gravel needed depends on the dimensions of the wall and the depth of the backfill space behind the wall. The steps below will show you how to make the calculations.

Multiply the length of the wall (in feet) by the height of wall (also in feet) to get the square footage of the outside face of the wall. Then, multiply 1.12. This allows for having 12% of the wall's height under the surface of the ground to give the wall more stability.
For example, if the wall is to be 24 feet long and 2.5 feet high, then the square footage is (24)(2.5)(1.12) = 67.2 sq.ft.

Determine the square footage of the outside face of each stone block. Measure its area in square inches and then divide by 144 to convert to square feet.
For example if each stone block is 10 inches wide and 6 inches wide, then the square footage of each stone face is (10)(6)/144 = 0.4167 sq.ft.

Divide the square footage of the retaining wall's face by the square footage of each stone's face, and this gives the total number of stones needed.
Using the sample numbers above, we would need 67.2/0.4167 = 162 blocks.
To account for breakage, you should buy several more. 
Estimate the amount of gravel needed to pack the trench at the base of the retaining wall. If the wall is L feet long, H feet high, and T inches thick, then the trench needs to be L feet long, 0.12H feet deep, and 2T inches wide.
In cubic yards, the volume of the trench is LHT/1350, and about half of the volume will be filled with coarse gravel. Thus, the cubic yards of gravel needed for the base is LHT/2700.
For example, if the wall is 24 ft. long, 2.5 ft high, and 8 inches thick, then we need (24)(2.5)(8)/2700 = 0.1778 cubic yards of gravel for the base.

Estimate the cubic yards of gravel needed for backfill behind the retaining wall. Assuming that there is half a foot of space between the stones and the earth, the volume of gravel needed is
0.5LH cubic feet, or
LH/54 cubic yards.For example, for a retaining wall that is 24 feet long and 2.5 feet high, we will need (24)(2.5)/54 = 1.111 cubic yards of gravel for backfill.

Calculate the number of capstones needed by 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 24 feet long and the capstones are 8 inches wide, then the number of capstones needed is 24/8 = 36. You may need to buy a few more in case some stones should break or need to be cut.