Number of Blocks to Build a Retaining Wall

Wall Length

• Before you can determine how many cinder blocks you need for your retaining wall, you'll need to measure the length of your proposed retaining wall, whether you're building on a straight line or a curve. Write down the footage.

Block Size

• When you've decided exactly where you want your retaining wall, you'll need to know the size of the cinder blocks you'll be using for construction. Measure the size of the cinder block or blocks you want for your retaining wall. Retaining walls are commonly constructed of cinder blocks fitted with rebar for strength and support and to prevent leaning. Use a tape measure to determine the length, height and width of the block. Cinder blocks typically come in 16-inch lengths and are 8 inches in height.

Measurement Conversion

• First, convert the length of the proposed wall to inches. If your retaining wall is going to be 40 feet in length, for example, multiply 40 by 12 (the number of inches in a foot), which equals 480. Then divide by the length of the cinder block. Standard cinder blocks are 16 inches long; 480 divided by 16 equals 30, which is the number of cinder blocks it will take to construct the first row of the 40-foot wall.

Height

• You'll also need to know how high you want your retaining wall to be. For example, if you want your wall to be 6 feet tall, convert feet into inches. Multiple 6 by 12, which equals 72 inches. Divide 72 by 8, or the height of your cinder block, and you come up with nine. This means you'll need nine cinder blocks stacked one on top of each other to reach your 6-foot height.

Footing

• It's advisable to place 3 to 4 feet of your retaining wall below ground level for stability. Your local building codes may also specify a certain depth for your retaining wall. Measure the number of blocks for your footings the same way you measure the above-ground number of cinder blocks and add to your final tally.

Total Number of Cinder Blocks

• To figure out the total number of cinder blocks, add the number of cinder blocks for the height of the wall above the ground to the number of blocks for the footing. Multiply the total by the number of cinder blocks it takes for the length of the wall.

Adjusting for Mortar

• The height of your retaining wall will be affected by the mortar applied between the rows of cinder block. The mortar in cinder-block walls typically measures 1/2 inch. For a nine-row cinder-block wall, the mortar will add 5 inches to the height of the wall. (The mortar in the below-ground footing will also need to be taken into consideration for the final height of the wall, from its base to the top.)