Calculating the exact number of concrete blocks for a project comes down to one simple formula: total wall surface area divided by the face area of a single block, plus a standard 10% safety buffer for cuts and waste. First, calculate the wall”s square footage by multiplying its length by its height in feet. Be sure your height measurement includes any below-ground rows, since many retaining walls require at least one buried layer to properly anchor the structure. If your wall includes doors or windows, calculate their area (width multiplied by height) and subtract that from the total wall area.
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Once you have the net wall area, you can then determine how many blocks you”ll need. It”s easy to mix them up, but understanding keeps you from buying the wrong material for a heavy-duty wall. Most standard units have nominal dimensions of 8 by 8 by 17 inches, which account for a standard 3/8-inch mortar joint. To calculate the surface area of a single block, multiply its height by its length in inches, and divide that number by 144 to find the square footage. This gives you roughly 0.89 square feet per standard block. As a rule of thumb, it takes 1.125 blocks per square foot of wall surface area.
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