Project managers who must estimate concrete block for large construction projects hire trained estimators to do the job. These estimators use specific tables along with specialized computer programs to obtain accurate results. Fortunately for the homeowner who needs to estimate concrete block for a smaller project such as the foundation of a house, a simple rule of thumb is more than adequate when using standard concrete block and is accurate and based on many years of practical experience.
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Add the wall lengths to determine the total linear length. A standard block is 16 inches long, which means that one foot equals threequarters of a block. Multiply the total linear length (in feet) by 0.75 to obtain the total number of blocks needed for one course.

Calculate the number of courses in your foundation. A course of standard concrete block, including the mortar joints, is eight inches high. Divide the height of the foundation (in inches) by eight to figure out how many courses you need.

Multiply the number of courses by the number of blocks in each course. This is the total number of blocks required for your foundation. If you have any openings in your foundation, deduct some blocks to account for them. Multiply your total by 1.05 to allow for any breakage or waste.

If your foundation will be 10 or 12 inches thick, you need special Lshaped corner blocks at the corners of each course. Multiply the number of courses by the number of corners in your foundation to learn how many L blocks you need, and subtract this amount from the total number of regular blocks.
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References
 Building with Masonry; Richard Kreh; 1998