If you are planning to build a garage out of concrete block, you should know how many blocks you will need before you shop around at local block suppliers. Fortunately, a concrete block is standardized, with a face 8 inches high by 16 inches long, so you can use simple math to estimate the number of blocks your garage project will require.
Find the perimeter of your planned garage by adding together the planned length of all four walls in feet. Subtract from that figure the width in feet of the garage door or doors since you are not going to put a wall in the door opening. For instance, if your garage will be 24 feet wide by 20 feet deep, with two 9-foot-wide doors, you add 24 + 24 + 20 + 20 to get 88 feet, then subtract 18 feet for the doors to get a 70 foot net perimeter.
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Multiply the net perimeter by the planned height in feet to get the total wall area in square feet. If your garage walls are to be 10 feet tall, multiply 10 by the 70-foot net perimeter to get 700 square feet of total wall area. For this rough estimate, you don't need to deduct side door or side window area.
Multiply the total wall area by 1.125 to get the approximate number of blocks you'll need to build the garage walls. Multiplying 700 square feet of wall area by 1.125 is 788, meaning you will need around 788 concrete blocks to build your garage walls. Add 5 percent to the total to account for spoiled blocks, which in this example is 40 blocks. Adding 40 to 788 results in 828 blocks needed to build your garage walls.