Anatomy of a concrete masonry unit — face shells, webs, cores, and nominal vs actual size

A CMU is a two-core hollow unit: two face shells tied by three webs, with two cores to grout. Actual size is nominal − ⅜″, and one ⅜″ joint restores the 8×8×16 module — which is why it works out to 1.125 blocks per square foot.

A plan view of a standard two-core hollow concrete masonry unit (CMU), the block people call a cinder block. The two long walls are the face shells, at least 1¼ inches thick on an 8-inch unit; three cross webs connect them, leaving two hollow cores that get grouted when the wall is reinforced. The actual unit is 15⅝ inches long by 7⅝ inches wide by 7⅝ inches tall — nominal minus ⅜ inch in every direction, so one ⅜-inch mortar joint restores the 8-by-8-by-16-inch module. Because the nominal 8-by-16 face is 0.889 square feet, a wall needs 1.125 blocks per square foot, which is 119 units per 100 square feet with 5 percent waste. — Source: ASTM C90; NCMA/CMHA TEK 04-02A (119 units / 100 ft²)

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A plan view of a standard two-core hollow concrete masonry unit (CMU), the block people call a cinder block. The two long walls are the face shells, at least 1¼ inches thick on an 8-inch unit; three cross webs connect them, leaving two hollow cores that get grouted when the wall is reinforced. The actual unit is 15⅝ inches long by 7⅝ inches wide by 7⅝ inches tall — nominal minus ⅜ inch in every direction, so one ⅜-inch mortar joint restores the 8-by-8-by-16-inch module. Because the nominal 8-by-16 face is 0.889 square feet, a wall needs 1.125 blocks per square foot, which is 119 units per 100 square feet with 5 percent waste.

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    Diagram: <a href="https://remodelcalculators.com/construction-diagrams/cmu-anatomy" target="_blank" rel="noopener">Anatomy of a concrete masonry unit — face shells, webs, cores, and nominal vs actual size</a> — courtesy of <a href="https://remodelcalculators.com" target="_blank" rel="noopener">RemodelCalculators.com</a>
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