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.
What this diagram shows
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.
Cinder Block / CMU Calculator
Estimate cinder block (CMU) quantities: blocks, mortar bags, grout, bond-beam & lintel units, and rebar — from ASTM C90 & NCMA TEK. Free, no signup.
Related diagrams
Masonry, Stucco & HardscapeBrick bond patterns compared — running, ⅓ running, stack, common, English, and Flemish
Masonry, Stucco & HardscapeWhere bricks-per-square-foot comes from — the nominal brick-plus-joint cell
Masonry, Stucco & HardscapeAnchored brick-veneer wall section — air space, WRB, ties, flashing, and weep holes
Masonry, Stucco & HardscapeWhat goes under pavers — compacted base, bedding sand, joint sand, and edge restraint
Masonry, Stucco & HardscapePaver laying patterns — running bond, herringbone, basketweave, and waste factors
Masonry, Stucco & HardscapeWhy pavers need edge restraint — the perimeter creeps and unravels without it