Concrete Block Calculator

Block estimating rests on modular coordination. A standard hollow concrete masonry unit measures 7 and 5/8 by 7 and 5/8 by 15 and 5/8 inches, deliberately undersized so that a 3/8 inch mortar joint brings the module to a nominal 8 by 8 by 16 inches. Layout arithmetic therefore works in whole 8 inch increments: each unit consumes 16.375 inches of wall length including one head joint, each course adds 8.375 inches of height, and one unit face with its joints covers about 0.89 square feet, which produces the familiar figure of 1.125 blocks per square foot of wall. Cutting is minimized when wall lengths and opening dimensions are set to multiples of 8 inches, and that economy is the main reason the module has survived unchanged for a century. A waste allowance of 5 percent is normal for a plain wall, rising to 7 to 10 percent where openings, corners, and control joints force cuts. Mortar for face shell bedding of standard units runs roughly 8 to 9 cubic feet per 100 blocks, which works out to about one 80 pound bag of preblended mortar for every 10 to 15 units. Type S mortar, at 1,800 psi, is specified below grade and for structural walls, while Type N at 750 psi suits above-grade veneer and partitions. Units meeting ASTM C90 carry a minimum net area compressive strength of 2,000 psi. Cost and labor outputs vary widely by region and wall complexity, so local unit rates should replace any default figures on a priced job.

Cast concrete blocks appeared commercially in the United States around the turn of the twentieth century. Harmon S. Palmer patented a practical hollow block machine in 1900, an iron mold with removable cores that let a small operator tamp out units on the job site. Within a decade mail order catalogs including Sears were selling block machines, and hollow concrete block became a common substitute for quarried stone in foundations, retaining walls, and small commercial buildings. Early units were frequently cast with a rock-face pattern on the exposed side to imitate the cut stone they were displacing. The industry organized and standardized quickly. The National Concrete Masonry Association was founded in 1918, and ASTM issued specifications for load-bearing units that evolved into ASTM C90, the standard still cited on drawings today. The nominal 8 by 8 by 16 inch unit displaced a variety of earlier sizes as dimensional coordination spread through American building practice in the 1920s and 1930s. Structural design moved from empirical tables to engineered methods with publication of the first Building Code Requirements for Masonry Structures by the Masonry Standards Joint Committee, designated ACI 530 and ASCE 5, in 1988; that document is now published as TMS 402. Fully reinforced and grouted block construction became dominant in seismic regions after West Coast earthquake experience, notably the 1971 San Fernando event, exposed the vulnerability of unreinforced masonry.