Concrete Block / CMU Calculator
Enter your wall dimensions and block size to calculate how many CMU blocks, mortar bags, and rebar cells your masonry project requires. Includes mortar joint spacing and a configurable waste factor.
What is Concrete Block / CMU?
A concrete block calculator (also called a CMU calculator) estimates the number of concrete masonry units, mortar, and reinforcement needed to build a block wall. Concrete blocks, often called cinder blocks or CMUs, are the standard building material for foundation walls, retaining walls, garden walls, and structural partitions. The standard CMU measures 16 inches long by 8 inches tall by 8 inches wide. However, the actual dimensions are 15-5/8 by 7-5/8 by 7-5/8 inches to allow for a 3/8-inch mortar joint on each side. When laid with mortar, each block occupies a 16 by 8 inch module in the wall face. This modular sizing means a standard block wall works out to approximately 1.125 blocks per square foot. Mortar consumption depends on block size and joint thickness. For standard 8x8x16 CMUs with 3/8-inch joints, one 70-pound bag of Type S or Type N mortar lays approximately 12 blocks. Type S mortar is standard for below-grade and structural walls, while Type N is acceptable for above-grade non-load-bearing walls. Rebar reinforcement is typically required by code in seismic zones and for any wall over 4 feet tall. The standard pattern is vertical rebar every 4 feet (every third cell in a standard block) with the cells grouted solid. Horizontal rebar is placed in bond beam blocks every 4 feet of wall height. This calculator estimates vertical rebar cells assuming one rebar every 4 cells horizontally, multiplied by the number of courses. The waste factor for block walls is lower than for most materials because blocks are uniform and breakage is minimal if handled properly. A 5 percent waste factor is standard, though complex walls with many corners or openings may need 8 to 10 percent. Accurate block estimation saves significant money because CMUs are heavy and expensive to deliver, and returning unused pallets often incurs restocking fees.
How to Calculate
- Measure the total wall length in feet
- Measure the wall height in feet
- Confirm the block size (standard is 16 inches long by 8 inches tall)
- Enter the mortar joint width (3/8 inch is standard)
- Set the waste factor (5% is standard for straightforward walls)
- Review total blocks, mortar bags, and rebar cells needed
Formula
Wall Area = Wall Length (ft) x Wall Height (ft) Effective Block Area = ((Block Length + Mortar Joint) / 12) x ((Block Height + Mortar Joint) / 12) sq ft Blocks per Sq Ft = 1 / Effective Block Area Total Blocks = ceiling(Wall Area / Effective Block Area x (1 + Waste%/100)) Mortar Bags = ceiling(Total Blocks / 12) Rebar Cells = ceiling(Cells per Course / 4) x Number of Courses Where 12 blocks per 70 lb bag is the standard mortar coverage, and rebar is placed every 4th cell vertically through all courses.
Example Calculation
A 20 ft long by 8 ft tall wall using standard 16x8 blocks with 3/8-inch mortar joints and 5% waste: Wall Area = 20 x 8 = 160 sq ft Effective Block Area = (16.375 / 12) x (8.375 / 12) = 1.365 x 0.698 = 0.953 sq ft Blocks per Sq Ft = 1 / 0.953 = 1.05 Blocks (raw) = 160 / 0.953 = 167.9 Blocks with 5% waste = ceiling(167.9 x 1.05) = ceiling(176.3) = 177 blocks Mortar Bags = ceiling(177 / 12) = 15 bags Courses = ceiling(8 / 0.698) = 12 courses Cells per Course = ceiling(20 / 1.365) = 15 Rebar Cells = ceiling(15 / 4) x 12 = 4 x 12 = 48 cells
Frequently Asked Questions
What is the difference between a cinder block and a concrete block?
Modern concrete blocks (CMUs) are made from Portland cement and aggregate. The term cinder block originally referred to blocks made with coal cinders as aggregate. Today the terms are used interchangeably, but nearly all blocks sold are true concrete blocks. CMUs are denser and stronger than old-style cinder blocks.
How much does a standard concrete block weigh?
A standard 8x8x16 hollow-core CMU weighs approximately 35 to 38 pounds. Lightweight blocks weigh about 28 pounds. Solid blocks can weigh 50 pounds or more. Weight matters for planning labor and delivery — a pallet of 90 standard blocks weighs approximately 3,400 pounds.
What type of mortar should I use for a block wall?
Use Type S mortar for foundation walls, retaining walls, and any below-grade application. Type S has a minimum compressive strength of 1,800 PSI. Type N mortar (750 PSI) is suitable for above-grade, non-load-bearing walls. Type M (2,500 PSI) is used for heavy-load applications but is rarely needed in residential construction.
Do I need to fill all the block cells with grout?
Not always. Building codes typically require grouting only the cells that contain rebar. However, fully grouting all cells significantly increases wall strength and is required in high-seismic zones. Check your local building code for specific requirements.
How tall can I build a block wall without rebar?
Most codes limit unreinforced block walls to 4 feet in height. Any wall taller than 4 feet typically requires vertical rebar at 48-inch spacing and horizontal bond beams at 48-inch vertical intervals. Retaining walls always require reinforcement regardless of height.
Should I subtract openings like doors and windows from the wall area?
Yes, for a more precise estimate. A standard door opening is about 21 sq ft (3x7 ft) and a standard window is about 15 sq ft (3x5 ft). However, blocks around openings require lintel blocks and extra cutting, so the savings from fewer blocks may be offset by additional specialty blocks and labor.