Concrete Block Calculator
Calculate the number of concrete blocks (CMU) or cinder blocks needed for any wall, including mortar and waste. Estimate total project cost.
Wall Dimensions
How to Use This Calculator
Measure Your Wall
Measure the total length and height of the wall you are building in feet. For walls with windows or doors, measure the full wall area and subtract opening areas, or calculate each section separately.
Select Block Size and Mortar Joint
Standard CMU (concrete masonry unit) blocks are 8x8x16 inches nominal. The 3/8-inch mortar joint is standard practice in residential and commercial masonry. Use the same joint thickness throughout the wall for consistent coursing.
Choose a Waste Factor
A 10% waste factor is standard for most block wall projects and accounts for cuts, breaks, and alignment adjustments. For walls with multiple corners, arches, or many openings, increase to 15-20% to avoid running short.
Enter Block Cost and Calculate
Enter the price per block from your supplier. Standard 8x8x16 CMU blocks typically cost $2-$4 each. Click Calculate to get total blocks needed, courses, and estimated material cost.
How We Calculate
The concrete block calculator uses standard masonry coursing arithmetic to determine the number of CMU (concrete masonry units) or cinder blocks required for a wall. Effective block dimensions account for mortar joint thickness: a standard 8x8x16 CMU with a 3/8-inch mortar joint has an effective height of 8.375 inches and effective length of 16.375 inches. The calculator determines blocks per horizontal course (row) by dividing wall length by effective block length, and number of courses by dividing wall height by effective block height, then multiplies these together for total block count.
The distinction between concrete blocks (CMU) and cinder blocks is primarily historical. True cinder blocks used industrial cinder aggregate from coal combustion and were common before 1950. Modern "cinder blocks" are actually lightweight CMU made with expanded shale, pumice, or slag aggregate — they are not made from cinders. For calculation purposes, standard, lightweight, and cinder blocks use the same nominal dimensions (8x8x16 inches is most common), so this calculator applies equally to all types. Structural requirements differ, however: exterior load-bearing walls typically require hollow-core or solid CMU with reinforcing steel and grout fill.
Waste factors account for several sources of material loss: cut blocks at corners, window and door jambs, bond pattern adjustments, and blocks broken during delivery or installation. The Masonry Contractors Association of America (MCAA) recommends a minimum 10% waste allowance for standard rectangular walls. Complex layouts with many openings or non-right-angle corners may require 15-20% waste allowance to avoid material shortages mid-project.
Sources & References
- Masonry Contractors Association of America — CMU Installation Guide (masoncontractors.org)
- National Concrete Masonry Association — CMU Dimensions and Properties (ncma.org)
- Portland Cement Association — Concrete Masonry Handbook (cement.org)
Data last verified:
Frequently Asked Questions
For a 100 square foot wall using standard 8x8x16 CMU blocks with 3/8-inch mortar joints, you need approximately 112-115 blocks before waste. Each block covers approximately 0.89 square feet of wall face when installed with standard mortar. Adding a 10% waste factor brings the total to approximately 123-127 blocks. The exact count depends on your specific block size and mortar joint thickness.
Modern "cinder blocks" are actually lightweight concrete masonry units (CMU) made with expanded shale, pumice, or lightweight aggregate. True cinder blocks, made from coal combustion byproducts, are rarely used today due to structural limitations and environmental concerns. For sizing and calculation purposes, they are identical — both use 8x8x16 nominal dimensions. CMU blocks are stronger and are the current construction standard for structural walls.
For a standard 8x8x16 CMU block with a 3/8-inch mortar joint, the effective length is 16.375 inches. One lineal foot (12 inches) equals approximately 0.73 blocks — so you need approximately 0.73 blocks per lineal foot per course. For a 40-foot wall, that is approximately 29-30 blocks per course. Multiple the blocks per course by the number of courses for total blocks needed.
An 8-foot (96-inch) wall built with standard 8x8x16 CMU blocks and 3/8-inch mortar joints requires approximately 11 courses. Each course is 8.375 inches tall (8-inch block + 3/8-inch mortar). Multiply 11 courses x 8.375 inches = 92.125 inches (7.68 feet). For a true 8-foot wall, you would need 12 courses (100.5 inches, or 8.375 feet) or adjust the mortar joint thickness.
Concrete block material costs range from $2-$4 per standard 8x8x16 CMU block. For a 40x8-foot wall (320 square feet), you need approximately 360-380 blocks including waste, costing $720-$1,500 in materials alone. Labor costs for professional masonry work run $10-$20 per square foot, bringing total installed cost to $5,600-$9,900 for this size wall. DIY installation eliminates labor costs but requires skill and proper tools.
Not always. For non-structural garden walls, retaining walls under 3 feet, or decorative applications, hollow-core blocks without fill are often adequate. For structural walls, load-bearing applications, or tall retaining walls, building codes typically require filling the cores with grout and installing vertical rebar for reinforcement. A structural engineer should specify fill and rebar requirements for any load-bearing wall.
For standard 8x8x16 CMU blocks with 3/8-inch mortar joints, plan on approximately 0.5-0.6 cubic feet of mortar per 100 blocks laid. One 80-pound bag of masonry cement and sand mix typically yields about 0.5 cubic feet of mortar. For a 100-block section of wall, you would need approximately 1-2 bags of mortar mix. Pre-mixed mortar bags list coverage on the packaging for more precise estimation.
The standard CMU block is 8 inches wide x 8 inches tall x 16 inches long (nominal dimensions). The actual dimensions are slightly smaller (7-5/8 x 7-5/8 x 15-5/8 inches) to allow for a 3/8-inch mortar joint on each face. Other common sizes include 4x8x16 (partition blocks), 6x8x16 (medium), 12x8x16 (heavy-duty), and 8x4x8 (half blocks). All follow the same modular sizing system.
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