Concrete Block Calculator
Concrete masonry units (CMU) are sized so that the nominal face includes the mortar joint. Standard 8x16-inch nominal blocks (7.625 x 15.625 inches actual) with a 3/8-inch joint fit 1.125 blocks per square foot. This calculator multiplies the wall face area by blocks per square foot, adds a waste percentage, and calculates mortar bags. Enter wall dimensions in feet and block nominal face dimensions in inches. For openings, subtract door and window areas from the total wall area before calculating.
Concrete block quantity formula
Blocks per sq ft = 144 / (Nominal height x Nominal length)
Wall area = Length x Height
Total blocks = ceil(Area x Blocks per sq ft x (1 + waste%))
Mortar bags = ceil(Total blocks / 100 * 2.2) (approx 1 bag per 45 blocks)
Nominal block dimensions already include the mortar joint, so this formula gives blocks per square foot of wall face directly. The mortar estimate (approximately 1 cubic foot per 100 blocks, 0.45 cf per 60-lb bag) is an approximation; actual mortar use depends on joint thickness and workmanship.
CMU wall construction notes
- Standard CMU per ASTM C90 has a minimum net area compressive strength of 1,900 psi.
- Lay CMU in running bond (staggered joints) for maximum strength unless otherwise specified.
- Control joints should be placed every 25 feet horizontally and at changes in wall height to control cracking.
- Reinforced cells should be filled with ASTM C476 grout, not concrete mix.
- All CMU below grade or exposed to moisture should be waterproofed per IBC Section 1805.
Concrete block calculator: frequently asked questions
What is the standard concrete block size?
The nominal size of a standard concrete masonry unit (CMU) is 8x8x16 inches (actual dimensions 7.625 x 7.625 x 15.625 inches). The nominal size includes the 3/8-inch mortar joint. Standard blocks cover 0.889 square feet of wall face each.
How many standard blocks per square foot?
Standard 8x16-inch nominal blocks (including 3/8-inch joint) yield approximately 1.125 blocks per square foot of wall face. That equals about 112.5 blocks per 100 square feet of wall.
How much mortar do I need for concrete block?
Approximately 7 to 8 cubic feet of mortar per 100 square feet of wall (single-wythe), or about 1 cubic foot per 100 blocks. One 60-pound bag of mortar mix yields about 0.45 cubic feet of mixed mortar.
Do concrete block walls need rebar?
Reinforced CMU walls are required in most seismic zones and for tall or heavily loaded walls. Vertical rebar is placed in hollow block cores and filled with grout. Check IBC Section 2104 and local seismic requirements.
What is the difference between 4-inch, 6-inch, 8-inch, and 12-inch block?
The inch designation refers to the actual depth (thickness) of the block. 8-inch block is the most common for structural walls. 4-inch and 6-inch blocks are used for non-structural partitions. 12-inch block is used for heavily loaded walls and retaining walls.
Official sources
- ASTM International: ASTM C90 Standard Specification for Loadbearing Concrete Masonry Units.
- National Concrete Masonry Association: NCMA TEK Notes.
Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.