Concrete Block (CMU) Wall Estimating: Blocks, Mortar, and Grout per Square Foot
October 21, 2026
Concrete masonry units (CMU), commonly called cinder blocks, are the building blocks of foundations, retaining walls, garages, and boundary walls. A standard block is nominally 8 inches high, 8 inches thick, and 16 inches long, but the actual dimensions are 3/8 inch smaller (7⅝ × 7⅝ × 15⅝) to allow for a 3/8-inch mortar joint. The nominal face is 8 × 16 = 128 square inches, or 0.889 square feet, which is 1.125 blocks per square foot of wall.
For a wall 20 feet long and 8 feet high, the area is 160 square feet, and 160 × 1.125 = 180 blocks, before subtracting openings and adding waste.
Skip the math and get your answer instantly:
Open the Area Calculator →Counting Blocks
Courses = height in inches ÷ 8, so an 8-foot wall has 12 courses. Blocks per course = length in inches ÷ 16, so a 20-foot wall has 15 blocks per course. Multiply: 12 × 15 = 180. Add 5% for breakage and cuts: 189. Subtract any openings for doors and windows. A 3 × 7 foot door opening removes 21 square feet or about 24 blocks, though lintels and jamb blocks are needed.
Corners, half blocks, bond beam blocks at the top of walls, and special shapes for jambs and sills are counted separately. Make a block schedule from your drawings.
| Wall (length x height) | Area | Blocks (1.125/sq ft) | With 5% waste |
|---|---|---|---|
| 10 ft x 4 ft | 40 sq ft | 45 | 48 |
| 20 ft x 8 ft | 160 sq ft | 180 | 189 |
| 30 ft x 6 ft | 180 sq ft | 203 | 213 |
| 40 ft x 8 ft | 320 sq ft | 360 | 378 |
Mortar and Grout
Mortar is used for the horizontal bed joints and the vertical head joints. A rough guide is that one 80-pound bag of pre-mixed mortar lays about 30 to 40 blocks, and an 8-by-8-by-16 wall needs about 1 bag per 30 square feet. For 160 square feet, you would need roughly 5 to 6 bags. Mortar mixes are classified by strength (types M, S, N, O), and the type depends on loads and exposure; check the building plans.
Grout fills the cores of blocks that contain reinforcing steel, as in foundations and retaining walls, and requires an engineered design. One cubic yard of grout fills about 100 to 135 cores of 8-inch blocks, depending on the core size. Reinforcing bars are placed vertically through the cores at spacing specified by the structural design, often 32 or 48 inches.
Foundations and Footings
A block wall needs a concrete footing wide enough for the load, typically twice the wall thickness, and extending below the frost line. The first course is laid on a full bed of mortar on the footing, checked for level and plumb, and then each subsequent course is built using a line. Vertical expansion and control joints are needed for long walls to reduce cracking. Waterproofing is essential for below-grade walls.
Cost and Labor Planning
Blocks cost a few dollars each, but total installed costs include labor, mortar, rebar, grout, footings, and finishes. A skilled mason lays perhaps 100 to 150 blocks a day, and a helper tends mortar. For a DIY wall of 180 blocks, expect several days of work, including cure times. Check local codes for permits, setbacks, and structural requirements, especially for retaining walls and walls over certain heights.
Frequently Asked Questions
How many concrete blocks per square foot of wall?
1.125 for standard 8 x 16 inch blocks.
How many blocks for a 20 x 8 foot wall?
180, before subtracting openings and adding waste.
How much mortar do I need?
Roughly one 80 lb bag per 30 to 40 blocks.
Why are actual block dimensions smaller than nominal?
To allow for the 3/8 inch mortar joint so that the finished module is 8 x 16 inches.
Do block walls need reinforcement?
Often yes. Reinforcing steel and grout requirements depend on the height, loads, and local codes.