How many concrete blocks do I need? Calculate CMU blocks, mortar, rebar, and costs for walls, foundations, and retaining structures. Enter dimensions for instant estimates.
10% for simple walls, 15% for walls with openings
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Last updated March 31, 2026 by our expert review team
Block cost only. Add mortar ($8-12/bag) and rebar ($8-12/stick) separately.
Ehsan Ghazanfari
Licensed Structural Engineer
FISE-certified structural engineer with 11+ years designing bridges, retaining walls, and foundations. MSc from Aalto University.
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Certified Cost & Estimating Professional
AACE-certified estimator working with 20+ insulation companies including the two largest franchises in America.
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How we verify our calculatorsStagger blocks in a running bond pattern for maximum strength. Each course should offset by half a block length from the course below.
Use Type S mortar for below-grade and load-bearing walls (stronger than Type N). Type N is only suitable for non-structural above-grade partitions.
Fill every cell with rebar and grout for structural walls. For non-structural walls, fill every other cell to save on material costs.
Building a retaining wall? Use our retaining wall calculator for projects that need to hold back soil and handle lateral pressure.
Need to fill block cells with concrete? Our concrete bags calculator estimates the right number of bags for grout fill.
Dry-stack the first course without mortar to check layout and spacing before committing. Adjust block positions to minimize cuts at the ends.
This calculator estimates the number of CMU (concrete masonry unit) blocks needed based on your wall dimensions and selected block size. The standard 8x8x16 block covers 1.125 square feet of wall area when you include the 3/8-inch mortar joint on each side. The formula multiplies your wall area by this coverage rate, then adds your chosen waste factor to account for cuts and breakage.
Mortar is calculated at roughly 33 blocks per 80-lb bag of pre-mixed mortar, which assumes standard bed and head joints. If you are building a structural wall, the calculator also estimates rebar at every 4 feet vertically and horizontally (bond beam), plus grout fill for the reinforced cells. For retaining wall projects, every cell typically needs to be grouted. Use our concrete bags calculator to estimate grout quantities more precisely.
Cost estimates include blocks, mortar, and optionally rebar. Actual prices vary by region and supplier. The 10% waste factor works for simple rectangular walls, but increase to 15-20% for walls with windows, doors, or corners that require more cuts.
Formula
Wall Area (sq ft) x Blocks per sq ft x (1 + Waste %) = Total Blocks
Total Blocks / 33 = Mortar Bags (80 lb)
How many concrete blocks per square foot?
Standard 8x8x16 blocks cover 1.125 blocks per square foot of wall area. This accounts for the block face plus the 3/8-inch mortar joint on each side.
How much mortar do I need per block?
Plan for roughly 33 blocks per 80-lb bag of pre-mixed mortar. For a 100-block wall, you need about 3 bags. Always round up since partial bags go to waste.
How far apart should rebar be in a block wall?
Building codes typically require vertical rebar every 4 feet and horizontal rebar (bond beam) every 4 feet of wall height. Always check your local code for specific spacing requirements.
How much does a concrete block cost?
Standard 8x8x16 blocks cost $1.50 to $2.50 each at most home improvement stores. Larger 12-inch blocks run $2.00 to $3.50. Prices vary by region and quantity.
Should I use concrete blocks or bricks?
Blocks are faster to lay (fewer units per sq ft), cheaper for structural walls, and easier to reinforce with rebar. Bricks offer better aesthetics and weather resistance for veneer and decorative work.
Do concrete block walls need a foundation?
Yes. Block walls require a poured concrete footing at least twice the width of the block and a minimum of 8 inches thick. The footing must extend below the frost line in cold climates.
How do I cap or finish a block wall?
Use solid cap blocks or pour a concrete bond beam on the top course. Cap blocks prevent water from entering the hollow cells and provide a finished look.
How do I cut concrete blocks?
Score the block on all sides with a masonry chisel and hammer, then strike firmly to split. For precise cuts, use a diamond blade on a circular saw or masonry saw with water to control dust.
Garden Wall
10 ft x 4 ftA small garden wall does not need rebar in most cases. Use Type N mortar and cap the top course with solid cap blocks to keep water out of the cells.
Garage Foundation
80 ft x 4 ft perimeterFoundation walls require Type S mortar, rebar every 4 feet, and fully grouted cells. Waterproof the exterior below grade with a bituminous coating.
Retaining Wall
20 ft x 6 ftRetaining walls over 4 feet tall usually need an engineer's stamp. Add weep holes every 6 to 8 feet and drainage gravel behind the wall to relieve hydrostatic pressure.
Full Basement
120 ft x 8 ft perimeterBasement walls need full rebar reinforcement, waterproofing, and a perimeter drain tile system. Hire a structural engineer for walls this size.
Not accounting for mortar joint thickness
Concrete blocks are sized to include the 3/8-inch mortar joint. An 8x8x16 block actually measures 7-5/8 x 7-5/8 x 15-5/8 inches. Do not add extra for joints in your calculations.
Skipping rebar in structural walls
Building codes require vertical rebar every 4 feet in load-bearing walls. Without rebar, the wall cannot resist lateral loads from wind, soil, or seismic forces.
Using Type N mortar below grade
Type S mortar is required for below-grade and wet applications. Type N does not have enough compressive strength (750 psi vs 1,800 psi) to handle soil pressure and moisture.
Not ordering enough mortar
Running out of mortar mid-wall creates cold joints that are weaker than continuous pours. Buy 10-15% extra mortar bags so you never have to stop and run to the store.
Laying blocks without a level string line
Stretch a mason line between corner poles for every course. Without a guide, walls develop bows and leans that are impossible to fix once the mortar sets.
Important Disclaimer
These estimates are for planning purposes only. Actual costs vary by location, material availability, and project complexity. Always get at least 3 local quotes. This calculator does not replace professional advice.