Concrete Block (CMU) Count Calculator
Find how many standard concrete blocks you need for a wall, from its length and height.
Reviewed by the ToolNestr Editorial Team — July 2026
How the concrete block count calculator works
A standard concrete masonry unit (CMU) is nominally 8×8×16 inches. Its actual size is slightly smaller, 7 5/8 × 7 5/8 × 15 5/8 inches, because the missing 3/8 inch on each face is filled by the mortar joint when it's laid — so on the wall, each block plus its joint occupies a clean 8×16 inch footprint. That works out to 0.89 square feet of wall face per block (8 × 16 ÷ 144).
To size a wall, multiply its length by its height to get the total area, then divide by 0.89 square feet to get the raw block count. Real walls always need more than this raw number because of cut blocks at corners and openings, breakage, and layout adjustments, so a waste allowance of roughly 5 to 10% is added on top — this calculator defaults to 10%, adjustable for simple versus complex layouts.
The tool also estimates courses (the number of block rows in the wall height) and a rough mortar bag count, using a common shop rule of about 3 blocks per 80-pound bag of mortar mix for a standard 3/8 inch joint. For a load-bearing or structural CMU wall — one carrying roof, floor, or lateral loads — verify block type, reinforcement, and grouting requirements against your local building code and a qualified professional.
Measure the wall
Length and height in feet.
Set the waste
Adjust for a simple wall or one with lots of corners.
Order blocks
Plus a rough mortar bag estimate.
The formula explained
Raw block count
= (L × H) ÷ 0.89. A 20 × 6 ft wall: 120 ÷ 0.89 = 134.8 → 135 blocks.
With waste
× 1.10 = 135 × 1.10 = 149 blocks to order.
Courses
= height(in) ÷ 8 = 72 ÷ 8 = 9 courses tall.
Worked example
A 30 ft × 4 ft garden wall, 5% waste.
Where CMU block counts matter
Foundation and basement walls
The most common large-scale use, often paired with core-fill grouting and vertical rebar at engineer-specified spacing for below-grade loads.
Garden and boundary walls
Low, unreinforced walls for property lines and raised beds are the simplest case — just area divided by coverage, plus a modest waste allowance.
Commercial and shop buildings
CMU is a common structural shell for warehouses and shops, where large wall areas make even a small per-block cost difference add up across the order.
Fire-rated separation walls
CMU's mass and non-combustible material make it a standard choice for fire walls separating units or occupancies, where exact block counts feed directly into cost estimates.
Real-world context
A full basement or foundation wall shows this calculator's most common large-scale use: an 80 ft by 8 ft wall is 640 square feet, needing 719 raw blocks and 792 after a 10% waste allowance — a wall this size spans 12 courses tall and is typically ordered by the pallet (usually 90 to 108 blocks per pallet) rather than counted out individually.
A small garden or planter wall shows the low end: a 12 ft by 3 ft wall needs just 43 blocks at a lighter 5% waste allowance — small enough that a homeowner can pick the exact block count up from a single supply-yard trip without needing a full pallet delivery.
A long property-line retaining or screen wall shows how quickly block counts scale with length: a 200 ft by 10 ft wall needs 2,472 blocks after waste — at roughly 100 blocks per pallet, that's about 25 pallets, a delivery that needs site access and staging planned well before the first course goes down.
In every case, the block count feeds directly into other material estimates too: the course count tells you how many rows of horizontal joint reinforcement wire to buy if your design calls for it, and the total block count is what a mason or estimator uses to price labor, since CMU walls are commonly quoted per block laid rather than per square foot.
Common misconceptions
"An 8x8x16 block is exactly 8 inches wide, 8 inches tall, and 16 inches long." That's the nominal, not actual, size. The physical block measures 7 5/8 × 7 5/8 × 15 5/8 inches; the extra 3/8 inch on each dimension is the mortar joint, which is why the block only "becomes" a clean 8×16 footprint once it's laid in the wall with its joint.
"Since it's just a count of identical blocks, one flat waste percentage fits any wall shape." Waste scales with complexity, not just size. A simple straight run of wall wastes far fewer blocks to cuts than a wall with several corners, a door or window opening, or a stepped top — the 5-10% range exists precisely because layout complexity, not wall area, drives how much extra you actually need.
Related calculators
Frequently asked questions
How many concrete blocks do I need for a wall?
Divide the wall area by the face area of one standard block. A standard 8×8×16 in block (with a 3/8 in mortar joint) covers about 0.89 square feet of wall face, so a 20 ft × 6 ft wall (120 sq ft) needs about 120 ÷ 0.89 ≈ 135 blocks.
What is a standard CMU block size?
The most common concrete masonry unit (CMU) is nominally 8×8×16 inches, though its actual size is 7 5/8×7 5/8×15 5/8 inches — the extra 3/8 inch on each dimension is filled by the mortar joint, making the coursed dimension a clean 8×16 inches.
How much waste allowance should I add?
A common allowance is 5 to 10% for cut blocks, corners, and breakage. This calculator applies a default 10% waste factor, which you can adjust for a simple rectangular wall versus one with many corners or openings.
How much mortar do I need per block?
A rough industry estimate is about 3 standard blocks per 80 lb bag of mortar mix for a typical 3/8 in joint. For an exact quantity by joint size, use a dedicated mortar mix calculator alongside this block count.
Sources & references
This tool uses standard formulas and reference values from:
- • American Concrete Institute — ACI 318, Building Code Requirements for Structural Concrete. concrete.org
- • ICC — International Residential Code (IRC), span, footing and framing tables. codes.iccsafe.org
- • APA – The Engineered Wood Association, allowable span and load guidance.
Estimates for planning only. Span, load and code values vary by jurisdiction — verify against your local adopted code and a licensed engineer before building.