Construction · Concrete Tools
Concrete Block CalculatorWork out how many concrete blocks you need instantly for walls, footings, and builds.
Example — 20 × 8 ft garage wall
180 blocks
8″ × 8″ × 16″ CMU
A 20 × 8 ft garage wall needs around 180 × 8″ CMU blocks including 5% waste — roughly $540 in materials at $3 per block.
How to calculate how many concrete blocks you need
The calculation is straightforward: work out the wall face area, divide by the face area of one block, and round up. The only number that trips people up is the block face area — remember that the nominal 8 × 16 inch dimension already includes a ⅜-inch mortar joint on each side, so the actual coverage is very close to the nominal figure.
Step 1 — wall area
Wall area (sq ft) = Length (ft) × Height (ft)
Step 2 — blocks needed
Blocks = Wall area ÷ Block face area (0.889 sq ft for 8×16)
Step 3 — apply waste factor
Final count = Blocks × (1 + waste%) — always round up
For a clean wall with no openings this gives an accurate result. For walls with windows or doors, calculate the gross wall area first, then subtract each opening area before dividing by the block face area.
Worked example: 24 × 6 ft retaining wall
A 24-foot retaining wall at 6 feet high is a typical residential project — a garden terrace or low retaining structure. Here is how to work through the block count step by step using standard 8 × 8 × 16 inch CMU blocks:
Calculate wall area
24 ft × 6 ft = 144 sq ft
Divide by block face area
144 ÷ 0.889 = 161.9 → 162 blocks
Add 5% waste
162 × 1.05 = 170.1 → 171 blocks
Estimate material cost
171 × $3 = $513
That's a manageable block order. Note that this figure is for blocks only — a retaining wall also needs rebar, grout fill, footing concrete, and drainage material. Use our concrete calculator to size the footing pour separately.
Standard US concrete block sizes
Concrete masonry units (CMUs) in the US are specified by nominal dimensions that include the mortar joint. The actual block is ⅜ inch smaller on each face. The most common sizes you'll find at any masonry supplier:
8 × 8 × 16 in
≈ 0.889 sq ft / block
The standard block. Used for foundations, basements, retaining walls, and load-bearing walls. Most widely available and cheapest per block.
8 × 4 × 16 in
≈ 0.444 sq ft / block
Half block (course closer). Used at corners and as course starters. Same 16-inch face length, half the height — maintains running bond at transitions.
12 × 8 × 16 in
≈ 0.889 sq ft / block
Heavy-duty block for thicker walls and load-bearing applications. Same face area as 8×8×16 but 12-inch width — used where higher compressive strength or extra wall thickness is specified.
If your project uses a non-standard block size, measure the actual block face (height × length in inches), convert to square feet by dividing by 144, and enter the cost-per-block manually in the calculator's advanced panel.
How much do concrete blocks cost in the US?
Prices vary by region and supplier but these are typical 2025–2026 ranges at masonry yards and big-box stores:
Standard 8×8×16 CMU
$2–$4 / block
Most widely available. Pallet discounts common at 100+ units.
Split-face or decorative
$4–$8 / block
Textured finish for exposed walls. Higher labour cost too.
Installed cost (labour + materials)
$10–$20 / sq ft
Masonry contractor rates for block wall construction, full install.
The calculator defaults to $3 per block — a reasonable mid-range for standard CMU. Update this in the "Adjust waste & price" panel to match your supplier's quote. Always get at least two quotes and ask about pallet pricing if you're buying 200 blocks or more.
Common mistakes when calculating concrete blocks
Not accounting for openings
Door and window openings reduce the block count significantly. A single 36-inch door opening in an 8-foot wall removes about 24 square feet — nearly 27 blocks. Always subtract openings from gross wall area.
Forgetting the waste factor
Block walls always have cut blocks at corners, around openings, and at course ends. Without a waste factor you will almost certainly run short. 5% is a minimum; use 10% for complex walls with multiple openings.
Confusing nominal and actual dimensions
Nominal 8×16 blocks are actually 7⅝ × 15⅝ inches. The nominal size is used for calculator purposes because it already includes the mortar joint. Don't mix nominal and actual measurements in the same calculation.
Buying for one wall and forgetting others
Run the calculator separately for each wall of a structure and add the totals. Garage walls, basement walls, and interior partitions all have different dimensions and opening layouts.
Where this calculator is used
Garage foundations
CMU perimeter walls on poured footings
Basement walls
Below-grade block construction
Retaining walls
Garden terracing and slope stabilisation
Privacy walls & fences
Boundary and screen walls
Outbuilding bases
Workshops, sheds, storage structures
Commercial structures
Tilt-up and load-bearing CMU buildings
Frequently asked questions
How many concrete blocks per square foot?
A standard 8×8×16 CMU covers 0.889 square feet of wall face, meaning you need about 1.125 blocks per square foot, or 113 blocks per 100 square feet before any waste allowance.
How do I calculate blocks needed for a wall?
Multiply wall length by height (in feet) to get square footage. Divide by 0.889 for 8×16 blocks and round up. Add 5–10% for waste. The calculator above does this automatically.
What size are standard US concrete blocks?
The standard CMU is nominally 8×8×16 inches, with actual dimensions of 7⅝×7⅝×15⅝ inches. The nominal size includes a 3/8-inch mortar joint on three faces.
Do I need to subtract doors and windows?
Yes. Calculate gross wall area first, subtract the area of all openings, then divide the net area by the block face area. The calculator works on the area you enter — deduct openings manually.
Is a cinder block the same as a concrete block?
Technically no — original cinder blocks used coal ash aggregate while concrete blocks (CMUs) use stone or sand aggregate. In everyday US usage the terms are interchangeable; virtually all modern 'cinder blocks' sold today are concrete masonry units.
How many blocks do I need for a 100 square foot wall?
Using 8×8×16 blocks: 100 ÷ 0.889 = 112.5, round up to 113 blocks at exact volume. With a 5% waste factor: 119 blocks. With 10% waste: 125 blocks.
Disclaimer
This calculator provides estimates only and should be used for planning purposes. Actual quantities and costs may vary based on block size tolerances, mortar joint thickness, site conditions, and local pricing. Always verify your order with your supplier before purchasing.