Concrete Block Calculator

Calculate how many concrete blocks (CMU) you need for a wall or foundation. Get block count with 5% waste, mortar bags, sand, grout fill volume, and cost estimate. Supports standard and custom block sizes.

πŸ—οΈ Concrete Block Calculator

Blocks

Wall Area160.00 sq ft
Blocks (exact)180 blocks
Blocks (w/ 5% waste)189 blocks

Mortar & Fill

Mortar Mix (80 lb bags)6 bags
Portland Cement6 bags
Sand0.9 cu yd
Grout / Core Fill2.32 cu yd

πŸ’‘ How to Estimate Concrete Blocks for a Wall

Standard CMU (concrete masonry unit) blocks are 16 inches long Γ— 8 inches high Γ— 8 inches wide (nominal size including a β…œ-inch mortar joint). The actual block is slightly smaller (15⅝ Γ— 7⅝). A standard 8Γ—16 block covers 0.89 sq ft of wall β€” so you need about 1.125 blocks per square foot.

To estimate blocks: measure wall length Γ— height = wall area (sq ft), subtract any door/window openings, then divide by 0.89. Add 5% for waste (cuts, breakage, corners). For mortar, plan about 3 bags of Portland cement per 100 blocks, plus 1 cubic yard of sand per 7 bags of cement.

For poured concrete, use our concrete calculator. Calculate wall area with our square footage calculator. For brick walls, see our brick calculator.

What Are Concrete Blocks?

Concrete blocks (also called CMUs β€” concrete masonry units, or cinder blocks) are precast rectangular building units made from Portland cement, aggregate, and water. They are used to construct load-bearing walls, foundations, retaining walls, partition walls, and fencing.

CMUs come in two basic types: hollow blocks (the most common, with two open cells that are filled with grout/concrete during installation) and solid blocks (used for retaining walls, filled walls, and special applications). The hollow cells allow reinforcing rebar to be inserted vertically, making the wall much stronger.

In the United States, the standard CMU is 8Γ—8Γ—16 inches (nominal). The actual dimensions are 7⅝ Γ— 7⅝ Γ— 15⅝ inches β€” the β…œ-inch difference is filled by the mortar joint during construction.

Step-by-Step: How to Calculate Concrete Blocks

Step 1: Find Wall Square Footage

Measure the length and height of the wall in feet. Multiply them together to get the gross wall area. If there are doors, windows, or other openings, measure those separately and subtract them.

Example: A 30 ft long Γ— 8 ft tall wall with one 3Γ—7 ft door and two 3Γ—4 ft windows: gross = 240 sq ft, openings = 21 + 24 = 45 sq ft, net = 195 sq ft.

Step 2: Find Block Square Footage

Calculate the face area of one block. For a standard 8Γ—16 block: (8 Γ— 16) Γ· 144 = 0.89 sq ft per block. Different block sizes have different face areas β€” the table below lists common sizes.

Step 3: Calculate Number of Blocks

Divide the net wall area by the block face area, then add 5% for waste (cuts, breakage, corner pieces).

blocks needed = (wall sq ft Γ· block sq ft) Γ— 1.05

How to Estimate Mortar for a Block Wall

Mortar is the paste that bonds blocks together. It's made from Portland cement, sand, and water (ratio: 1 part cement to 3 parts sand).

  • Pre-mixed mortar (80 lb bags): 1 bag covers about 33 standard blocks. Divide your block count by 33 to get the number of bags.
  • Site-mixed mortar: Plan about 3 bags (94 lb) of Portland cement per 100 blocks. For sand, use 1 cubic yard of masonry sand per 7 bags of cement.
  • Mortar joint thickness: The standard joint is β…œ inch (for CMU walls). Thicker joints require more mortar.

How to Estimate Grout / Concrete Fill

Hollow CMU blocks are designed to be filled with grout (a fluid concrete mix) after rebar is placed. To estimate fill volume:

  1. Calculate the cell volume per block: subtract the shell thickness (typically 1.25 inches on each side) and web thickness (typically 1 inch, 3 webs per standard block) from the block dimensions.
  2. For a standard 8Γ—8Γ—16 block: cell width β‰ˆ 5.5 in, cell length β‰ˆ 6.5 in, height = 8 in β†’ ~572 cu in per block (2 cells).
  3. Multiply by total blocks and divide by 46,656 cu in/cu yd to get cubic yards of grout.

Rule of thumb: Standard 8Γ—8Γ—16 CMU blocks require about 0.007 cubic yards of fill per block, or roughly 1 cubic yard per 145 blocks.

Standard Concrete Block Sizes

Block Size (WΓ—HΓ—L)Nominal (in)Actual (in)Face Area (sq ft)Blocks per sq ft
Standard8Γ—8Γ—167⅝ Γ— 7⅝ Γ— 15⅝0.891.125
Half-Height8Γ—4Γ—167⅝ Γ— 3⅝ Γ— 15⅝0.442.25
Half-Length8Γ—8Γ—87⅝ Γ— 7⅝ Γ— 7⅝0.442.25
Thick Wall12Γ—8Γ—1611⅝ Γ— 7⅝ Γ— 15⅝0.891.125
Thin Partition4Γ—8Γ—163⅝ Γ— 7⅝ Γ— 15⅝0.891.125
Wide10Γ—8Γ—169⅝ Γ— 7⅝ Γ— 15⅝0.891.125

Note: The nominal size includes the β…œ-inch mortar joint. When estimating, always use the nominal dimensions.

Concrete Block Wall Cost (2025 US Pricing)

Cost ComponentTypical Range
Standard 8Γ—8Γ—16 CMU block$1.50–$3.00 each
Half block (8Γ—8Γ—8)$1.00–$2.00 each
Mortar (80 lb pre-mix bag)$5–$10 per bag
Portland cement (94 lb bag)$12–$16 per bag
Masonry sand$25–$40 per cubic yard
Installed cost (material + labor)$10–$17 per sq ft of wall
100 sq ft wall (about 113 blocks)$1,000–$1,700 installed

Tools Needed for a Block Wall

  • Brick trowel β€” for spreading mortar on bed and head joints
  • Mason line and line blocks β€” to keep courses straight and level
  • 4-ft level β€” to check plumb and level on every course
  • Jointer / striking tool β€” to finish mortar joints for a clean appearance
  • Mixing tub and hoe β€” to mix mortar on site
  • Block chisel and hammer β€” to score and split blocks for corners and ends
A 20 ft Γ— 8 ft wall = 160 sq ft Γ· 0.89 = 180 blocks. With 5% waste = 189 blocks. Mortar: 6 bags cement + 0.9 cu yd sand. Grout fill: about 1.3 cubic yards.

Concrete Block Calculator FAQ

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