Concrete Block Calculator
Calculate how many concrete blocks (CMU) or cinder blocks you need for any wall project. Enter the wall dimensions and block size for accurate estimates of blocks, mortar bags, and total cost.
How to Calculate Concrete Blocks for a Wall
Accurately estimating the number of concrete blocks (also called CMU or cinder blocks) you need is essential for budgeting and planning any masonry project. Whether you are building a retaining wall, foundation, garage, or garden wall, this concrete block calculator provides a reliable material estimate including blocks, mortar, and cost.
The Block Calculation Formula
The calculation follows the standard masonry estimating method:
- Calculate wall area: Wall Length (ft) × Wall Height (ft) = total square footage.
- Determine blocks per square foot: A standard 8×8×16 block covers an 8" × 16" face area (0.89 sq ft nominal with mortar joints). This works out to 1.125 blocks per square foot of wall.
- Calculate base blocks: Wall area × 1.125 = blocks needed (before waste).
- Add waste factor: Multiply by (1 + waste percentage) to account for breakage, cuts, and damaged blocks.
- Estimate mortar: Approximately 1 bag (80 lb) of mortar mix per 33 standard blocks.
Standard vs. Half-Height Blocks
The two most common concrete block sizes used in residential and commercial construction are:
- Standard 8×8×16: The workhorse of masonry construction. Nominal dimensions are 8 inches wide, 8 inches tall, and 16 inches long. Actual dimensions are 7-5/8" in each direction to allow for 3/8" mortar joints. Used for foundation walls, structural walls, retaining walls, and most load-bearing applications. Each block weighs approximately 35–40 lbs.
- Half-height 8×4×16: Same width and length as standard blocks but only 4 inches tall (nominal). Used for garden walls, cap courses, decorative walls, and height adjustments. Requires 2.25 blocks per square foot due to the shorter height. Each block weighs approximately 20–24 lbs.
Mortar Requirements
Mortar is the binding material between concrete blocks. For standard 3/8-inch mortar joints:
- Standard blocks (8×8×16): One 80 lb bag of mortar mix covers approximately 33 blocks (bed and head joints).
- Half-height blocks (8×4×16): One 80 lb bag covers approximately 40 blocks because the bed joints use less mortar.
- Mortar type: Type S mortar is standard for below-grade and structural walls. Type N is used for above-grade, non-load-bearing walls. Always use the mortar type specified by your building plans or local code.
Concrete Block Costs
Material costs vary by region and supplier, but typical ranges include:
- Standard gray CMU (8×8×16): $1.50–$2.50 per block
- Lightweight CMU: $2.00–$3.00 per block
- Split-face decorative: $2.50–$4.50 per block
- Half-height CMU (8×4×16): $1.00–$2.00 per block
- Mortar mix (80 lb bag): $5.00–$10.00 per bag
- Professional labor: $10–$17 per square foot of wall face
Planning Tips for Block Wall Projects
- Check local codes: Most block walls over 4 feet tall require a building permit, rebar reinforcement, and grout-filled cores.
- Foundation requirements: Block walls need a concrete footing at least twice the width of the block (16 inches minimum for 8-inch blocks) and extending below the frost line.
- Reinforcement: Structural walls require vertical rebar in filled cores every 4 feet and horizontal reinforcement (ladder wire or rebar in bond beam blocks) every 2–4 courses.
- Drainage: Retaining walls need drainage behind them — install perforated pipe and gravel backfill to prevent hydrostatic pressure buildup.
- Order extra: Always order 5–10% more blocks than calculated. Blocks break during delivery and cutting, and having extras avoids costly project delays.
Common Block Wall Projects
Here are rough material estimates for common project sizes using standard 8×8×16 blocks at 10% waste:
- Garden wall (20 ft × 4 ft): ~100 blocks, 3 bags mortar
- Garage wall (24 ft × 8 ft): ~238 blocks, 7 bags mortar
- Retaining wall (40 ft × 4 ft): ~198 blocks, 6 bags mortar
- Full basement (120 linear ft × 8 ft): ~1,188 blocks, 36 bags mortar
Frequently Asked Questions
How many concrete blocks do I need per square foot?
For standard 8x8x16 inch concrete blocks (CMU), you need 1.125 blocks per square foot of wall area. This accounts for the block face area (128 sq in) plus a standard 3/8-inch mortar joint. For a 100 sq ft wall, you would need approximately 113 blocks before adding waste factor.
How much mortar do I need for concrete blocks?
You need approximately 3 standard bags (80 lb) of mortar mix per 100 concrete blocks for standard 8x8x16 blocks with 3/8-inch joints. Each 80 lb bag of mortar mix yields enough mortar for approximately 33-35 standard blocks. For half-height 8x4x16 blocks, you need slightly less mortar per block.
How much does a concrete block wall cost?
Standard 8x8x16 concrete blocks cost $1.50-$3.00 each depending on type and location. An 80 lb bag of mortar costs $5-$10. For a typical 100 sq ft wall, materials cost roughly $200-$400. Professional installation adds $10-$17 per square foot for labor, bringing total costs to $1,200-$2,100 per 100 sq ft.
What is the difference between a cinder block and a concrete block?
Traditional cinder blocks used coal cinders as aggregate and were lighter but weaker. Modern "cinder blocks" are actually concrete masonry units (CMU) made with portland cement, aggregate, and water. The terms are used interchangeably today, but true cinder blocks are no longer manufactured. CMUs are stronger, more durable, and meet current building codes.
How much waste factor should I add for concrete blocks?
Add 5-10% waste factor for concrete block projects. Use 5% for simple straight walls with few cuts. Use 10% for walls with corners, openings (doors and windows), or complex layouts that require more cutting. Blocks can break during handling and transport, so having extras on site prevents project delays.
What size concrete blocks are available?
The most common concrete block sizes are: Standard 8x8x16 inches (actual 7-5/8 x 7-5/8 x 15-5/8) used for most walls and foundations; Half-height 8x4x16 inches for shorter walls and cap courses; 12x8x16 inches for thicker foundation walls; and 4x8x16 inches for veneer and non-structural walls. Nominal sizes include the mortar joint.