Free concrete block calculator to estimate CMU blocks, mortar bags, and sand needed for walls. Includes cost estimates for retaining walls, foundations, and block fences.
Most common size for walls and foundations
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Planning a block wall, retaining wall, or foundation? Our concrete block calculator (also called CMU or cinder block calculator) determines exactly how many blocks you need, plus mortar bags and sand quantities. Get accurate material estimates with cost projections before you start building.
Concrete blocks (CMU - Concrete Masonry Units) are measured by their nominal dimensions. Standard blocks are 8" × 8" × 16" (actual: 7-5/8" × 7-5/8" × 15-5/8") to account for mortar joints. You need approximately 1.125 blocks per square foot of wall area. Mortar requirements average 3 bags (80 lb) per 100 blocks, and sand is needed at about 1 ton per 7-8 bags of mortar.
Block Calculation Formula
Blocks = Wall Area (sq ft) × 1.125 × (1 + Waste %)Know exactly how many blocks to order—avoid shortages or excess.
Calculate supplementary materials needed for complete installation.
Account for breakage, cuts, and installation waste automatically.
Budget your project with accurate material cost estimates.
Calculate for standard 8", 4" partition, or 12" foundation blocks.
Build decorative garden walls or retaining walls for landscaping projects.
Calculate blocks for basement or crawl space foundation walls.
Estimate materials for detached garage or workshop block walls.
Build durable block privacy fences that last for decades.
Create solid block foundations for sheds and outbuildings.
Use 4" partition blocks for interior non-load-bearing walls.
For standard 8×8×16 inch blocks with 3/8" mortar joints, you need approximately 1.125 blocks per square foot of wall area. This accounts for the mortar joints reducing the effective block size.
Plan for approximately 3 bags of 80 lb mortar mix per 100 standard concrete blocks. This varies slightly based on joint thickness and block type. Type S mortar is recommended for most applications.
8" standard blocks are most common for walls and foundations. Use 4" partition blocks for interior non-load-bearing walls. Use 12" foundation blocks for basement walls or where extra strength is needed.
For walls over 4 feet tall, vertical rebar is recommended every 4 feet, filled with grout. Horizontal rebar (bond beam) should be placed every 4 feet of height. Always check local building codes for requirements.
Add 7-10% for straightforward walls with few openings. Add 10-15% for walls with doors, windows, or complex layouts. Keep extra blocks for future repairs—matching blocks can be hard to find later.
Modern 'cinder blocks' are actually concrete blocks (CMU). True cinder blocks used coal cinders as aggregate and are rarely made today. The terms are often used interchangeably, but CMU is the industry standard.
Standard 8×8×16 inch blocks weigh about 38 lbs each. Half blocks weigh about 19 lbs. 12" foundation blocks weigh approximately 55 lbs. Lightweight blocks are available but cost more.
Mix sand with mortar at approximately 3:1 ratio (sand to mortar). For pre-mixed mortar, you'll need about 1 ton of sand per 7-8 bags of mortar mix for proper consistency.