Concrete Block Calculator
Estimate the number of concrete blocks (CMU) and bags of mortar mix needed to build a retaining wall, foundation, or structure.
Calculation Results
The overall square footage of the block wall.
Total number of standard concrete blocks needed.
Bags of structural Type S mortar mix needed (assumes roughly 1 bag per 30 blocks).
How This Calculator Works
To calculate concrete masonry materials:
- Enter the total length and height of the wall.
- The calculator assumes standard 8x8x16 concrete blocks, which are the most common blocks used in retaining walls and foundation structures.
- Add a waste buffer (10% is standard; use 12-15% if you need to cut blocks to accommodate corners or half-block ends).
- Review your results: the calculator displays total wall area, CMU blocks, and structural Type S mortar bags.
Formula & Calculations
Wall surface area is calculated by multiplying length by height:
Wall Area = Length × Height
A standard 8" × 8" × 16" concrete masonry unit (CMU) measures nominally 7.625" × 7.625" × 15.625". When laid with a standard 3/8-inch mortar joint, the block covers exactly 8" × 16" of wall surface area. This equals exactly 0.8889 square feet.
The total count of blocks needed is computed as:
Blocks = ceil((Wall Area / 0.8889) × (1 + Waste / 100))
Mortar requirements are estimated based on standard blocklaying: laying 30 concrete blocks typically requires **one 80lb bag of Type S masonry mortar mix**.
Step-by-Step Calculation Examples
Retaining Wall (30ft long x 6ft high)
Calculating blocks and mortar for a backyard retaining wall using standard 8x8x16 CMU blocks and a 10% waste buffer.
- length: 30
- height: 6
- waste: 10
- area: 180.0 sq ft
- blocks_needed: 223
- mortar_bags: 8
Frequently Asked Questions
What is the actual size of an 8x8x16 block?
The actual size of a standard block is 7-5/8 inches wide, 7-5/8 inches high, and 15-5/8 inches long. The nominal size refers to the dimensions after laying it in a 3/8-inch mortar joint, which rounds out to exactly 8"x8"x16".
What type of mortar is best for concrete blocks?
Type S mortar is recommended for below-grade foundations, structural load-bearing walls, and retaining block walls. It has higher compressive strength (1,800 psi) and handles wind and soil pressure better than Type N.