Concrete Block Calculator
Calculate standard 8x8x16 CMU blocks, mortar bags, vertical rebar, and core fill grout for masonry foundation and retaining walls.
Standard 8" × 8" × 16" Stretcher Blocks (Includes 5% waste)
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What Is a Concrete Block Calculator?
A concrete block calculator determines the quantity of standard 8"×8"×16" Concrete Masonry Units (CMU) needed to build retaining walls, foundation stem walls, and structural masonry enclosures. It computes blocks, mortar bags, vertical rebar, and core fill grout.
How to Use This Calculator
- Enter wall length and wall height in feet.
- Select core fill reinforcement interval (solid core fill, or reinforced every 16", 32", or 48" OC).
- View the total 8x8x16 CMU blocks needed (including 5% waste).
- Check Type S/N mortar bags (70 lb / 80 lb), vertical rebar, and grout volume.
CMU Block & Mortar Equations
\text{Blocks} = (L \times H \times 1.125) \times 1.05, \quad \text{Mortar Bags (70lb)} = \left\lceil \frac{\text{Blocks}}{100} \times 3.3 \right\rceilA standard 8x8x16 block with a 3/8" mortar joint covers 0.8889 sq ft (1.125 blocks per sq ft). Approximately 3.3 seventy-pound mortar bags are needed per 100 blocks.
Worked Example
Scenario: Building a 30 ft long by 6 ft high retaining wall with reinforced grouted cores every 32" OC.
Wall area: 30 × 6 = 180 sq ft.
Blocks required: 180 × 1.125 = 202.5 blocks (+ 5% waste = 213 blocks).
Mortar: (213 ÷ 100) × 3.3 = 7.02 -> 8 bags of 70-lb Type S mortar.
Core fill grout: ~0.89 cubic yards.
Tips & Key Notes
- Always lay concrete blocks on a solid concrete footing poured twice the width of the block (16" wide for an 8" block) and below the frost line.
- Use Type S mortar for exterior and below-grade walls requiring high compressive and flexural strength.
- Stagger vertical head joints by half a block length (running bond) for maximum structural interlock.
Frequently Asked Questions
How many concrete blocks are in a square foot of wall?
There are 1.125 standard 8"×8"×16" concrete blocks in one square foot of wall surface area.
How many blocks do I need for an 8x10 wall?
An 8 ft high by 10 ft long wall is 80 square feet. 80 × 1.125 = 90 blocks (+ 5% cuts = 95 blocks).
How many bags of mortar do I need for 100 concrete blocks?
You need approximately 3 to 3.5 bags (70-lb or 80-lb bags) of Type S or Type N masonry mortar per 100 blocks.
What is the actual size of an 8x8x16 CMU block?
The actual dimensions are 7-5/8" high × 7-5/8" deep × 15-5/8" long. When laid with a standard 3/8-inch mortar joint, the nominal dimension is exactly 8" × 8" × 16".
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