Concrete Block Calculator
Calculate the number of concrete blocks (CMU), mortar bags, and core-fill grout required for walls, foundations, and retaining structures.
Calculation Results
Total wall face area minus window and door cutouts.
Total concrete blocks needed (based on standard 8"x16" nominal face area = 0.889 sq ft).
Commercial supplier pallets (typically 108 units per pallet for 8x8x16).
Bags of Type S structural masonry mortar mix (approx. 1 bag per 30 blocks).
Total pea-gravel grout volume needed to fill vertical reinforcement cores.
How This Calculator Works
Plan your masonry block wall with these simple steps:
- Select CMU Thickness: Standard 8x8x16 is used for general structures and foundations. 12x8x16 is preferred for tall retaining walls resisting severe lateral earth pressure.
- Input Dimensions: Enter total wall length and wall height.
- Deduct Wall Openings: Subtract total square footage of windows, entry doors, or garage portals to prevent over-ordering.
- Select Core Fill Strategy: Choose whether the wall is un-grouted, 100% solid filled, or reinforced at 32" or 48" intervals.
- Review Bill of Materials: Get instant block counts, commercial pallet quantities, 80lb mortar bags, and grout volume.
Formula & Calculations
Concrete masonry unit (CMU) calculations comply with ASTM C90 specifications for load-bearing masonry:
1. Block Face Coverage
Standard CMU blocks have an actual face dimension of 7-5/8" high × 15-5/8" long. When laid with a standard 3/8-inch mortar joint, the nominal coverage is exactly 8 inches by 16 inches:
Face Area = (8 / 12) × (16 / 12) = 0.8889 sq ft per block
To cover 100 square feet of wall, exactly 112.5 blocks are required before waste allowance.
2. Mortar Consumption (ASTM C270)
Masonry mortar (Type S or Type M) is used for bedding face shells and cross webs:
- One 80 lb bag of pre-mixed Type S mortar lays approximately 30 to 33 standard 8x8x16 blocks.
- For 100 blocks, order 3 to 3.5 eighty-pound bags of mortar mix.
3. Core-Fill Grouting Volume
Structural retaining walls and foundation basements insert vertical rebar into hollow block cores and fill them with fine grout (ASTM C476):
- 8x8x16 block:
0.28 cu ftof grout per block when fully filled (approx. 96 blocks per cubic yard of grout). - 12x8x16 block:
0.42 cu ftof grout per block. - 6x8x16 block:
0.20 cu ftof grout per block.
Step-by-Step Calculation Examples
Basement Foundation Wall (30ft x 8ft, 8x8x16 CMU)
Standard foundation wall with one 3ft x 4ft window opening, 10% waste, and every 2nd core grouted.
- block_size: 8x8x16
- length: 30
- height: 8
- openings_area: 12
- core_fill: half
- waste: 10
- net_wall_area: 228.0 sq ft
- blocks_needed: 282
- pallets: 3
- mortar_bags: 10
- grout_volume: 1.46 cu yd
Backyard Garden Retaining Wall (40ft x 4ft)
4-foot tall landscape retaining wall with no openings, 10% waste, hollow cores.
- block_size: 8x8x16
- length: 40
- height: 4
- openings_area: 0
- core_fill: none
- waste: 10
- net_wall_area: 160.0 sq ft
- blocks_needed: 198
- pallets: 2
- mortar_bags: 7
- grout_volume: 0.00 cu yd
Frequently Asked Questions
How many concrete blocks are in a 100 sq ft wall?
A 100 square foot wall requires approximately 113 standard 8x8x16 blocks without waste. With a recommended 10% waste allowance for cuts and half-blocks at corners, budget 125 blocks per 100 square feet.
What is the actual size versus nominal size of a cinder block?
The nominal size is 8" x 8" x 16", which includes the standard 3/8-inch mortar joint. The actual physical dimensions of the block are 7-5/8" high, 7-5/8" wide, and 15-5/8" long.
How many bags of mortar do I need for 100 concrete blocks?
You will need approximately 3.3 to 3.5 bags (80 lb each) of Type S mortar mix for every 100 standard 8x8x16 blocks. For 200 blocks, order 7 bags.
When should I fill concrete block cores with grout?
Cores should be grouted with concrete when building structural retaining walls, earth-retaining basement foundations, or high-wind load walls. Grouting encloses vertical steel rebar, turning the individual masonry units into a monolithic reinforced structure.
What is the difference between concrete block and cinder block?
Historically, cinder blocks were made using coal cinders and industrial ash, making them lighter but structurally weaker. Modern concrete masonry units (CMU) are manufactured with Portland cement, sand, and crushed stone aggregate, offering far higher compressive load ratings (typically 1,900 to 2,500+ PSI).
How many concrete blocks fit on a standard pallet?
A standard pallet of 8x8x16 concrete blocks contains either 108 or 120 blocks, weighing approximately 3,800 to 4,200 lbs.