Concrete Block Calculator: Estimate Blocks Needed
Calculate how many concrete blocks a wall needs from its length, height and thickness, using the block-count formula, a worked example and FAQ answers.
Concrete Block Quantity Estimator
Calculate the number of concrete blocks needed for your construction project. Enter the dimensions of your wall to get an estimate.
Wall Dimensions
Enter the length of the wall in feet
Enter the height of the wall in feet
Enter the width (thickness) of the wall in feet
Calculation Results
You will need approximately 180 concrete blocks for a wall that is 20 ft long, 8 ft high, and 0.67 ft thick.
Calculation Details
Wall Visualization
Additional Information
This calculator uses standard concrete block dimensions of 8"×8"×16" (width × height × length) with 3/8" mortar joints.
The calculation rounds up to whole blocks, as partial blocks are typically not used. Actual quantities may vary based on specific block sizes and construction methods.
Documentation
Concrete Block Calculator
A concrete block calculator works out how many concrete blocks a wall needs, based on the wall's length, height and thickness. It also shows how many blocks go in each row and how many rows the wall has.
Concrete blocks, also called concrete masonry units (CMUs) or cinder blocks, are hollow rectangular blocks made of cast concrete. Builders stack them in rows, called courses, with mortar between each block, to build a wall.
How to calculate concrete blocks needed
A concrete block wall is normally built one block thick. Builders call this single-wythe construction. The wall's thickness decides which block width to buy — common widths are 4, 6, 8, 10 and 12 inches — but it does not change how many blocks the wall needs. Every standard block shows the same 16-by-8-inch face, so the block count depends only on the wall's area, not its thickness.
The formula is:
Total blocks = Blocks per row × Number of rows
- Blocks per row = ceiling(wall length in inches ÷ 16)
- Number of rows = ceiling(wall height in inches ÷ 8)
The ceiling function rounds any result up to the next whole number, since a builder cannot buy part of a block.
Block sizes and mortar joints
A block sold as "8x8x16" is not actually that size. Under ASTM C90, the U.S. standard for concrete masonry units, its true, pre-mortar dimensions are 7-5/8 by 7-5/8 by 15-5/8 inches. Adding a standard 3/8-inch mortar joint brings the block up to a full 8 by 8 by 16 inches once it sits in a wall. This laid size is called the nominal size, and it is already the number the formula above uses. The mortar joint is built into that number, so it should not be added again on top of it.
Worked example
Take a wall 25 feet long and 6 feet high, built with 8-inch-thick blocks.
- Convert to inches: length = 25 × 12 = 300 inches; height = 6 × 12 = 72 inches.
- Blocks per row = ceiling(300 ÷ 16) = ceiling(18.75) = 19
- Number of rows = ceiling(72 ÷ 8) = ceiling(9) = 9
- Total blocks = 19 × 9 = 171 blocks
The wall's 8-inch thickness only tells the builder which block width to buy. It does not change the total of 171.
How to use the concrete block calculator
Enter the wall's length, height and thickness in feet. The calculator rounds the length and height up to whole rows of blocks and reports the total, along with the number of blocks per row and the number of rows.
The tool does not subtract space for doors or windows, and it does not add extra blocks for breakage. Most builders add 5 to 10 percent more blocks than the raw total, to cover cuts, waste and mistakes.
Common uses
Builders use this kind of block count for foundation walls, retaining walls, garden walls, garages and workshop walls. Homeowners use the same formula for smaller jobs, such as raised garden beds, fire pits and compost bins.
Concrete block compared with other wall materials
Poured concrete walls are stronger and have fewer seams than block walls, but they need temporary forms and longer curing time. Brick walls usually cost more and take longer to build, since bricks are smaller than blocks and need more of them per square foot. Insulated concrete forms combine the formwork with built-in insulation, at a higher material cost. None of these methods use a block calculator: poured concrete needs a volume calculator, and brick needs a brick calculator sized to the smaller brick module.
History of concrete block construction
Cast concrete building elements date back to ancient Rome, which used a material called opus caementicium, poured into wooden forms. The modern hollow concrete block was patented by American inventor Harmon S. Palmer in 1900, after roughly ten years of experiments. Its hollow core saved weight and material compared with a solid block, a feature still standard today. Portland cement, the binding ingredient in modern concrete, was patented separately by Joseph Aspdin in 1824.
Block manufacturing grew fast after Palmer's patent. By 1905, an estimated 1,500 companies made concrete blocks in the United States. Automated block-making machines later in the century lowered costs, and concrete block became one of the most common wall materials in North America.
Frequently asked questions
What is the standard size of a concrete block?
The most common concrete block has a nominal size of 8 by 8 by 16 inches (width by height by length). Its true, pre-mortar size is 7-5/8 by 7-5/8 by 15-5/8 inches, under the ASTM C90 standard. Blocks also come in other widths, including 4, 6, 10 and 12 inches, all sharing the same 8-by-16-inch face.
How many blocks do I need for a 10 by 10 foot wall?
For a wall 10 feet long and 10 feet high: length = 120 inches, height = 120 inches. Blocks per row = ceiling(120 ÷ 16) = 8. Number of rows = ceiling(120 ÷ 8) = 15. Total blocks = 8 × 15 = 120 blocks, before adding extra for waste or subtracting any openings.
Does wall thickness change the number of blocks?
No. A block wall is normally built single-wythe, one block thick. Thickness only decides which block width to buy. Every standard block has the same 8-by-16-inch face, so the block count depends on wall area, not wall thickness.
How do I account for doors and windows?
Calculate the block count for the full wall as if it had no openings. Then calculate the blocks for each opening the same way, and subtract that number. A 3-foot by 7-foot door, for example, covers ceiling(36 ÷ 16) × ceiling(84 ÷ 8) = 3 × 11 = 33 blocks to subtract.
Should I add extra blocks for waste?
Yes. Adding 5 to 10 percent extra blocks covers cuts, breakage and mistakes. Walls with many corners or openings may need 10 to 15 percent extra.
What is the difference between a concrete block and a cinder block?
The two names are often used for the same product today, but they once meant different things. Concrete blocks use sand and gravel as the aggregate. Cinder blocks originally used coal cinders as the aggregate. True cinder blocks are rarely made now, so most "cinder blocks" sold today are actually concrete blocks.
References
- ASTM International. ASTM C90: Standard Specification for Loadbearing Concrete Masonry Units.
- National Concrete Masonry Association. TEK 14-13C: Concrete Masonry Wall Weights.
- International Code Council. International Building Code (IBC).