Concrete Block Wall Calculator
This free concrete block wall calculator tells you how many CMU (cinder) blocks to buy for your wall. Enter the wall length and height, and it instantly estimates the block count — rounded up, with a 10% waste option — plus a rough estimate of the 60-lb mortar bags you will need.
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Results update as you type.
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Enter your wall dimensions to see the estimate.
How to use this calculator
Measure your wall's length and height in feet and enter them. If the wall has windows or door openings, measure each wall segment separately and add the results together — this calculator counts full wall face area, so openings would inflate the estimate.
Keep the 10% waste allowance box checked: it covers cut blocks at corners and ends, breakage, and a few spares for future repairs. Uncheck it if you prefer to plan with zero waste and order extras yourself. Results update instantly as you type.
How it works
A standard 16 × 8 in concrete masonry unit (CMU) face measures 16/12 × 8/12 = 0.889 sq ft per block, assuming 3/8 in mortar joints. The wall area is divided by the block face area and rounded up to a whole block (a tiny epsilon guards against floating-point results like 134.0000000001 that would otherwise round up wrongly):
blocks = ceil(wall sq ft ÷ 0.889)
The waste option adds 10% and rounds up again:
blocks to buy = ceil(blocks × 1.10)
For example, a 30 ft by 4 ft wall is 120 sq ft: 120 ÷ 0.889 = 135 blocks, plus 10% waste = 149 blocks to buy.
The mortar estimate uses a rule of thumb of about 0.5 bags of 60-lb mortar per 20 blocks:
mortar bags (rough estimate) = ceil(blocks ÷ 20 × 0.5)
This calculator is for CMU (cinder block) walls laid with mortar. It is different from our brick wall calculator, which covers smaller clay bricks (about 7.5 per sq ft), and our retaining wall block calculator, which covers dry-stacked segmental retaining blocks with no mortar. All results are estimates: the block count assumes standard 3/8 in joints, and the mortar rule of thumb is a planning figure — confirm quantities with your supplier before ordering.
Frequently asked questions
How many concrete blocks do I need for a 30 ft by 4 ft wall?
A 30 ft × 4 ft wall is 120 sq ft. A standard 16 × 8 in concrete block (CMU) covers about 0.889 sq ft of wall face, so 120 ÷ 0.889 = 135 blocks. Adding a 10% waste allowance gives 149 blocks to buy (135 × 1.1 = 148.5, rounded up). Enter any wall size in the calculator above and it does this math instantly.
What is a standard concrete block (CMU) size?
The standard U.S. concrete masonry unit (CMU), often called a cinder block, is nominally 8 × 8 × 16 in. Each block covers about 0.889 sq ft of wall face with a 3/8 in mortar joint — roughly 1.125 blocks per square foot. The calculator uses this standard size and mortar joint as its baseline.
How much mortar do I need for a concrete block wall?
A common rough estimate is about 0.5 bags of 60-lb mortar per 20 blocks. For example, 149 blocks need roughly 4 bags (149 ÷ 20 × 0.5 = 3.7, rounded up). This is a planning estimate only: actual mortar use varies with joint width and how much mortar ends up wasted on site, so buy a bag or two extra. Confirm with your supplier before ordering.
Is this the same as a brick wall or retaining wall block calculator?
No — they cover different materials. This concrete block wall calculator is for mortared CMU (cinder block) walls. Our brick wall calculator covers smaller clay bricks at about 7.5 per sq ft, and our retaining wall block calculator covers dry-stacked segmental retaining blocks that use no mortar. Use the tool that matches your wall material.
Should I add a waste allowance for concrete blocks?
Yes — 10% is the standard allowance for cut blocks, breakage, and job-site mistakes. Walls with corners, window or door openings, or lots of cuts may need more. The calculator adds the 10% automatically; uncheck the box if you prefer to plan with zero waste and order spares yourself.