Concrete Block Calculator
Work out how many concrete blocks (CMU), bags of mortar and how much core-fill grout a block wall needs, from its length and height, with openings and waste.
How to Use
- Enter the wall length and height, and the area of any doors or windows to leave out.
- Choose the block size. The usual US block is 8×8×16 (nominal); the joint is ⅜ in (10 mm).
- Choose how the cores are filled: not at all, every cell (a solid wall), or every second, third or fourth cell for a reinforced wall.
- Keep about 5% for waste and cut blocks.
- Read the blocks, mortar bags, courses and grout; add a price per block for the cost.
Worked Example
A wall 30 ft long and 8 ft high in 8×8×16 block. Each block with its ⅜ in joint covers 16 × 8 in = 128 in², so a square foot (144 in²) takes 144 ÷ 128 = 1.125 blocks. The wall is 30 × 8 = 240 ft², so 240 × 1.125 = 270 blocks; with 5% waste, 284. Courses: 8 ft = 96 in, ÷ 8 in = 12 courses of 22.5 blocks. Mortar at about 13 blocks per 80 lb bag: 284 ÷ 13 = 22 bags.
The common mistake: using the actual block size. 144 ÷ (15⅝ × 7⅝) gives 1.21 blocks per square foot, so 290 blocks before waste instead of 270, because the joint is left out. Nominal sizes already include it.
Show Work
Formulas
How Block Walls Are Built
Concrete masonry units are laid in running bond, each course offset by half a block, on a mortar bed placed on the face shells. Hollow cores make the blocks light enough to lift and leave room for steel: in reinforced walls, vertical rebar runs through chosen cells, which are then filled with grout, a fluid concrete that bonds the steel and block into one unit. Horizontal joint reinforcement or bond-beam courses add strength along the wall.
The hollow concrete block was patented in the US by Harmon S. Palmer in 1900, with a machine that cast them on site. The 8-inch module that block shares with modular brick lets the two be built together: three courses of brick match one course of block.
About This Calculator
This calculator counts concrete blocks from the block size and joint, so it handles 4, 6, 8 and 12 in block, half-high block, UK and Australian sizes. It deducts openings, adds waste, gives the courses, the bags of mortar mix and the core-fill grout for solid or partly grouted walls, and works in feet or metres.
Everything runs in your browser; nothing is sent anywhere. Structural block walls need an engineer’s or code-based design for footing, rebar and grout spacing.
Related calculators: Brick Calculator, Concrete Calculator, and Rebar Calculator.
Frequently Asked Questions
How many concrete blocks are in a square foot?
A standard 8×8×16 block is really 7⅝ × 15⅝ in on its face; with a ⅜ in joint it covers 8 × 16 in, which is 0.889 ft², so a square foot takes 1.125 blocks. 100 ft² of wall needs 112.5 blocks before waste. Half-high 4×16 blocks need twice as many.
How much mortar do concrete blocks need?
An 80 lb bag of mortar mix lays about 13 standard 8×8×16 blocks, so 100 blocks take about 8 bags. Block is normally laid with mortar only on the face shells, which is what that figure assumes.
How much grout fills the cores of a block wall?
An 8 in block wall filled solid takes about 0.26 ft³ of grout per square foot, roughly 1 yd³ per 100 ft² of wall. Reinforced walls are often grouted only in the cells with rebar, for example every 32 or 48 in, which cuts the grout to a quarter or a sixth of that.
What is the difference between nominal and actual block size?
Nominal sizes include the ⅜ in joint: an “8×8×16” block is actually 7⅝ × 7⅝ × 15⅝ in. That way two blocks and their joints make exactly 32 in, and walls lay out on an 8 in grid. UK blocks are 440 × 215 mm (450 × 225 mm with joints).
Are cinder blocks and concrete blocks the same?
Not quite. Cinder blocks were made with coal cinders (fly ash) as the aggregate; they were lighter and weaker. Modern concrete masonry units (CMU) use sand and gravel or lightweight aggregate, but most people still call them cinder blocks.
How do I use the Concrete Block Calculator?
Simply type your numbers and read the result, which refreshes the instant you change something. There is nothing to submit and nothing to wait for.
Do I need to install or sign up for anything?
Not at all — it runs in the browser with nothing to install and no account. After it loads once, it even works without an internet connection.
Is my information private?
Yes. Everything happens in your browser. Nothing you type is sent to a server or saved anywhere.
Common Use Cases
Garage or shed wall
A 30×8 ft wall of 8×8×16 block: 270 blocks, 284 with 5% waste, laid in 12 courses.
Foundation stem wall
A 120 ft perimeter, 3 ft high: 405 blocks before waste; with 5% waste, 426 blocks and 33 bags of mortar.
Reinforced wall
A 40×6 ft wall grouted in every third cell needs about 0.76 yd³ of grout; fully grouted, 2.29 yd³.
Wall with a door
A 24×8 ft wall less a 3×7 ft door is 171 ft² net: 193 blocks before waste.
UK blockwork
A 6×2.4 m wall of 440×215 mm blocks is 14.4 m² × 9.88 = 143 blocks before waste.
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