Work out how many concrete blocks, cinder blocks or CMUs you need for a wall, plus core fill volume, mortar bags and material cost — metric or imperial.
Openings (doors, windows)
Mortar (optional)
Cost (optional)
| Item | Value |
|---|
Written and reviewed by Mohsin Iqbal · Last reviewed August 2026
This concrete block estimator works out the number of concrete blocks, cinder blocks or CMUs needed for a wall, based on wall dimensions, block size, door and window openings, and a waste allowance. Use it as a cinder block wall calculator for US-style block, or switch presets for AU/NZ and UK sizes. Concrete blocks are also called cinder blocks and CMUs (concrete masonry units) in the US, breeze blocks in the UK, and besser blocks in Australia and New Zealand — all calculated the same way. Switch between metric and imperial units at any time.
The short answer: divide the net wall area (gross area minus any openings) by the area one block covers including its mortar joint, then add a waste allowance. For a standard US 16in x 8in block with a 3/8in joint, that works out to roughly 1.125 blocks per square foot. For a standard AU/NZ 390mm x 190mm besser block with a 10mm joint, it's roughly 12.5 blocks per square metre. Block size varies by region and manufacturer, so use the calculator above with your actual block dimensions for an exact figure.
Wall Area = Wall Length × Wall Height
Net Wall Area = Wall Area − Total Opening Area
Block Face Area = (Block Length + Joint) × (Block Height + Joint)
Blocks Required = Net Wall Area ÷ Block Face Area
Final Blocks = Blocks Required × (1 + Waste %)
The mortar joint matters more than people expect. A block plus its joint is called the coursing module — for a 390mm x 190mm block with a 10mm joint, the module is 400mm x 200mm, not 390mm x 190mm. Leaving the joint out of the calculation overstates the block count, because dividing the wall area by the smaller bare-block area returns more blocks than the wall will actually take.
| Region | Nominal face size | Actual size | Common thicknesses |
|---|---|---|---|
| US / Canada | 16in x 8in | 15⅝in x 7⅝in (3/8in joint) | 4in, 6in, 8in, 10in, 12in |
| Australia / New Zealand | 400mm x 200mm (module) | 390mm x 190mm (10mm joint) | 90mm, 140mm, 190mm |
| UK | 450mm x 225mm (module) | 440mm x 215mm (10mm joint) | 100mm, 140mm, 215mm |
These are the most common sizes, but manufacturers do vary. If your supplier's block differs from these presets, use the "Custom size" option and enter the actual dimensions printed on the product specification sheet.
| Block type | Blocks per m² | Blocks per ft² |
|---|---|---|
| US 16in x 8in (3/8in joint) | ~12.1 | ~1.125 |
| AU/NZ 390mm x 190mm (10mm joint) | ~12.5 | ~1.16 |
| UK 440mm x 215mm (10mm joint) | ~9.9 | ~0.92 |
| Step | Working | Result |
|---|---|---|
| Wall size | 6m × 2m | 12 m² |
| Block module | 0.4m × 0.2m | 0.08 m² |
| Blocks (before waste) | 12 ÷ 0.08 | 150 blocks |
| +5% waste | 150 × 1.05 | 158 blocks |
| Step | Working | Result |
|---|---|---|
| Wall size | 20ft × 8ft | 160 ft² |
| Block coverage | ~1.125 blocks/ft² | — |
| Blocks (before waste) | 160 × 1.125 | 180 blocks |
| +5% waste | 180 × 1.05 | 189 blocks |
| Step | Working | Result |
|---|---|---|
| Gross wall area | 8m × 2.4m | 19.2 m² |
| Door opening | 0.9m × 2.1m | 1.89 m² |
| Window opening | 1.2m × 1.0m | 1.2 m² |
| Net wall area | 19.2 − 1.89 − 1.2 | 16.11 m² |
| Blocks (before waste) | 16.11 ÷ 0.08 | 202 blocks |
| +10% waste (openings present) | 201.4 × 1.10, rounded up | 222 blocks |
Add each opening's width and height in the calculator above; multiple openings of the same size can be entered with a quantity. The calculator subtracts the total opening area from the gross wall area before working out the block count, so you don't need to do this manually. As a rule of thumb, walls with several openings and more cutting around frames typically warrant a 10% waste allowance rather than 5%, since offcuts around openings are harder to reuse elsewhere in the wall.
How much waste to add depends on the wall's complexity:
| Waste % | When it applies |
|---|---|
| 0% | Only if you're ordering to an exact, pre-confirmed block count from a detailed plan |
| 5% | Simple rectangular wall, few or no openings |
| 10% | Walls with several openings, corners or a first-time builder |
| 15% | Complex layouts, curved walls, or significant cutting expected |
Waste depends on cuts, breakage, wall layout, corners, and workmanship — there is no single figure that's correct for every project, so treat these as starting points rather than fixed rules.
Some block walls need their cores filled with concrete or grout — for structural reinforcement, fire rating, sound dampening, or to anchor rebar. Switch to the Block Fill / Core Grout tab above to estimate the fill volume. Enter the number of blocks (or pull it straight from the Quantity tab), the fill volume per core, how many cores your block has, and what percentage of blocks are being filled — some walls only fill every few cells or a bond beam course rather than every core.
Actual core void volume varies by manufacturer, block design and face shell thickness — there is no single correct number that applies to every 8x8x16 block on the market. The default of 0.15 cu ft per core is a general approximation; check your supplier's published fill-volume specification, or measure a sample block, for an order you're committing to.
The Quantity tab includes an optional mortar coverage field (blocks per bag). Enter the figure from your mortar product's bag or spec sheet and the calculator will divide your block count by it to estimate bags needed. There is no single bags-per-block figure that applies everywhere — bag size, mix yield, block width and bedding method (face-shell only vs full mortar bed) all affect real consumption — so without a coverage figure entered, the calculator shows guidance instead of a number, rather than guessing.
Enter a price per block to see the estimated material cost. This calculator shows blocks-only cost — it does not include labor, mortar, reinforcement, footings or foundation work, delivery, or tax, all of which are typically required for a finished wall and commonly add up to several times the raw block cost. Enter your own local block price rather than relying on a built-in figure, since prices vary significantly by region, block type and supplier.
These three terms cause more confusion than the actual construction, because they largely describe the same product:
| Term | Meaning |
|---|---|
| CMU | Concrete masonry unit — the formal, manufacturer-correct and engineering term. |
| Concrete block | The common everyday name for the same product, made from Portland cement, sand and aggregate. |
| Cinder block | Technically a specific older type made with coal-cinder aggregate for lighter weight; genuine cinder blocks are rarely manufactured today, but the term is still used informally in the US for any concrete block. |
| Besser block | Common Australian/NZ term, from the Besser Company, a block-machinery manufacturer — same product as above. |
| Breeze block | Common UK term, historically referring to a lightweight block made with breeze (furnace ash) aggregate, now often used loosely for concrete block generally. |
For calculation purposes, all of these are estimated the same way — this calculator doesn't need to know which term you use, only the actual dimensions of the block you're buying.
This calculator estimates material quantities for planning and budgeting purposes. It does not replace advice from a structural engineer, building designer or quantity surveyor, and it does not check your local building code requirements for reinforcement, footings, wind or seismic bracing, or fire rating — all of which can affect block selection, core-filling requirements and wall design. Waste, mortar and fill-volume figures are estimates based on commonly cited trade rules of thumb and typical manufacturer specifications; always confirm exact quantities with your block supplier before placing a large order, and consult a qualified professional for anything structural.
Divide the net wall area (length x height, minus door/window openings) by the face area of one block including its mortar joint, then add a waste allowance of 5-10%. For a standard US 16in x 8in block with a 3/8in joint, that's roughly 1.125 blocks per square foot. Enter your dimensions above for an exact count.
About 12.5 blocks per square metre for a standard 390mm x 190mm block laid with a 10mm mortar joint, since each block plus its joint covers roughly 0.4m x 0.2m = 0.08 square metres. This calculator computes the exact figure for whatever block size you enter.
Around 1.125 blocks per square foot for a standard US CMU with a 15-5/8in x 7-5/8in actual face and a 3/8in mortar joint, since one block plus its joint covers exactly the 16in x 8in nominal module, or about 0.889 sq ft.
CMU (concrete masonry unit) is the technical, manufacturer-correct term. Concrete block is the common name for the same product, made from Portland cement, sand and aggregate. Cinder block technically refers to an older type made with coal-cinder aggregate, rarely manufactured today, but the term is still used informally in the US for any concrete block. All three are calculated the same way.
In the US, the standard CMU has a nominal face size of 16in x 8in (actual size 15-5/8in x 7-5/8in, with a 3/8in mortar joint making up the difference), in thicknesses of 4in, 6in, 8in, 10in or 12in. In Australia and New Zealand, the standard block is 390mm x 190mm face with a 10mm joint, in 90mm, 140mm or 190mm thickness. The UK commonly uses 440mm x 215mm or 440mm x 100mm blocks.
CMU stands for concrete masonry unit - the formal industry and engineering term for what's commonly called a concrete block or cinder block. It's a precast rectangular block made from cement, aggregate and water, produced hollow (with one or two cores) or solid.
A standard US 8x8x16 CMU with two cores takes roughly 0.15-0.20 cubic feet of grout per core when fully filled - so about 0.3-0.4 cubic feet for both cores - but this varies by manufacturer, face shell thickness and block design. Use the Block Fill mode above with your supplier's published fill volume for an accurate figure.
5% is reasonable for a simple rectangular wall with few cuts. 10% suits walls with several openings or corners. 15% is safer for complex layouts, curved walls or if you're inexperienced with cutting block, since broken and miscut blocks are common on a first attempt.
Calculate the gross wall area (length x height), then subtract the area of each door and window opening before dividing by the block face area. This calculator lets you add multiple openings and automatically works out the net area for you.
It depends on your mortar product's coverage, which varies by bag size, mix yield, block width and bedding method - there's no single figure that applies to every product. Enter the blocks-per-bag coverage from your mortar's packaging or spec sheet in the optional Mortar field above for an estimate, or check the manufacturer's published yield chart.
For calculation purposes, yes - they're the same size and are estimated identically. Technically a cinder block used coal-cinder aggregate and a concrete block uses sand, gravel or crushed stone aggregate, but true cinder blocks are rarely manufactured today. Most people use cinder block and concrete block interchangeably in the US.
Besser block is the common Australian and New Zealand term for a standard concrete masonry block, named after the Besser Company, a manufacturer of block-making machinery. It refers to the same product called a concrete block or CMU elsewhere, typically 390mm x 190mm face size.
Breeze block is the common UK term for a lightweight concrete block, historically made with breeze (furnace ash/clinker) aggregate. In modern UK usage it's often used loosely for any concrete building block, similar to how cinder block is used in the US.
A standard pallet (cube) of 8x8x16 CMU typically holds 90 to 108 blocks in the US, though this varies by manufacturer and block weight. Check with your local supplier for their exact pallet quantity when ordering.
Material cost is the block quantity multiplied by the price per block, which varies significantly by region and block type. This calculator shows blocks-only material cost from the price you enter - it does not include labor, mortar, reinforcement, footings, delivery or tax, which commonly add 2-4 times the material cost for a professionally built wall.
Yes. Select the AU/NZ or custom block preset, or enter your block's actual face dimensions and mortar joint thickness directly - the calculation works identically regardless of what the block is called locally.