This calculator is US-oriented but works for Canada, the United Kingdom, Australia and other markets. Switch between metric and US customary / imperial units, then choose a regional block preset or enter the specified dimensions from the product data sheet.
Estimate concrete blocks, CMUs and masonry materials
Enter one wall or several separate masonry sections, add doors and windows, select the concrete-block size and choose how mortar should be estimated. The result includes net wall area, block quantity, course count, mortar bags or cement and sand, allowance and optional material cost.
What the calculator returns
| Result | Meaning |
|---|---|
| Net blockwork area | Gross wall area after the entered openings are deducted |
| Blocks before allowance | Area divided by the modular face area of one block |
| Blocks to buy | Calculated quantity plus the selected allowance, rounded up |
| Mortar materials | Preblended bags or masonry cement and sand from editable coverage assumptions |
| Cost | Only the materials for which a price has been entered |
Measure each wall section separately
- Enter the length and height of each rectangular wall or section.
- Use the quantity column for repeated walls with identical dimensions.
- Divide stepped, sloped or irregular masonry into measurable rectangles.
- Enter doors, windows and open passages in the separate openings list.
- Confirm the dimensions against the plan and a second site measurement.
Do not deduct every outlet box, pipe hole or small penetration. These openings rarely reduce the number of complete blocks that must be delivered and cut on site.
Nominal size and specified size are different
In the United States and Canada, a nominal 8 × 8 × 16 in CMU normally fits an 8 × 16 in masonry module. The specified block is commonly 3/8 in smaller in each modular direction so that the block plus a 3/8 in mortar joint reaches the nominal size. Use specified or actual dimensions in a custom entry, not the marketing name alone.
Regional block modules included as presets
| Region | Specified face size | Typical joint | Modular face |
|---|---|---|---|
| United States / Canada | 15 5/8 × 7 5/8 in | 3/8 in | 16 × 8 in |
| United Kingdom | 440 × 215 mm | 10 mm | 450 × 225 mm |
| Australia / metric markets | 390 × 190 mm | 10 mm | 400 × 200 mm |
Block thickness does not normally change the number of standard units per square foot or square metre when the face dimensions remain the same. It does affect weight, structural use, fire performance and the wall specification, which this quantity calculator does not design.
How the block count is calculated
The net wall area is divided by the modular face area of one unit. Modular length equals specified block length plus joint thickness, and modular height equals specified block height plus joint thickness. The selected block allowance is added only after openings have been deducted.
How many nominal 8 × 8 × 16 in CMUs are needed for 100 ft² with 5% allowance?
Answer: Approximately 119 blocks.
Explanation: An 8 × 16 in module covers 8/9 ft², so 100 ft² requires 112.5 units before allowance. Multiplying by 1.05 gives 118.125, which rounds up to 119 complete units.
Choose the mortar-estimating method that matches your information
Three mortar options
| Mode | Input used | Result |
|---|---|---|
| Preblended mortar | Average blocks laid per bag | Whole bags of dry masonry mortar |
| Masonry cement and sand | Blocks per cement bag, cement bag weight and sand per bag | Cement bags, cement weight and sand weight |
| No mortar estimate | No coverage data | Concrete blocks only |
The default US planning values use approximately 16 conventional hollow CMUs per 80 lb preblended mortar bag, or approximately 30 hollow CMUs per 70 lb masonry-cement bag with about 250 lb of masonry sand per cement bag. These are editable field estimates, not universal product coverage.
Mortar consumption changes with unit configuration, face-shell or full bedding, joint dimensions, tooling, waste, weather and workmanship. Replace the defaults with the exact product yield, project specification or an experienced mason's established job rate whenever available.
Concrete block, CMU, cinder block and breeze block
Regional search and buying terms
| Term | Where it is common | Planning meaning |
|---|---|---|
| Concrete masonry unit / CMU | United States and Canada | Technical term for manufactured concrete masonry units |
| Concrete block | All four target markets | Broad general term for block masonry |
| Cinder block | United States, usually colloquial | Often used for modern CMU even when cinders are not present |
| Breeze block | United Kingdom and some Commonwealth usage | A colloquial term that may also refer to decorative or lightweight units |
| Concrete masonry block | Australia and international specifications | Common formal description for concrete blocks |
How much block allowance should be added?
Practical starting points
| Project conditions | Possible starting allowance | Reason |
|---|---|---|
| Long modular walls with a planned bond | About 3–5% | Repeated courses and reusable cuts may keep loss low |
| Typical walls with doors, windows and corners | About 5–10% | Allows for cutting, breakage and special locations |
| Many short returns, angles or decorative patterns | Check a block layout | One percentage may not represent the actual cutting pattern |
The allowance does not create corner units, bond-beam units or lintel blocks automatically. Those product types should be scheduled separately from the general block quantity.
Optional cost follows the purchasing quantity
- Enable cost in Advanced settings.
- Choose USD, CAD, GBP, AUD, EUR or UAH.
- Enter the price per concrete block or CMU.
- For preblended mortar, enter the price per bag.
- For site-mixed mortar, enter the cement-bag price and sand price per metric tonne in metric mode or per US short ton in imperial mode.
The total excludes sales tax unless already included in the entered rate. It also excludes pallets, deposits, freight, unloading, equipment, labor and any material that is not represented by a price field.
What the estimate deliberately excludes
- corner, half, jamb, lintel and bond-beam units;
- vertical and horizontal reinforcement;
- grout or concrete used to fill selected cells;
- wall ties, anchors, connectors and movement-joint materials;
- foundations, damp-proofing, drainage and finishes;
- delivery, lifting equipment, scaffolding and labor.
A material quantity is not a structural design. Load-bearing, retaining, reinforced, tall, fire-rated, wind-exposed or seismic masonry must be specified and checked under the applicable local code by a qualified professional.
Common concrete-block estimating mistakes
Mistake, consequence and correction
| Mistake | Likely result | Better approach |
|---|---|---|
| Entering nominal dimensions as specified dimensions | The block count is understated | Use actual or specified block dimensions |
| Ignoring mortar-joint thickness | The modular face area is wrong | Enter the designed joint thickness |
| Deducting every small penetration | The calculated saving exceeds the real full-block saving | Deduct substantial openings only |
| Using one US mortar rule for every market and block | Mortar quantity may be seriously wrong | Replace coverage with product or project data |
| Treating all units as standard stretchers | The total may be close but the order is incomplete | Schedule special units separately |
Technical references behind the defaults
CMHA: concrete masonry unit nominal and specified dimensionsCMHA: estimating concrete masonry units, mortar, masonry cement and sandForterra: UK concrete block example with a 440 × 215 mm faceNational Masonry: Australian 390 × 190 mm block example and 12.5 units/m²Read how HomDera handles assumptions, editable values, rounding and calculator limitationsFinal ordering check
- Wall dimensions include every section to be built.
- Large openings have been entered once and only once.
- The selected block dimensions match the supplier's specified product size.
- Joint thickness matches the project detail.
- The allowance reflects the actual bond pattern and cutting complexity.
- Mortar coverage comes from the selected product or a verified site rate.
- Special units, reinforcement, grout and lintels have separate quantities.
The calculator provides a strong preliminary purchasing estimate when the wall area, specified block size and mortar coverage are reliable. The final order should still be checked against the bond layout, structural drawings, supplier pack quantities and local installation requirements.
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