Concrete Block Calculator
This calculator works out how many concrete blocks you need for a wall, along with the mortar and total material cost, so you can order the right quantities before a build starts. Enter your wall length and height, the number of walls, and any doorway or window openings to subtract, then choose your block size (standard 390 x 190 mm NZ block, half-height, half-length, or custom), mortar joint thickness, and a waste allowance to cover cuts and breakages. You can also add block and mortar prices, blocks per mortar bag, and whether the wall is single or double skin, to get a full cost estimate. It returns the total blocks needed, net wall area, number of 20 kg mortar bags required, and estimated material cost, with a breakdown showing gross and net area, blocks per square metre before and after waste, and the split between block cost and mortar cost. Use it to plan a garden wall, boundary wall or retaining wall, checking how block size, joint thickness and waste allowance each change your order quantity. A 10 percent waste allowance suits most straight walls, while curved or heavily cut layouts need more. This tool estimates material quantities only, not structural adequacy, so get an engineer's sign-off for retaining walls over 1.5 m or any load-bearing structure, and confirm current block and mortar prices with your supplier before ordering.
1. Wall Dimensions
2. Block Size and Options
3. Cost Inputs (optional)
4. Wall Layers
Single skin walls are most common for garden retaining walls and boundary walls. Double skin provides greater structural strength for load-bearing applications.
Note: This calculator estimates surface area requirements. Always consult a structural engineer for retaining walls over 1.5 m high or any load-bearing structure.
Block Calculation Breakdown
Mortar and Cost Breakdown
How to Calculate Concrete Blocks for a Wall
The core calculation is straightforward: divide the total wall area by the area of one block (including mortar joints), then add a waste factor for cuts and breakages.
For a standard 390 x 190 mm block with a 10 mm mortar joint, the co-ordinating size is 400 x 200 mm. Each block covers 0.08 m² of wall face. This gives 12.5 blocks per square metre before waste.
The formula is:
- Net wall area (m²) = (length x height x number of walls) minus openings
- Blocks per m² = 1 / ((block length + joint) x (block height + joint))
- Base blocks = net area x blocks per m²
- Blocks to order = base blocks x (1 + waste fraction) x wall layers
Standard Concrete Block Sizes in New Zealand
| Block Type | Face Size (L x H) | Width | Co-ordinating Size | Blocks per m² |
|---|---|---|---|---|
| Standard (full) | 390 x 190 mm | 190 mm | 400 x 200 mm | 12.5 |
| Half-height | 390 x 90 mm | 190 mm | 400 x 100 mm | 25.0 |
| Half-length | 190 x 190 mm | 190 mm | 200 x 200 mm | 25.0 |
| Knock-out / lintel | 390 x 190 mm | 190 mm | 400 x 200 mm | 12.5 |
Blocks are manufactured to NZS 3122. Common widths are 90 mm, 140 mm, and 190 mm depending on structural requirements. This calculator uses face dimensions only (not block width), so it works for any block depth.
Mortar Estimation
The amount of mortar needed depends on block size, joint thickness, and mortar type. A standard rule for 390 x 190 mm blocks with 10 mm joints is approximately 17 blocks per 20 kg bag of pre-mixed mortar. For site-mixed mortar (1 part cement, 3 parts sand), approximately 0.012 cubic metres of mortar is used per square metre of wall.
Always add 10 to 15 percent to your mortar estimate to allow for wastage during mixing and application. Left-over mortar should not be stored once mixed, so slight over-ordering is better than running short mid-course.
Waste Allowance
A 10 percent waste allowance is suitable for most straight wall projects where blocks are cut at corners or around openings. Increase to 15 percent for L-shaped or U-shaped walls with multiple corners, and to 20 percent for curved walls or decorative features where many half-blocks are needed.
Worked Example
A garden wall 6 m long and 2 m high, one skin, standard 390 x 190 mm blocks with 10 mm joints, 10% waste:
- Wall area: 6 x 2 = 12 m²
- Co-ordinating block size: 0.400 x 0.200 = 0.08 m²
- Blocks per m²: 1 / 0.08 = 12.5
- Base blocks: 12 x 12.5 = 150
- With 10% waste: 150 x 1.10 = 165 blocks
- Mortar bags (at 17 blocks per bag): ceil(165 / 17) = 10 bags
- Material cost at $4.50/block and $18/bag: (165 x $4.50) + (10 x $18) = $742.50 + $180 = $922.50
This matches the default output of the calculator above.
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Sources and method: Block dimensions per NZS 3122 (Precast concrete masonry units). Mortar coverage rates based on industry standard of 17 standard blocks per 20 kg bag. Waste allowances follow standard NZ trade practice. Cost inputs are indicative only; check current prices with your local supplier.
This calculator provides material estimates for planning purposes only. Actual quantities may vary depending on block dimensions, mortar consistency, wall design, and site conditions. Always verify with your building supplier and seek engineering advice for structural or retaining walls.