How many concrete blocks do I need per square metre of wall?

For standard 390 x 190 mm concrete blocks laid with 10 mm mortar joints, each block occupies 400 mm x 200 mm of wall face area, which is 0.08 m². That means you need 12.5 blocks per square metre of wall (1 divided by 0.08). Always add a waste allowance of 5 to 10 percent for cuts, breakage, and off-cuts, so a practical figure is around 13 to 14 blocks per square metre.

How much mortar do I need per concrete block?

A common rule of thumb is approximately 0.012 to 0.015 cubic metres of mortar per square metre of block wall, or roughly one cubic metre of mortar per 80 to 85 square metres of wall. This accounts for bed joints, perpend joints, and normal wastage. For a single standard block (390 x 190 mm face), approximately 0.001 cubic metres of mortar is used. Pre-mixed mortar bags (20 kg) typically cover around 15 to 20 blocks each.

What is the standard concrete block size in New Zealand?

The most common concrete masonry unit (CMU) in New Zealand is 390 mm long x 190 mm high x 190 mm wide (nominal). When laid with a 10 mm mortar joint, the co-ordinating size becomes 400 mm x 200 mm, giving 12.5 blocks per square metre. Other common sizes include half blocks (190 x 190 mm) and knock-out blocks used around reinforcing steel. Blocks are manufactured to NZS 3122 and are available as standard hollow, semi-solid, or solid units.

Concrete Block Calculator

Work out how many concrete blocks you need for a wall or project. Enter the wall dimensions and block size, choose your waste allowance, and add block and mortar prices to get a complete material estimate.

Standard NZ concrete blocks are 390 x 190 mm face size. With a 10 mm mortar joint the co-ordinating size is 400 x 200 mm, giving 12.5 blocks per square metre.

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Updated June 2026 Based on NZS 3122 concrete masonry units and standard 10 mm mortar joints.

1. Wall Dimensions

Wall length (metres)

Wall height (metres)

Number of walls

walls

Openings to subtract (doors, windows)

m²

2. Block Size and Options

Block size (face dimensions)

Mortar joint thickness (mm)

Waste allowance

3. Cost Inputs (optional)

Block price ($ each incl GST)

$/block

Mortar bag price (20 kg bag)

$/bag

Blocks per mortar bag (20 kg bag covers)

blocks

4. Wall Layers

Wall construction type

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.

Your Concrete Block Estimate

Blocks Needed
-
Including waste allowance

Wall Area

Net area (m²)

Mortar Bags

20 kg bags needed

Estimated Cost

Blocks + mortar

Block Calculation Breakdown

Total wall length-

Wall height-

Number of walls-

Gross wall area-

Openings deducted-

Net wall area-

Wall layers (skins)-

Block face size (incl joint)-

Blocks per m² (net)-

Blocks before waste-

Waste allowance-

Total blocks to order-

Mortar and Cost Breakdown

Mortar bags needed-

Block cost-

Mortar cost-

Total material cost-

Cost per m² of wall-

Blocks per m² (with waste)-

Summary: Enter your wall dimensions above to get your block estimate.

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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Concrete Volume Calculator: volume for slabs, walls, and columns.

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.