How many blocks are in a square metre?

12.5. A standard block is 390 × 190 mm on face, which is 400 × 200 mm with its 10 mm joints, so 1 ÷ (0.4 × 0.2) = 12.5.

How much mortar do I need for blocks?

About 1 m³ of mortar for every 800 blocks or 64 m² of wall, wastage included. A 20 kg bag of premix mortar lays about 10 blocks.

How much concrete to core fill blocks?

About 1 m³ for every 120 blocks of 200 series, or 190 blocks of 150 series, when every core is filled. Which cores are filled is set by the engineer.

How much render do I need?

About 1.7 kg of render per m² for every mm of thickness, so a 6 mm coat takes about 10 kg per m². Thicker than 6 mm goes on in two coats.

Free calculator

Concrete block calculator

Blocks = wall area × 12.5 per m² (400 × 200 mm module), less doors and windows, plus about 10%. Mortar is about 1 m³ per 64 m² of wall; core fill about 1 m³ per 120 blocks of 200 series.

Your job

Walls

Wall 1

Length

Height

Doors and windows

Blocks

12.5 blocks per m² for the standard 390 × 190 mm face with 10 mm joints.

Wastage allowance10%

10% covers cuts and breakages, the allowance a block-and-mortar maker's calculator uses.

Core fill

Bar spacing and which cores to fill come from your engineer. This works out the concrete only.

Render

Render thickness

Up to 6 mm goes on in one coat; thicker in two. Worked out at about 1.7 kg of render per m² for every mm.

Optional

Blocks per pallet

Your price per block

Ask your block supplier. The greyed numbers are examples only and are never used in your result.

Blocks drawn to scale in stretcher bond. The right half shows the render; the block section shows the cores.

Blocks to buy

264blocks

200 series (190 mm), including 10% wastage

Wall area

19.20m²

No openings deducted

Mortar

0.30m³

Or about 27 × 20 kg premix bags

Render

11bags

20 kg bags for 19.2 m² at 6.0 mm, 1 coat

The working, with your numbers

1. Net area19.20 − 0.00 openings= 19.20 m²
2. Blocks19.20 × 12.5 × 1.10, rounded up= 264
3. Mortar19.20 ÷ 64= 0.30 m³
4. Render19.20 m² × 6.0 mm × 1.7 kg × 1.10 ÷ 20= 11 bags

Typed those measurements in by hand?MetreMate measures them straight off your PDF plans. Free to start.

The formula, in plain words

1Net area = length × height of each wall, less every door and window.
2Blocks = net area × 12.5 × (1 + wastage), rounded up. A block is 390 × 190 mm on face, 400 × 200 with its joints.
3Mortar m³ = net area ÷ 64 (about 1 m³ for 800 blocks, wastage included). Premix: about 10 blocks per 20 kg bag.
4Core fill m³ = blocks ÷ blocks filled per m³ (about 190 for 150 series, 120 for 200 series) × share of cores filled.
5Render = area × thickness (mm) × 1.7 kg, + wastage, in 20 kg bags. Up to 6 mm in one coat.

Worked example

An 8 × 2.4 m wall in 200 series blocks, no openings, 10% wastage, rendered one side at 6 mm.

1.Area: 8 × 2.4 = 19.2 m². Blocks: 19.2 × 12.5 = 240, × 1.1 = 264 blocks.
2.Mortar: 19.2 ÷ 64 = 0.3 m³, or about 27 premix bags.
3.Render: 19.2 × 6 × 1.7 = 196 kg, + 10% = 215 kg, eleven 20 kg bags.
4.Fully core filled, the same wall would take 240 ÷ 120 = 2 m³ of grout.

Answer: 264 blocks, 0.3 m³ of mortar and 11 bags of render.

How to measure it

Measure each wall's length and height in metres.
Add every door and window with its size: they come off the wall area.
Pick the block series: 100 (90 mm), 150 (140 mm) or 200 (190 mm) thick.
Ask your engineer which cores are filled if the wall is reinforced.

Where the numbers come from

Every default in the calculator is a number a supplier or manufacturer publishes. We list the source next to it so you can check.

What	Value	Source
Blocks are 400 × 200 mm nominal (390 × 190 face, 10 mm joints): 12.5 blocks per m²	12.5 per m²	Midland Brick, Utility Bricks & Blocks Technical Information Guide (2026) (checked 2026-10-01)
Blocks come 90 mm (100 series), 140 mm (150 series) and 190 mm (200 series) thick	190 mm	Midland Brick, Utility Bricks & Blocks Technical Information Guide (2026) (checked 2026-10-01)
A block-and-mortar calculator adds 10% to blocks, mortar and render	10 %	Cement Australia, mortar calculator (checked 2026-10-01)
About 1 m³ of mortar for every 800 blocks or 64 m² of wall, allowing for wastage	64 m² per m³	Midland Brick, Utility Bricks & Blocks Technical Information Guide (2026) (checked 2026-10-01)
A 20 kg bag of premixed mortar lays about 10 masonry blocks with a 10 mm joint	10 blocks per 20 kg bag	Cement Australia, Mortar Mix product data sheet (checked 2026-10-01)
Core fill: 1 m³ fills about 190 blocks of 15.01	190 blocks per m³	Midland Brick, Utility Bricks & Blocks Technical Information Guide (2026) (checked 2026-10-01)
Core fill: 1 m³ fills about 120 blocks of 20.01	120 blocks per m³	Midland Brick, Utility Bricks & Blocks Technical Information Guide (2026) (checked 2026-10-01)
Cement render: about 1.7 kg of powder per m² per mm of thickness	1.7 kg per m² per mm	Dulux AcraTex, RenderWall AcraBuild product data sheet (checked 2026-10-01)
One coat up to 6 mm; thicker work goes on in two coats	6 mm	Sika, Davco Render product data sheet (checked 2026-10-01)

Manufacturers named here are the source of the number, not a recommendation. Confirm quantities with your supplier.

Frequently asked questions

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