Blocks per square metre
Australian concrete blocks share one face size, 390 mm long by 190 mm high. With a 10 mm mortar joint on the end and under the bed, each block covers 400 × 200 mm of wall, which is 0.08 square metres, so there are 12.5 blocks in a square metre. That figure does not change between 10, 15 and 20 series, because the series number only describes the thickness: 90, 140 and 190 mm respectively, or roughly 10, 15 and 20 centimetres.
blocks per m² = 1 ÷ (0.400 × 0.200) = 12.5
Because blocks are 200 mm a course, wall heights fall naturally on 200 mm steps. A 1.2 m retaining wall is six courses; a 2.4 m garage wall is twelve. Heights in between mean a cut course, so it is worth setting the footing level to land the top of the wall on a full course.
Which series
| Series | Size | Typical use | Core fill, all cores |
|---|---|---|---|
| 10 | 390 × 90 × 190 | Non-loadbearing partitions, garden beds, veneer | ~0.03 m³/m² |
| 15 | 390 × 140 × 190 | Internal loadbearing walls, low fences | ~0.06 m³/m² |
| 20 | 390 × 190 × 190 | Retaining walls, reinforced walls, garages, pools | ~0.085 m³/m² |
Core fill figures are approximate and depend on the exact block, since manufacturers vary the core size. Use the figure on the block maker's data sheet when you have it.
Core filling
A reinforced block wall is a concrete wall built inside a block shell. Vertical bars go into the cores from starter bars in the footing, horizontal bars sit in knock-out or bond beam courses, and then the cores are filled with a grout mix, usually N20 with 10 mm aggregate so it flows down the core. Filling every core is standard for retaining walls and anything the engineer has designed. Every second core is common on lightly loaded walls where only the reinforced cores need filling. Unreinforced garden walls are often not filled at all. The calculator adds 10 percent to the fill volume because a certain amount goes down the outside of the wall and into the odd gap in the mortar.
Mortar
Blocks use less mortar per square metre than bricks because there are fewer joints. A common allowance is about 0.02 cubic metres of mixed mortar per square metre of wall, which becomes roughly 0.026 cubic metres of sand and about 0.2 of a 20 kg bag of cement per square metre. Over a whole wall the calculator rounds this up to whole bags.
Worked example
A retaining wall 8 m long and 1.2 m high in 20 series blocks, all cores filled. Area 9.6 m², so 120 blocks, 126 with 5 percent waste, which is two pallets of 72. Six courses. Core fill 9.6 × 0.085 = 0.82 m³, or 0.90 m³ with waste, so order 1.0 m³ of grout and have a pump on site. Mortar 0.19 m³, which is about 0.25 m³ of sand and two bags of cement.
Common questions
How many concrete blocks per m²?
12.5 for standard 390 × 190 face blocks with 10 mm joints, regardless of series.
How much concrete to core fill?
About 0.085 m³/m² for 20 series with every core filled, 0.06 for 15 series and 0.03 for 10 series. Halve it for every second core.
What do 10, 15 and 20 series mean?
Nominal thickness in centimetres. All are 390 long and 190 high; the thickness is 90, 140 or 190 mm.
How many blocks on a pallet?
Commonly 60 to 90 for 20 series, more for thinner blocks. Ask the supplier.
Quantities only. Reinforcement, footings, drainage and wall height limits for retaining walls come from your engineer and council. In Victoria a retaining wall over 1 m usually needs a building permit.