How the number of blocks is calculated
The calculator works out how many concrete blocks cover the wall's face, using the block plus a mortar joint on each side, multiplies by how many blocks deep the wall is, then adds wastage. A Standard 390 × 190 mm block face with 10 mm joints covers 0.40 × 0.20 m, so a wall one block thick needs 12.5 blocks per m².
The formula
- Wall volume = Length × Height × Thickness (in metres, shown for reference)
- Block + mortar volume = (L + joint) × (W + joint) × (H + joint)
- Blocks required = Length × Height × (Thickness + joint) ÷ Block + mortar volume, rounded up
- Total = Blocks required × (1 + wastage %), rounded up
One joint is added to the thickness because a wall n blocks thick has only n − 1 joints running through it. With that joint added, the thickness divided by (block width + joint) is exactly the number of leaves, so a single-leaf wall reduces to face area ÷ ((L + joint) × (H + joint)).
Defaults: wall 5 m × 3 m, thickness 200 mm, 10 mm joint (0 to 25 mm allowed), 5% wastage (0 to 50%). This is the same method as the brick calculator, with block-sized presets.
Worked example with the defaults
A wall 5 m long and 3 m high, 200 mm thick (one block deep), built with the Standard 390 × 200 × 190 mm block:
- Wall volume: 5 × 3 × 0.2 = 3.000 m³.
- Block with mortar: 0.40 × 0.21 × 0.20 = 0.0168 m³.
- Blocks: 5 × 3 × (0.2 + 0.01) ÷ 0.0168 = 187.5, rounded up to 188.
- With 5% wastage: 198 (10 extra).
That's the same as dividing the 15 m² face by 0.40 × 0.20 m per block. For a double-leaf wall of the same blocks, enter 410 mm (two 200 mm leaves and a 10 mm joint between them): the calculator gives 375 blocks, exactly twice 187.5, or 394 with wastage.
Nominal and actual block sizes
IS 2185 (Part 1):2005, the Indian standard for concrete masonry blocks, refers to blocks by their nominal dimensions, which include the mortar joint. Clause 4.2 says actual length and height are 10 mm short of nominal, and lists nominal lengths of 400, 500 or 600 mm, heights of 200 or 100 mm, and widths from 50 to 300 mm, plus half-length blocks.
So a block sold as "400 × 200 × 200" physically measures 390 mm long and 190 mm high, and that's what the Standard preset uses. The calculator adds the joint back on, so the block lays to a 400 × 200 mm module. Measure one before you choose:
- Actually 390 × 200 × 190 mm: use the Standard preset.
- Actually 400 × 200 × 200 mm: choose Custom and enter 400, 200, 200. The 5 m × 3 m wall then needs 175 blocks, or 184 with wastage.
Blocks per 100 m² of wall
These are the calculator's results for a 10 m × 10 m single-leaf wall with 10 mm joints, no wastage, and the thickness equal to the block width:
| Block (actual size, mm) | Face with joint (m) | Blocks per 100 m² |
|---|---|---|
| 390 × 200 × 190 (Standard) | 0.40 × 0.20 | 1,250 |
| 190 × 200 × 190 (Half) | 0.20 × 0.20 | 2,500 |
| 400 × 200 × 200 (Custom) | 0.41 × 0.21 | 1,162 |
| 390 × 150 × 190 (Custom, 150 mm thickness) | 0.40 × 0.20 | 1,250 |
Block width doesn't change the count for a single-leaf wall; it changes the weight and cost of each block.
Steps for a project
- Measure each wall's length and height, and subtract doors and windows.
- Measure a block and choose the matching preset or Custom.
- Enter the wall's real thickness: the block width for one leaf, or the leaves plus the joints between them.
- Set wastage for cutting and breakage.
- Add up the walls.
The room calculator gives wall areas for a whole room, and the plaster calculator estimates cement and sand to render the finished wall. For the footing under the wall, the concrete calculator works out the volume.
These are ordering estimates. Load-bearing block walls, reinforcement and wall thickness should be specified by a qualified structural engineer under your local building code.
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