Block and mortar calculator
- September 2026
Know your block count and the mortar to lay them, with the joints accounted for.
Enter the wall area with openings already taken out, and pick a block size. The tool counts blocks by the block plus its joint rather than by the block alone, because a 10 mm joint is added to both the length and the height, then works out the mortar needed to fill those joints.
- Runs in your browser
- No sign-up
- Nothing leaves your device
How is the block count calculated?
- Cell area = (block length + 10 mm) × (block height + 10 mm)
- Blocks per m² = 1 ÷ cell area
- Final count = wall area × blocks per m² × (1 + 5% breakage)
- Mortar volume = wall area × the joint share of the cell × wall thickness
Symbols
- joint
- 10 mm added to both the length and the height, which is why the count is below a straight division
- breakage
- 5% for handling and for cutting at openings
- mortar
- Multiplied by 1.3 for shrinkage, mixed 1:5 cement to sand
How many blocks for a 50 m² wall?
A 50 m² wall in 400 × 200 × 200 mm blocks at a 10 mm joint:
- Cell area: 0.41 × 0.21 = 0.0861 m²
- Blocks per m²: 1 ÷ 0.0861 = 11.6
- Before breakage: 50 × 11.6 = 581 blocks
- With 5 percent breakage: 610 blocks
- Joints take 7.1 percent of the face: 50 × 0.071 × 0.20 = 0.71 m³ of mortar
You need 610 blocks and about 0.92 m³ of dry mortar. Dividing the area by the block face alone would have given 625 before breakage, close to 8 percent more than you need to pay for.
Why is the count lower than area ÷ block face?
The usual error is dividing the wall area by the face of the block. That ignores the joint and gives a count higher than reality. A block in a wall does not occupy 400 × 200 mm, it occupies 410 × 210 mm, because the 10 mm joint is added in two directions.
So the tool works out the area of the cell, meaning the block plus its joint, and divides the wall area by that. The difference is not small: at 400 × 200 the count drops from 12.5 to 11.6 blocks per square metre.
The mortar volume comes from the same geometry: the share of the cell that the joints occupy, multiplied by the wall area and by the wall thickness. That is then multiplied by 1.3, because dry mortar shrinks when mixed, and split across the parts of a 1:5 mix.
- Cell area = (block length + 10 mm) × (block height + 10 mm)
- Blocks per m² = 1 ÷ cell area
- Final count = wall area × blocks per m² × (1 + 5% breakage)
- Mortar volume = wall area × the joint share of the cell × wall thickness
- Dry mortar = mortar volume × 1.3, at a 1:5 cement to sand mix
Questions to settle before ordering blocks
Why is the count lower than the one I worked out?
Most likely because you divided the area by the block face without the joint. A block plus its joint occupies more than the block alone, so fewer of them fit. The difference is about 8 percent at 400 × 200, which is real money on a large wall.
Does the quantity include plaster?
No. The mortar here is laying mortar, which only fills the joints between blocks. Plaster is a separate item that covers the whole face of the wall in a full coat and uses several times as much. Work it out separately from the coat thickness.
Which thickness should I use?
200 mm for external and load-bearing walls, 150 mm for main internal walls, and 100 mm for light partitions inside an apartment only. Thickness feeds straight into the mortar volume, because the joint runs the full depth of the wall.
Why a 5 percent breakage allowance?
Blocks break in loading and unloading, and get cut at openings and corners with part of each cut wasted. Five percent is the figure for an organised site. Raise it if the wall has many openings, or if delivery is a long haul over rough ground.
Do I deduct doors and windows?
Yes, take them off before entering the area. The tool uses the net wall area exactly as given. The room area calculator produces that figure directly, with openings already deducted.
Sources
Reviewed September 2026- The 10 mm mortar joint is standard blockwork practice, and is added to both the length and the height of every block, which is why the count differs from a straight division
- The registered block sizes are the nominal formats common across Gulf markets, with the 400 × 200 family the most widely used
- Mortar volume is derived geometrically from the share of the block cell that the joints occupy, multiplied by wall thickness, rather than from a rule-of-thumb figure per square metre
- A 1.3 shrinkage factor for mortar, lower than the 1.54 used for concrete because mortar has no coarse aggregate, at the usual 1:5 blockwork mix
- A 5 percent breakage allowance for handling and for cutting at openings and corners
Assumptions used
- A 10 mm mortar joint is standard blockwork practice. The joint is added to both the length and the height of every block, which is why the count is lower than area divided by block face.
- Nominal hollow concrete block formats sold across the Gulf. The 400x200 family is the common one; 390x190 is the equivalent metric size.
- A 1:5 cement-sand mortar, the usual blockwork mix. Mortar bulks less than concrete because it has no coarse aggregate, so the dry factor is 1.30 rather than 1.54.
- Breakage in handling and cutting at openings and corners.