Block Calculator
Estimate how many concrete masonry blocks a wall needs. Enter the net wall area in square metres and the face size of one block in millimetres — the defaults are 400 mm long by 200 mm high for a standard 8-inch unit — plus a waste percentage, and get the block count including that waste.
Your result will appear here.
Block Calculator online
Estimate how many concrete or clay blocks a wall needs from the wall area and the block face size — with the corner, opening and joint judgements that decide whether the last pallet is one row short.
How it works
The tool divides wall area by the face area of one block, then adds waste: Blocks = Wall area / ((Length / 1000) × (Height / 1000)) × (1 + Waste / 100). At the defaults, one block face covers 0.400 × 0.200 = 0.08 m², so each square metre needs 1 / 0.08 = 12.5 blocks before waste. The tool counts full-unit equivalents; it does not lay out bond patterns or subtract a separate 10 mm mortar joint — joint loss and cutting waste belong in the waste percentage.
Worked example
A 20 m² garage wall with 400 × 200 mm blocks and 5% waste: 20 / 0.08 = 250 blocks net, then 250 × 1.05 = 262.5, so 263 blocks to order. For a wall with several corners using 10% waste: 250 × 1.10 = 275 blocks. Subtract garage-door and window areas from the wall area first.
Units
Wall area is in square metres and block dimensions in millimetres (400 mm = 0.4 m). In imperial terms, a 16 × 8 in block face is about 0.086 m² including a 3/8 in joint, giving roughly 11.6 blocks per square metre — close to the tool’s 12.5, with the difference taken up by the waste allowance. Confirm whether your supplier counts nominal or actual unit sizes.
Practical tips
- Use 5% waste for long straight runs and 7–10% where there are corners, jambs, and short returns needing half blocks.
- Plan corners and openings on a 200 mm or 400 mm module to minimise cutting; order lintels, sills, and half units separately.
- Allow mortar separately — a 20 m² single-skin block wall typically needs around 0.2–0.3 m³ of mortar with 10 mm joints.
- Consider this a purchasing estimate only; load-bearing capacity, reinforcement, and moisture detailing require assessment by a licensed professional.
Edge cases and limitations
- Corners consume halves. Bonded corners and junctions are built from half blocks; a wall with many corners or short returns can need 3–5% more units than the flat area suggests, even though the extras arrive as cut halves. The waste percentage is where they live in this calculation.
- Piers are not wall. An attached pier (say 440 × 440 mm plan) is stacked solid: count it separately as pier height ÷ course height × blocks per course, and add it to the wall count instead of stretching the wall area.
- Hollow blocks count the same, weigh less. The count depends only on face size, so hollow, solid and cellular blocks of 400 × 200 mm all need 12.5 per m². What changes is handling and mortar — hollow cores filled with concrete for reinforcement are a separate volume entirely.
- Lintel and sill bands. Courses interrupted by lintels still use blocks above them; deduct the opening, not the band. A common under-order is deducting the full window height including the course that carries the lintel.
Privacy and browser processing
The calculation runs entirely in your browser — your wall dimensions are never uploaded.
Frequently asked questions
How many 400 × 200 mm blocks cover one square metre?
12.5 before waste, since 1 / (0.4 × 0.2) = 12.5. With 5% waste that is 12.5 × 1.05 = 13.125, so allow 14 blocks per square metre when ordering.
How is this different from the brick result?
Blocks are far larger: one square metre needs only 12.5 blocks versus 83.33 bricks at 200 × 60 mm. A 20 m² wall therefore needs about 263 blocks but would need roughly 1,750 bricks at the same 5% allowance.
Should mortar joints change the block size I enter?
No. Enter the actual unit face (for example 400 × 200) and let the waste percentage absorb the 10 mm joints and cuts. Increasing waste from 5% to 10% on the 20 m² example adds 250 × 0.05 = 12.5 extra blocks.
Do openings and corners need extra?
Yes. Deduct openings from the area, then use the higher end of waste (7–10%) for jambs and corners. A 20 m² wall with two corners at 10% needs 275 blocks versus 263 at 5%.