Concrete Block Calculator – Blocks for a Wall
Estimate how many concrete blocks you need for a wall. Enter the block dimensions in mm and the wall height and width in metres — the calculator divides the wall area by the block area and rounds up to the next whole block.
Calculate Number of Blocks Needed for Construction
Concrete Block Quantity Formula
Where:
- \(N\) — number of concrete blocks needed
- \(H_w\) — wall height (m)
- \(W_w\) — wall width, i.e. the length of the wall (m)
- \(h_b\) — block height (mm)
- \(w_b\) — block width (mm)
The formula divides the total face area of the wall by the face area of a single block. Because block dimensions are entered in millimetres while the wall is measured in metres, each block side is divided by 1000 first. The result is always rounded up to the next whole block, since you cannot buy a fraction of a block.
Concrete Block – Calculation Example
Example: a wall is 2.4 m high and 6 m long, and each block measures 200 mm high by 400 mm wide.
\[ A_{block} = \frac{200}{1000} \cdot \frac{400}{1000} = 0.08 \ \text{m}^2 \]
\[ A_{wall} = 2.4 \cdot 6 = 14.4 \ \text{m}^2 \]
\[ N = \frac{14.4}{0.08} = 180 \]
Result: 180 blocks are needed to cover the wall.
How to use the calculator
- Enter the Block height in millimetres — the vertical face dimension of one block (e.g. 200 mm).
- Enter the Block width in millimetres — the horizontal face dimension of one block (e.g. 400 mm).
- Enter the Wall height in metres, measured from the base to the top of the planned wall.
- Enter the Wall width in metres — this is the length of the wall along the ground.
- Read the result: the calculator shows the number of blocks, rounded up to the next whole block.
Use the face dimensions of the block as it will be laid in the wall, not its thickness. For a standard hollow block laid on its broad face, the height and width are the dimensions you see when looking straight at the finished wall.
What the concrete block calculator does
Concrete blocks (also called concrete masonry units, CMUs, or cinder blocks) are one of the most common materials for walls, foundations, garden walls, and outbuildings. Before ordering materials you need a realistic block count, because both the budget and the delivery size depend on it. This calculator gives you that estimate in seconds: it computes the face area of the wall, divides it by the face area of one block, and rounds up so you always have enough whole blocks.
The calculation is purely area-based, which is exactly how block quantities are estimated in practice. A wall of 14.4 m² built from 200 × 400 mm blocks always needs 180 blocks of face coverage, regardless of the wall's thickness — thickness only affects mortar, fill, and reinforcement quantities, not the number of blocks per square metre of face.
The result is always rounded up to the next whole number. If the division gives 180.3, you still need 181 blocks. Note that the calculator does not add a wastage allowance; for cutting, breakage, and damaged units, order an extra 5–10 % on top of the result (see the tips below).
Once you know the block count, you can move on to the supporting materials. To plan the wall itself, our Concrete Wall calculator helps with volume estimates, and the Cement and Sand for Plastering calculator covers the finishing layer. If you are laying paving instead of a wall, the Number of Paving Stones calculator works the same way, and the Floor Tile Estimator is the tile equivalent.
Common block sizes and blocks per m²
The blocks-per-square-metre figure below lets you sanity-check the calculator result: multiply the wall area by the value for your block size.
| Block face size (mm) | Block face area (m²) | Blocks per m² of wall |
|---|---|---|
| 200 × 400 | 0.08 | 12.5 |
| 200 × 200 | 0.04 | 25 |
| 150 × 400 | 0.06 | 16.7 |
| 190 × 390 (typical hollow CMU) | 0.0741 | 13.5 |
| 140 × 390 | 0.0546 | 18.3 |
Imperial "8 × 8 × 16" blocks have a face of roughly 203 × 406 mm (about 12.1 blocks per m²); entering 203 and 406 in the calculator reproduces that figure.
Assumptions, limitations and practical tips
- No mortar joints. The calculator uses the full nominal face size of the block. In reality, a 10 mm mortar joint makes each effective row and unit slightly larger, which usually cancels out the rounding — but for long walls, joints can change the count by a few blocks. Order the extra 5–10 % wastage and you are covered either way.
- No openings subtracted. Doors, windows, and vents are not deducted. For each opening, calculate its area (height × width), divide by the block face area, and subtract the result from the total.
- Rectangular walls only. Gable ends, stepped walls, and L- or U-shaped layouts should be split into rectangles, calculated separately, and added together.
- Add wastage when ordering. Multiply the result by 1.05–1.10. Blocks get damaged during delivery, and odd dimensions require cutting, which produces unusable offcuts.
- Blocks are sold in pallets. Check how many blocks a pallet contains at your supplier — a standard pallet often holds around 40–50 blocks — and round your order up to whole pallets.
- Estimate the mortar too. A rough planning figure is about 10 litres of mortar per m² of blockwork for 200 mm-thick blocks; for a full materials mix, see the Concrete Mix Ratio calculator.
Frequently Asked Questions
How do I calculate the number of concrete blocks by hand?
Divide the wall's face area by one block's face area. Multiply wall height by wall length in metres, then divide by \(\frac{h_b}{1000} \times \frac{w_b}{1000}\), where the block sides are in millimetres. Round up to the next whole block. For example, a 3 m × 5 m wall (15 m²) built with 190 × 390 mm blocks (0.0741 m² each) needs \(15 / 0.0741 \approx 202.4\), so 203 blocks.
How many concrete blocks are in a square metre?
It depends only on the block's face size: blocks per m² equals \(1 / A_{block}\). Common values are 12.5 blocks/m² for 200 × 400 mm faces, 25 for 200 × 200 mm, and about 13.5 for the typical 190 × 390 mm hollow CMU. Imperial "8 × 8 × 16" blocks give roughly 12.1 per m². Multiply the figure for your block by the total wall area in m² for a quick sanity check.
Should I subtract doors and windows from the block count?
Yes, for an accurate order. The calculator covers the full rectangular wall, so compute each opening's area (height × width), divide it by the block face area, and subtract the resulting block count from the total. For instance, a 0.9 m × 2.1 m door (1.89 m²) removes about 24 blocks of 200 × 400 mm size. Many builders still order openings' worth of blocks as spare instead of deducting.
How much extra should I order for wastage and cutting?
Add 5–10 % on top of the calculated count — multiply the result by 1.05 to 1.10. Breakage in delivery, misfits at corners and odd dimensions, and damaged units all create losses. For walls with many openings or non-rectangular shapes, lean toward the 10 % end. Since blocks are usually sold by the pallet (often 40–50 units), rounding up to whole pallets often covers the wastage automatically.
Why doesn't the block thickness change the number of blocks?
Because blocks are counted per square metre of wall face, not per volume. A 100 mm and a 200 mm block with the same face size cover the same wall area, so the count is identical. Thickness matters for other materials: thicker walls need more mortar, more concrete or insulation fill, and heavier reinforcement. It also affects structural strength, so for load-bearing walls follow local building codes or consult an engineer.
Does a 10 mm mortar joint change the block count?
Only slightly. A 10 mm joint makes each course effectively taller and each unit effectively wider, so a jointed wall actually needs marginally fewer blocks per m² than the raw calculation suggests — usually a difference of a few blocks on long walls. Because the calculator rounds up and you order 5–10 % extra wastage, the mortar joint effect is normally absorbed and safe to ignore for planning.
Can I use inches and feet instead of mm and metres?
The calculator expects block dimensions in millimetres and wall dimensions in metres, so convert first. Multiply inches by 25.4 to get millimetres, and multiply feet by 0.3048 to get metres. An 8-inch-tall, 16-inch-long block becomes 203 × 406 mm, and a 20-foot wall becomes 6.1 m. Alternatively, work entirely in one unit system by hand: wall area in ft² divided by block face area in ft² gives the same count.