How Many Bricks Per Square Metre

How the calculation works

How many hollow blocks do I need for a wall?

In modern UK construction, internal partition walls are typically built using hollow clay blocks or concrete blocks, selected for their lightweight properties and thermal performance compared to solid bricks. Calculating the exact quantity is straightforward in principle, but a common pitfall catches many builders: forgetting to account for the mortar joint thickness. Missing this step can mean ordering 15 to 20 blocks too few per square metre, a mistake discovered only when materials run short partway through construction.

The formula and what goes into it

The method is direct: divide the wall area by the effective area occupied by each block (including its mortar joint) and round up to the nearest whole number. Take a typical internal partition of 5 m length and 2.7 m height, common in UK terraced housing and semi-detached properties. The area is 13.5 m². With a standard hollow block of 30 × 20 cm and a 1 cm mortar joint, each block occupies 31 × 21 cm of wall surface, equal to 0.0651 m². Without any waste margin, the calculation gives 207 blocks. Add a 5% waste allowance for cutting and breakage: the result is 218 blocks.

The mortar joint thickness is often overlooked in informal calculations. A single centimetre of mortar might seem negligible, but across 200 blocks, it accounts for a real difference in area. BS EN 771-1 (the British Standard for masonry units) does not mandate mortar joint thickness; instead, it is specified by the design and site practice. In UK construction, 10 mm (1 cm) is the reference thickness for both horizontal bed joints and vertical perpend joints.

Hollow blocks versus solid bricks

Hollow clay blocks (the cellular type common in UK cavity walls and internal partitions) have cavities that reduce weight and improve insulation while maintaining structural strength. They are supplied in standard face dimensions but variable thicknesses:

• Block 30 × 20 × 7 cm: lightweight internal partitions with limited sound insulation; less common in new work because tighter acoustic requirements favour thicker blocks.
• Block 30 × 20 × 11 cm: the most common choice for internal partitions in new housing. Offers reasonable sound reduction and allows routing of electrical conduits through the cavities.
• Block 30 × 20 × 15 cm: used in cavity wall systems as the inner leaf, or as a structural internal wall where greater rigidity is needed.
• Block 30 × 20 × 22 cm: traditionally used as the inner leaf of external cavity walls, though modern Approved Document L (the energy conservation guidance within the Building Regulations) often requires thicker insulation, leading to thin blocks with external foam or mineral fibre.

Because the visible face is always 30 × 20 cm, the number of blocks per square metre of wall surface is identical for all thicknesses. What changes is the wall thickness and, consequently, the material cost per unit. A solid brick traditionally measures approximately 215 × 102 × 65 mm (face) and follows the same per-square-metre logic.

Waste margins in practice

Walls are rarely exact multiples of brick dimensions. There are always cuts needed at corners, around door and window frames, around electrical outlets, and where walls meet existing concrete columns or steel beams. A 5% waste margin handles most simple rectangular walls with no major openings. Increase to 10% for walls with multiple window or door openings, or when detailed cutting and fitting is expected. In older buildings undergoing refurbishment, where openings are irregular or where sections of the old wall are being retained, a 15% margin is prudent.

Waste also accounts for breakage during delivery and handling, and for occasional bricks that fail to meet specification when laid. Ordering slightly more than calculated is standard practice; leftover blocks are useful for repairs or later modifications, and their cost is far less than a rushed reorder.

Sourcing blocks in the UK

Hollow clay blocks are widely available through builders' merchants and from large DIY retailers, with next-day or same-week delivery in most populated areas. For larger projects (above 20 to 30 m² of wall area), direct orders from manufacturers or specialist suppliers often offer better unit pricing and allow bulk collection. National building suppliers hold stock in multiple regional warehouses, so finding the exact thickness you need is rarely an issue.

Concrete blocks (often preferred where robustness and cost are priorities) are equally available and sometimes cheaper than clay alternatives, though their acoustic properties and thermal mass differ. For aesthetic reasons in exposed work, facing blocks (bricks with a uniform colour and finish) may be specified, but for concealed internal partitions, utility blocks at lower cost are standard.

Common mistakes to avoid

Ignoring the mortar joint: Calculating the brick area as a simple 30 × 20 = 600 cm² and omitting the joint thickness of 10 mm leads to underestimating by nearly 9%. The effective footprint is 31 × 21 = 651 cm², a difference that multiplies across the entire wall.

Forgetting waste: Assuming an exact number of bricks with no margin for cuts or damage leaves no buffer. Most sites experience 5 to 10% wastage through normal work, broken units, and inevitable site modifications.

Buying exact quantities: Even when calculations are accurate, receiving a batch with slight dimensional variation or colour shift may require selective placement. Having a small surplus of 5 to 10% is practical insurance.

Using the same calculation for both leaves of a cavity wall: If your external wall is a cavity system (thick outer leaf plus inner leaf plus air gap and insulation), calculate each leaf separately. Both have the same per-square-metre block count, but different material costs and sourcing timelines.

When to seek professional advice

This calculator is suitable for non-load-bearing partitions, boundary walls, and simple infill masonry. However, whenever a wall is structural (load-bearing), in an earthquake zone (which includes much of the UK under Eurocode 8), or subject to planning and building control approval, the design must be carried out by a structural engineer. Masonry design under UK Building Regulations requires compliance with BS EN 1996 (Eurocode 6: Design of masonry structures), which specifies minimum wall thickness, block strength, and reinforcement based on loads, height, and environmental exposure.

The Building Research Establishment (BRE) and the Construction Industry Research and Information Association (CIRIA) publish guidance on masonry construction, material durability, and defect prevention. The Building Regulations Approved Document A (Structure) specifies load-bearing masonry requirements; Approved Document L addresses thermal performance and involves block selection based on climate region.

For any wall where you are uncertain of the structural implications, consult a qualified surveyor or structural engineer. A small design investment protects against costly mistakes.