What is the standard concrete block size in the UK?

The standard UK concrete block is 440 x 215 mm (face dimensions) with a coordinating size of 450 x 225 mm (including 10 mm mortar joints). The most common thickness is 100 mm, but blocks are also available in 75, 140, and 215 mm thicknesses. The standard block equates to 6 bricks in area (2 bricks wide x 3 bricks high with mortar joints), making it faster to lay than brickwork.

How many blocks per square metre?

Using standard 440 x 215 mm blocks with 10 mm mortar joints, you need approximately 10 blocks per m². This applies regardless of block thickness (100 mm, 140 mm, or 215 mm) as the face dimensions are the same. Add 5-10% for waste and cuts. For half-blocks or closers at corners and openings, the waste percentage may be higher.

What is the difference between dense and lightweight blocks?

Dense aggregate blocks (density ~2000 kg/m³, weight ~20 kg for 100 mm) are made with natural aggregate and are strong but heavy and poor insulators. Lightweight aggregate blocks (density ~1400 kg/m³, weight ~14 kg) use expanded clay or similar and offer better insulation. Aerated (Aircrete) blocks like Thermalite or Celcon (density ~460-750 kg/m³, weight ~5-8 kg) are the lightest and best insulating but have lower compressive strength.

What block type should I use for below DPC?

Below DPC (damp proof course), you must use dense concrete blocks with a minimum compressive strength of 7 N/mm² and low water absorption. Aircrete/aerated blocks are NOT suitable below DPC as they absorb water. Trench blocks (275 or 300 mm wide, dense concrete) are specifically designed for foundation walls below DPC and can replace brickwork for speed.

What thermal conductivity do blocks have?

Thermal conductivity varies dramatically by type: dense blocks have a lambda value of 1.13-1.33 W/mK (poor insulation), lightweight blocks 0.30-0.60 W/mK (moderate), and aerated blocks 0.09-0.19 W/mK (good). Thermalite Shield blocks achieve 0.09 W/mK, which can significantly contribute to meeting Part L U-value targets when used in the inner leaf of a cavity wall.

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UK Block Sizes — Concrete Block Dimensions and Weights

Standard UK concrete block sizes, weights, blocks per square metre, and common block types including dense, lightweight, and aerated blocks.

Standard Block Sizes and Weights

Type	Length (mm)	Height (mm)	Thickness (mm)	Weight (kg)	Blocks/m²
Dense aggregate	440	215	100	~20	10
Dense aggregate	440	215	140	~28	10
Dense aggregate	440	215	215	~42	10
Lightweight aggregate	440	215	100	~14	10
Lightweight aggregate	440	215	140	~19	10
Aerated (Aircrete)	440	215	100	~6	10
Aerated (Aircrete)	440	215	140	~8	10
Aerated (Aircrete)	440	215	215	~13	10
Trench block (dense)	440	215	275	~35	10

Block Compressive Strength

Type	Typical Strength (N/mm²)	Lambda (W/mK)	Suitable Below DPC?
Dense (7N)	7.3	1.13-1.33	Yes
Dense (10N)	10.4	1.13-1.33	Yes
Lightweight (7N)	7.3	0.30-0.60	Check spec
Aircrete (2.9N)	2.9	0.11-0.15	No
Aircrete (3.6N)	3.6	0.13-0.17	No
Aircrete (7N)	7.3	0.17-0.19	No

Related Calculators

Use the Block Calculator for quantities, or the Mortar Calculator for mortar volumes. The U-Value Calculator shows how block type affects wall U-values.

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.