UK Block Sizes — Concrete Block Dimensions and Weights

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.

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.

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.

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.

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.