Opening Deduction Calculator — Net Wall Area

When calculating materials like bricks, blocks, render or cladding, you should deduct the area of doors and windows to avoid over-ordering. This calculator gives you the net wall area after all deductions. Use our Brick Calculator or Block Calculator for full material quantities based on the net area.

A standard UK internal door opening is approximately 0.9m wide by 2.0m high (1.8 m²). External doors are typically wider at 0.9-1.0m. Always measure your actual openings for accuracy.

For most materials (bricks, blocks, render, cladding), yes. Under the UK formal method of measurement — RICS NRM2 (which superseded SMM7 in 2013) — brick and block walling is measured net by area on one face, and small voids below the stated threshold are not deducted. On site many trades simplify this to a practical rule of thumb: deduct full openings larger than about 0.5 m² and ignore anything smaller, since small offcuts create waste anyway (this is especially true for insulation). Use whichever approach your quantity surveyor or specification requires — this calculator lets you choose exactly what to deduct.