Why do I need to deduct openings?

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.

What is a standard door opening size?

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.

Should I always deduct openings from my material calculation?

For most materials (bricks, blocks, render, cladding), yes. However, for some materials like insulation, you may not deduct small openings as the offcuts create waste anyway. As a rule of thumb, deduct openings larger than 0.5 m².

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Opening Deduction Calculator — Net Wall Area

Calculate the area to deduct for doors, windows and other openings from your wall area. Get the net area for brickwork, cladding, rendering and more.

Wall Height

Standard storey height is 2.4m

Wall Length

Average length per wall

Number of Walls

Number of walls with the same dimensions

Number of Standard Doors

Each deducts 1.8 m²

Number of Windows

Each deducts 1.2 m²

Custom Opening Area (optional)

Additional area to deduct for non-standard openings

How We Calculate This

This calculator computes the gross wall area, subtracts all door, window and custom openings, and gives you the net area. It also shows the percentage deducted.

The formula

Net area = Gross area - (Door deductions + Window deductions + Custom deductions)

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.