How do I calculate net wall area for painting or plastering?

Multiply the total wall perimeter by height to get gross area, then subtract the area of doors and windows to get the net paintable/plasterable area — this calculator does this automatically.

What is a standard door and window size to assume?

A standard door is about 21 sqft (3ft x 7ft) and a standard window is about 12 sqft (4ft x 3ft), but you should use actual measurements where possible for accuracy.

Should I calculate each wall separately or the whole room together?

For best accuracy, calculate each wall separately if door/window placement varies significantly — otherwise, using the total room perimeter and height gives a quick, reliable estimate.

Wall Area Calculator — Net Area for Paint, Plaster & Tiling

Understanding Net Wall Area Calculations

Before you can estimate paint, plaster or tile quantities for a room, you need to know the net wall area — the actual surface area that will be covered after accounting for door and window openings. Many DIY estimates go wrong simply because they use the gross room perimeter × height without deducting openings, leading to over-ordering of materials. This calculator computes both the gross and net wall area for a room, making it the starting point for accurate paint, plaster and tile calculations.

How the Wall Area Calculator Works

The calculator first computes the room's perimeter from the length and breadth you enter, then multiplies by the wall height to get the gross wall area for all four walls. If you select fewer than 4 walls (useful for accent walls or partial painting), it adjusts the perimeter accordingly. It then deducts the area of doors and windows — using standard sizes (a door is approximately 21 sqft / ~2 sqm, and a window is approximately 12 sqft / ~1.1 sqm) multiplied by the number you enter — to arrive at the net wall area. If you choose to include the ceiling, its area (length × breadth) is added separately, since ceilings are often painted with a different product.

Standard Door and Window Sizes Used

Standard door — approximately 3 ft × 7 ft (0.9m × 2.1m) = ~21 sqft. Larger main doors or double doors should be measured individually for accuracy.
Standard window — approximately 4 ft × 3 ft (1.2m × 0.9m) = ~12 sqft. Bay windows, French windows or balcony doors are typically larger and should be measured separately.
For rooms with non-standard openings (large picture windows, sliding doors, ventilators), it's best to measure each opening individually and adjust the calculator's "gross area" result manually for full accuracy.

Factors That Affect Net Wall Area

Number of walls to cover — accent walls (1-2 walls) are common for feature paint colours or wallpaper, while plastering and priming usually cover all 4 walls.
Wall height — taller ceilings (above the standard 10ft / 3m) increase wall area proportionally and are often underestimated in quick visual guesses.
Door and window count and size — larger or additional openings (like balcony doors) reduce the net paintable/plasterable area significantly.
Purpose — paint calculations typically need net wall area (openings deducted), while plaster and tiling on walls also need net area, but skirting/dado areas need separate calculation.

How to Use This Calculator

Enter the room's length and breadth in metres.
Enter the wall height (default 3.0m).
Select the number of walls to cover — all 4, 3, 2 (opposite) or 1.
Enter the number of doors and windows.
Choose whether to include the ceiling area.
Select your purpose — Paint, Plaster or Tiling — to get a direct link to the relevant follow-up calculator.
Click Calculate to see the net wall area in both square feet and square metres.

Using This With Other Calculators

Once you have your net wall area, you can directly use it as an input for:

Paint Calculator — to estimate litres of paint and primer needed for the calculated area.
Plaster Calculator — to estimate cement and sand for plastering the net wall area.
Cement Calculator — for a more detailed material breakdown if plastering with a specific mix ratio.

This two-step approach (calculate area first, then materials) gives more accurate results than trying to estimate materials directly from room dimensions, especially for rooms with multiple openings or irregular shapes.

Common Mistakes to Avoid

Forgetting to deduct openings entirely, leading to material over-ordering of 10-20% in rooms with many doors/windows.
Using a single average room height for the whole house when some rooms (like double-height living rooms) have different heights.
Not separately accounting for skirting and dado areas when calculating tile or paint requirements near the floor.

Next Steps

Use your calculated wall area with our Paint Calculator to find litres of paint needed, or the Plaster Calculator to get cement and sand quantities for this area.

📐 Wall Area Calculator