Wall Area Calculator — Square Metres Minus Openings
Calculate gross and net wall area in square metres (m²) from room perimeter and wall height, then deduct doors, windows and other openings.
Enter your measurements
How to calculate wall area in square metres
For a room with walls of the same height, multiply the total wall perimeter by the wall height. That gives the gross wall area. Then subtract the combined area of doors, windows or other openings to get the net wall area.
Net wall area (m²) = gross wall area − openings (m²)
Worked example: 18 m perimeter × 2.4 m high
How to get the room perimeter
Measure the length of each wall and add the runs together. For a simple rectangular room, perimeter is 2 × (length + width). If the room has alcoves, chimney breasts or offsets, measuring each wall run individually is usually more reliable than forcing the space into a simple rectangle.
For a fuller measuring method, see the Measuring Walls & Openings Guide. If you need floor area instead, use the Floor Area Calculator.
How to measure doors and windows
Measure each opening as width × height, calculate its area in square metres, then add the openings together before entering the total. Keep the gross wall area and deductions separate so the estimate can be checked later.
Paint
Small openings may not always justify a precise deduction because cutting-in and extra coats can offset the saved area. Use the paint product’s coverage and your preferred allowance.
Plaster or render
Net wall area is a useful starting point, but reveals, returns and uneven backgrounds can add work that a simple flat-area calculation does not capture.
Boards, tiles and wallpaper
Area alone is not the final order quantity. Board size, tile module, roll coverage, pattern repeat and waste should be handled in the material-specific calculator.
When wall heights are different
If parts of the room have different heights — for example a stairwell, vaulted ceiling or stepped wall — calculate those wall sections separately and add the areas together. An average height can hide significant differences on irregular spaces.
Common wall-area measuring mistakes
- Using floor area as though it were wall area.
- Forgetting to include alcoves, returns or extra wall runs in the perimeter.
- Subtracting doors and windows twice.
- Mixing millimetres, centimetres and metres before multiplying.
- Using net wall area as the final material quantity without allowing for the actual product format or coverage.
Using wall area to estimate materials
Once you have the wall area, move to the calculator for the material you are actually buying. Useful next steps include the Paint Calculator, Plaster Calculator, Render Calculator, Tile Calculator, Wallpaper Calculator and Plasterboard Calculator.
Planning estimate only. Wall area is the geometry. Product coverage, coats, substrate condition, board/tile layout and waste still need to be checked separately.
Frequently asked questions
How do I calculate wall area from room length and width?
For a rectangular room, calculate perimeter as 2 × (length + width), then multiply by wall height. Subtract doors and windows afterwards if you need net wall area.
How do I calculate square metres of a wall?
For one rectangular wall, multiply width by height in metres. For a whole room, add the wall areas or use total perimeter × height.
Should I subtract doors and windows?
Subtract them when you need a net surface area, but the most useful treatment depends on the material. Small openings may have little effect on paint ordering, while precise finishes may justify accurate deductions.
What if my walls are different heights?
Calculate each different-height section separately and add the areas. Using one average height can distort the result on irregular rooms.
Can I use this for paint or plaster?
Yes as a starting area. Then use the relevant paint, plaster or render calculator because product coverage, coats, thickness and waste are separate assumptions.
Does the calculator include the ceiling?
No. This tool calculates wall area only. Measure the ceiling separately, usually as length × width for a simple rectangular ceiling.
Related measuring tools
For floor area, use the Floor Area Calculator. For unit conversions and other geometry tools, return to the Measuring & Conversions hub.