Wall Square Footage Calculator (net wall area minus doors & windows)

Understanding the wall area calculator — concept and principle

This calculator finds the net area of a room's walls: it adds up every wall, then subtracts doors and windows. The result is shown in square metres and square feet. Use it to buy paint, wallpaper, tiles, panelling or plasterboard.

How it is calculated

Wall area = width × height for each wall

Gross area = sum of all walls

Openings = sum of width × height for each door and window

Net wall area = gross area − openings

• Each line is one rectangle written as "width x height".
• In feet mode the lengths are converted first (1 ft = 0.3048 m); 1 ft² = 0.09290304 m².
• Openings as a share of the walls = openings ÷ gross × 100.

The principle

A flat wall is a rectangle, so its area is width times height. Area is additive, so the walls can be summed and the holes in them subtracted. The net figure is what actually gets covered. For a gable wall, split it into a rectangle and a triangle; a triangle has half the area of the rectangle with the same base and height, so you can enter it as a rectangle of half its height.

Worked example

Default inputs: metres; walls 4.2 × 2.4, 3.6 × 2.4, 4.2 × 2.4 and 3.6 × 2.4; door 0.9 × 2.1 and window 1.5 × 1.2.

1. Walls = 10.08 + 8.64 + 10.08 + 8.64 = 37.44 m² gross, from 4 walls.
2. Openings = 0.9 × 2.1 + 1.5 × 1.2 = 1.89 + 1.80 = 3.69 m², from 2 openings.
3. Net = 37.44 − 3.69 = 33.75 m².
4. In square feet: 33.75 ÷ 0.09290304 = 363.28 ft².
5. Openings share = 3.69 ÷ 37.44 = 9.9 %.

How to read the result

For paint, divide the net area by the spread rate on the tin and multiply by the number of coats. For wallpaper and tiles, add the waste the product recommends, more for large pattern repeats. Small openings are often ignored by decorators because cutting in around them uses extra material anyway.

Common mistakes

• Measuring height to the top of the skirting or cornice when you only paint between them.
• Subtracting openings for wallpaper with a large pattern repeat, which wastes more around openings.
• Counting a wall twice in an open-plan space.
• Forgetting the ceiling if you are painting it too; it is a separate area.

Frequently asked questions

How do I calculate the wall area of a room?

Add the perimeter × wall height, then subtract doors and windows. For a 4.2 × 3.6 m room 2.4 m high, that is 37.44 m² before openings.

What size is a standard door?

Sizes vary by country; measure your own. The default here uses 0.9 × 2.1 m as an example.

How do I include a sloped ceiling wall?

Enter the rectangular part, then the triangle as a rectangle with half its height.

Sources: NIST Special Publication 811 (2008), Appendix B (1 ft = 0.3048 m, 1 ft² = 0.09290304 m²)