Net Wall Area Calculator for Plaster, Tiles and Finishes
The net wall area calculator works out the wall surface a plaster, tile or cladding estimate starts from. Enter the room’s inside size and wall height, or list walls one by one, then add each door and window. You can carry the finish above the ceiling line, take a skirting strip off and leave small openings in. Every wall and opening is listed in square meters and square feet, and the walls are drawn laid out flat. For a quick paint or wallpaper figure from the room size, the wall area calculator for paint and wallpaper is simpler.
Net wall area calculator
Bare numbers are meters; 190 mm works too.
Assumptions
Change any value to match your product or local rules.
0 subtracts every opening
Basis: Example value (0: subtract everything). Some painters and estimators skip small openings; set your own threshold.
0 stops the finish at the wall height
Basis: Example value (0). Where a false ceiling hides the top of the wall, some drawings carry plaster or tiles up past it by a set height; use the figure on yours.
0 leaves the skirting strip in
Basis: Example value (0). Enter your skirting or tile skirting height to take that strip off. It runs the wall length less the door widths.
- Wall length (perimeter)
- 15.60 m
- 51′ 2″
- Gross wall area
- 42.12 m²
- 453.4 sq ft
- Openings subtracted
- 3.69 m²
- 39.7 sq ft
- Net wall area
- 38.43 m²
- 413.7 sq ft
- Skirting length (less door widths)
- 14.70 m
- 48′ 3″
- Ceiling (not in the wall total)
- Not included
Net wall area: 38.43 m² (413.7 sq ft), from 42.12 m² (453.4 sq ft) of wall less 3.69 m² (39.7 sq ft) of openings.
Each wall
- Wall 1: 4.20 m × 2.70 m = 11.34 m²
- Wall 2: 3.60 m × 2.70 m = 9.72 m²
- Wall 3: 4.20 m × 2.70 m = 11.34 m²
- Wall 4: 3.60 m × 2.70 m = 9.72 m²
Floor Planner
- “Design this room” opens a room whose inside is 4.20 m (13′ 9″) × 3.60 m (11′ 10″).
How this was calculated
- Perimeter = 2 × (width + length); gross wall area = perimeter × wall height (listed walls: Σ length × height × count)
- Height = wall height + any finish above the ceiling
- Openings = Σ width × height × count of each opening at least the threshold in size
- Skirting length = wall length − Σ door width × count; skirting strip = skirting length × skirting height
- Net wall area = gross wall area − openings − skirting strip
- Perimeter = 2 × (4.20 m + 3.60 m) = 15.60 m
- Gross wall area = 15.60 m × 2.70 m = 42.12 m²
- Door 1 = 0.90 m × 2.10 m × 1 = 1.89 m²
- Window 2 = 1.50 m × 1.20 m × 1 = 1.80 m²
- Net wall area = 42.12 m² − 3.69 m² = 38.43 m²
How the wall area is worked out
Perimeter
For a rectangular room, the walls run 2 × (width + length) around it.
Gross area
The perimeter times the wall height, plus any height the finish runs above the ceiling. In “Only the walls I list” mode, each wall is its length times its height times how many of it there are.
Openings
Each door, window or other opening is its width times its height times how many. Openings smaller than the threshold you set are left in the wall.
Skirting
The skirting runs the wall length less the door widths. With a skirting height entered, that strip is taken off too.
Net area
Gross area less the openings and the skirting strip. If they add up to more than the walls, the calculator stops and asks you to check them.
Worked example in meters and in feet
A room 4.2 m by 3.6 m with 2.7 m walls has a perimeter of 15.6 m and 15.6 × 2.7 = 42.12 m² of wall. A door 0.9 × 2.1 m (1.89 m²) and a window 1.5 × 1.2 m (1.80 m²) take out 3.69 m², leaving 38.43 m², about 413.7 sq ft.
In feet, a room 14 by 12 ft with 9 ft walls has a 52 ft perimeter and 468 sq ft of wall. A 3 × 7 ft door (21 sq ft) and a 5 × 4 ft window (20 sq ft) leave 427 sq ft, about 39.67 m².
Finish above the ceiling and skirting
Where a false ceiling hides the top of the wall, drawings often carry plaster or tiles a set height past it so the edge is never seen. Enter that height in “Finish runs above the ceiling by” and it is added to every wall. With the room above, a 0.2 m extra height makes the gross area 15.6 × 2.9 = 45.24 m².
Skirting covers a strip at the foot of the wall, so the finish behind it can often be left out. Enter the skirting height and the calculator takes off the skirting length (the wall length less the door widths) times that height. A 0.1 m skirting along 15.6 − 0.9 = 14.7 m is 1.47 m², so the net area becomes 45.24 − 3.69 − 1.47 = 40.08 m². Check your specification: some plaster runs right down to the floor behind the skirting.
Gross and net wall area
Gross wall area is every wall from floor to ceiling, as if it had no openings. Net wall area is what is left once doors, windows and other openings are taken out: the surface you would paint, paper or panel.
Which one you need depends on the job. A supplier quoting by the wall may want gross; a paint or wallpaper estimate starts from net. The calculator shows both, so you can give whichever is asked for.
Sloped ceilings, stairs and walls that aren’t rectangles
Switch to “Only the walls I list” and enter each wall on its own. A wall under a sloping ceiling can be entered at its average height (the low and high ends added and halved), which is exact when the top edge is a straight line. A wall beside a stair can be split into a rectangle and a triangle; enter the triangle as a wall of the same length at half its height.
Bay windows, chimney breasts and alcoves add short walls: measure each face and list it.
Doors and windows: what to deduct
Measure openings at the frame: the outside edges of the casing or the plaster opening, whichever bounds the wall surface you are working out. Glass doors count as doors; a serving hatch or an open archway goes in as “Other opening”.
Some estimators leave small openings in, because cutting around them takes as long as covering them. Set “Leave openings smaller than” to your own figure and those openings stay in the wall; the step list shows which ones were left in.
Using the result for plaster, tiles or paint
For plaster, divide the net area by the coverage per bag on the product’s data sheet. For wall tiles, work from the net area and add for cuts and breakage as the tile maker suggests. For paint, multiply by the number of coats and divide by the coverage per litre or gallon. The ceiling area is shown on its own line when you ask for it, so it is never mixed into the wall total.
Frequently asked questions
How do I calculate wall area?
Add up the lengths of the walls (for a rectangular room, 2 × (width + length)) and multiply by the wall height. That is the gross area. Take off each door and window, width times height, to get the net area.
Do I subtract doors and windows?
For anything that covers the wall surface, such as paint, wallpaper or panelling, yes: subtract them to get the net area. Some estimators leave small openings in; set a threshold in the calculator if you work that way.
How do I measure a wall with a sloped ceiling?
Measure the height at both ends and use the average, which is exact when the top edge is straight. Enter the wall in “Only the walls I list” mode with that average height.
What is the wall area of a 12 × 12 room with 8 ft ceilings?
The perimeter is 48 ft, so the gross wall area is 48 × 8 = 384 sq ft, about 35.67 m², before any doors or windows are taken off.
How do I add a finish that runs above the ceiling?
Enter the extra height in “Finish runs above the ceiling by”. It is added to every wall’s height before the openings come off. A 4.2 × 3.6 m room with 2.7 m walls and 0.2 m above the ceiling has 15.6 × 2.9 = 45.24 m² of gross wall.
Does wall area include the ceiling?
No. The ceiling is a separate surface. The calculator can show the ceiling area (width × length) on its own line so you can add it if your job covers it.
Ready to plan your space?
It is free, runs in your browser and needs no account. Projects are saved privately on this device.