Areas
Room wall and ceiling area less openings
- where
- room perimeter (m)
- room length (m)
- room width (m)
- gross wall area (m²)
- wall height, floor to ceiling (m)
- net wall area (m²)
- total area of doors and windows (m²)
- ceiling area (m²)
- include ceiling: 1 = yes, 0 = no
- total area to plaster (m²)
Explain this equation
This works out the plastering area of a rectangular room: the walls less doors and windows, and the ceiling if you are doing it. Use it as the first step before ordering plaster, boards or render.
P is the distance round the room, so P × H is the total wall area as if there were no openings. Aop is the combined area of doors and windows, deducted to give the net wall area. Ac is the ceiling, switched on by C = 1 or off by C = 0.
A rectangular room’s walls unfold into one long strip as long as the perimeter and as tall as the room, which is why wall area is perimeter times height. The ceiling is simply length times width.
Sense check: a 4 × 3.5 m room 2.4 m high has 36 m² of wall. Take off a door and window (about 3.5 m²) and add the 14 m² ceiling to get 46.5 m².
Watch out: reveals, sills and soffits around openings are extra surface not counted here.
Works out the plastering area of a rectangular room from its length, width and height, deducting doors and windows and optionally adding the ceiling.
Enter your values to see the result.
Good to know
Measure inside the room, wall face to wall face. Window and door reveals are extra surface: add their area (depth × perimeter) as a separate job, or leave small openings in. Many plasterers do not deduct openings under about 0.5 m², because the reveals use the plaster saved.
Common mistake
Deducting the full opening and then forgetting the reveals, soffits and sills, which can add 0.3–0.6 m² per window.
Limits
Rectangular rooms with flat ceilings only. For chimney breasts, bays, sloping ceilings or stairwells, measure each surface separately and add them.
Sources
- Geometry: perimeter × height for walls, length × width for the ceiling
Results are calculations from the published methods and manufacturer figures shown, and depend on the data you enter. Always follow the manufacturers’ system and product data, BS 8212, BS EN 13914 and Building Regulations. Coverage rates vary by product, background and thickness. Check prices, VAT and CIS rules with your own records and HMRC.