Perimeter × height, minus the openings
Perimeter, wall area with windows and doors taken out, plus floor and ceiling area. The number every decorating estimate starts from.
Almost every job starts here: wallpaper, paint, plaster and tile quantities all follow from wall area. The sum is simple — perimeter times ceiling height — and then the details start to matter.
The main one is whether to deduct openings. For wallpaper and paint, always deduct or you will overbuy. For plaster and primer it is usually better not to: the reveals around windows need covering anyway and eat the difference.
What this calculator shows
- Perimeter and total wall area before any deductions
- The area actually to be decorated once openings come out
- Floor and ceiling separately, which is what flooring and ceiling work need
What to keep in mind
- The room is treated as rectangular. Break an irregular room into rectangles and add the results.
- Alcoves, projections and boxed-in pipes are ignored — add or subtract those by hand.
FAQs
Should I deduct windows and doors?
For wallpaper, paint and tile, yes. For plaster, primer and filler usually not, since the reveals need finishing and the deduction just muddies the estimate.
How big is a typical window or door?
A 1.5 by 1.4 m window is about 2.1 m²; an internal door at 0.8 by 2 m is 1.6 m². Add up your own openings and enter the total.
How do I handle an L-shaped room?
Split it into rectangles, work each out and add them. For bays and sloped walls, measure each wall separately and add the lengths into the perimeter.
Why show floor and ceiling area?
The floor drives flooring and screed; the ceiling drives paint or a stretch ceiling. They are equal, so the combined figure is shown too.
Worked example
A typical bedroom
Input: 5 × 3.5 m, 2.7 m ceiling, 4.5 m² of window and door
Output: Perimeter 17 m, 41.4 m² to decorate
Note: Without the deduction it would read 45.9 m². Four and a half square metres is roughly a roll and a half of wallpaper — not nothing.