Wall area is the figure every decorating quantity hangs off — paint, wallpaper, plaster, boarding, tiling — and it is always the same two measurements: the perimeter of the room, times the height, less the openings. The perimeter is the one people get wrong, because a room is rarely four plain walls. A chimney breast adds two returns and a bay adds three, and both add surface that a length-times-width calculation never sees. This takes the room dimensions, subtracts the openings, and draws one wall to scale so the shape of the number is visible.
Room, height and openings
Perimeter times height, minus whatever is not wall.
Floor to ceiling, not floor to the top of the skirting.
Doors and windows.
Chimney breast returns, bay window cheeks, reveals. Each counts as one extra narrow wall.
Result
Perimeter first, always
Measure the perimeter along the skirting rather than measuring each wall. It is one tape run instead of four, it adds up correctly whether or not the room is square, and — critically — it follows the wall face, so it picks up the returns into a chimney breast and the cheeks of a bay automatically. Measuring length plus width and doubling it only works on a plain rectangle, and most rooms are not one.
The height is floor to ceiling, not floor to the top of the skirting, and it is worth taking in three places on an old room. Ceilings sag and floors slope, and a room that is 2.38 at one end and 2.42 at the other is not unusual. Use the average, and use the maximum where the material is cut to fit — wallpaper drops have to be cut to the longest.
What counts as an opening and what does not
A door or a window is subtracted, because it is not wall. But the reveal — the surface going back from the face of the wall to the frame — is wall, and on a thick wall it is a meaningful amount of it. A 300 mm reveal on a 2 m door opening is 0.6 square metres, and four of them in a room is two and a half square metres that a naive subtraction throws away.
That is what the returns field is for. Each return is counted as a narrow wall 300 millimetres wide and full height, which is a reasonable figure for a chimney breast return or a bay cheek. If the reveals are deeper, or if there is a lot of them, measure them properly and add the real number rather than counting returns.
In a room of ordinary proportions with a 2.4 m ceiling, the walls come to about twice the floor area — which is a useful check on any measurement.
If the answer is much less than double, the openings have been over-subtracted or the height is wrong. If it is much more, the ceiling is higher than 2.4 m or the room is long and narrow.
Old rooms are not rectangles
A period room is rarely square, rarely level and rarely plumb, and each of those changes the figure. Walls that are out of square make the two long walls different lengths, which is invisible to the eye and a real difference to the tape. Floors that slope make the height vary around the room. Ceilings that sag make it vary more at the middle than at the edges.
None of this is a reason to measure more precisely, because the material quantities do not need it — paint and paper have allowances built into them for exactly this. It is a reason to measure the perimeter rather than to compute it, because the perimeter absorbs every one of those errors automatically, where length times two plus width times two does not.
The ratio, and what it tells you
Wall area divided by floor area is a shape measure, and it is surprisingly useful. A square room has the lowest ratio for a given area, because a square minimises perimeter. A long narrow room has a much higher one. A very small room has a higher one again, because the perimeter does not shrink as fast as the area does.
That matters for cost. Two rooms of the same floor area can have very different wall areas, and therefore very different decorating costs — which is why a hallway and a box bedroom of the same square metreage do not cost the same to paint. The ratio is the quick way to see it before any measuring of materials happens.
| Room | Floor | Wall | Ratio |
|---|---|---|---|
| 4 x 4 m square | 16.0 m² | 35.5 m² | 2.22 |
| 5 x 4 m | 20.0 m² | 39.6 m² | 1.98 |
| 8 x 2.5 m corridor | 20.0 m² | 47.4 m² | 2.37 |
| 2 x 2 m cloakroom | 4.0 m² | 15.4 m² | 3.85 |
| 5 x 4 m, 3 m ceiling | 20.0 m² | 50.4 m² | 2.52 |
| 5 x 4 m, four openings | 20.0 m² | 36.0 m² | 1.80 |
A worked example
- PerimeterTwo lengths of 5.0 and two of 4.0 is 18.0 m.
- GrossAt 2.4 m high that is 43.2 m² of wall.
- OpeningsTwo openings at 0.9 x 2.0 m take off 3.6 m².
- Net39.6 m² of wall, which is 1.98 times the 20 m² floor.
- FacesTwo long walls at 12.0 m² each and two short ones at 9.6 m².
Nearly forty square metres of wall in a twenty square metre room. That single relationship — the walls are about twice the floor — is the most useful thing to carry away, because it turns a guess into a check on every room you measure.
What the figure feeds
Paint comes first: the paint figure takes this area and turns it into cans, with the spreading rate and the number of coats. Wallpaper takes it differently — the wallpaper figure works from the perimeter rather than the area, because paper is hung in drops and the perimeter is what sets how many.
Boarding takes the area again but not directly, because board comes in sheets and the count follows the layout of each wall rather than the total: the drywall sheet figure does that properly. Plastering and rendering are priced off this area too, and the labour hours figure is where the area turns into days.
Do
- Measure the perimeter along the skirting, not four walls separately.
- Take the height in three places on an old room and use the average.
- Add the reveals back in rather than subtracting the whole opening.
- Check the answer against roughly twice the floor area.
- Use the maximum height where material is cut to fit.
Avoid
- Measuring four walls and missing the returns into a chimney breast.
- Subtracting the reveal as well as the opening.
- Using one height measurement in a room with a sagged ceiling.
- Forgetting that a small room has proportionally more wall than a large one.
- Using the net wall area to count sheets of board, which follows layout instead.
Where the numbers stop being true
This treats the room as a rectangle with vertical walls of constant height. A room with a sloping ceiling, a dormer, a vaulted section or a stair wall is not that, and each of those needs the surface working out shape by shape rather than from a perimeter.
The openings are counted as rectangles of one size. A bay window, a patio door or an arched opening is not, and a room where the openings are a large share of the wall should have them measured individually. On a garden room with glazing on three sides, the openings are most of the wall and the subtraction is the calculation.
And nothing here covers the ceiling, which is a separate surface, usually in a different material, and usually quoted separately. The ceiling area figure does that one, and it is worth doing at the same time because the two are normally decorated together.
How the figures are produced
The perimeter is twice the length plus the width. Gross wall area is the perimeter times the height. Openings are the count times the width times the height, and returns are the count times 0.3 metres times the height. Net is gross less openings plus returns. The ratio is the net wall area divided by the floor area, which is length times width.
Questions people ask
How do I measure a room for painting?
Perimeter times height, less the openings, plus the reveals. Follow the skirting with the tape so the returns are included.
Do I subtract the windows?
Yes, the opening itself. But add the reveal back, because the surface going back to the frame is wall.
What about a chimney breast?
It adds two returns and removes a small area from the face. Net, it adds surface — count the returns.
How much wall in a 5 x 4 m room?
About 39.6 m² with two openings, against 43.2 m² gross and a 20 m² floor.
Is the ceiling part of the wall area?
No. It is a separate surface, in a different material, and it is measured and quoted separately.
Why does a small room cost more per square metre?
Because the perimeter does not shrink as fast as the area. A small room has proportionally far more wall for its floor.