The openings are not a rounding error
Almost every room calculator asks for length, width and height and stops there. That gives you the gross wall area, which is the area of a room with no way in and nothing to look out of.
Worked on a 4 by 3 metre room at 2.4 metres, which is 33.6 m² of gross wall:
- A door and a medium window: 2.95 m², 9% off
- A door and a large window: 3.67 m², 11% off
- A door and a patio door: 5.29 m², 16% off
- Glazing on two sides: 7.56 m², over a fifth off
The widely repeated figure is "about 15% for a door and a window", and as the table shows that is only reached once there is a patio door in the room. Either way the error runs in one direction: it overstates the wall, and the wall is what you buy paint and paper against.
One distinction worth keeping. Use the net figure for anything sold by coverage, and the gross figure for plastering, because a plasterer works to the reveals around every opening and the holes do not save the labour they appear to.
An L-shaped room has exactly the same perimeter as the rectangle it came from
This one is genuinely counter-intuitive and it costs people money on every renovation.
Take a 5 by 4 metre room and notch 2 by 1.5 out of a corner. The floor area drops from 20 m² to 17 m². The perimeter stays at 18 metres, exactly as it was.
The reason is that the two faces of the notch replace precisely the wall length the notch removed. Going in one direction you lose 2 metres and gain 2; going the other you lose 1.5 and gain 1.5. It nets to nothing, every time, whatever size the notch is.
So skirting, coving, pipe runs and cable trunking all need the full perimeter. Because the area went down, people assume everything did, order against the smaller figure, and come up short.
A sloped ceiling is not a room with a lower ceiling
Loft conversions are exactly where somebody is buying materials without a plan drawing to work from, and exactly where a box measurement is furthest out.
Take a room 3 metres wide where the ceiling rises from 1.5 metres at the eaves to 2.4 at the ridge. The sloping surface is 3.13 metres across, not 3, because it is the hypotenuse rather than the plan width. That is about 4% more plasterboard, insulation and paint than the floor plan suggests.
Then there are the two gable ends, the triangles above the low wall at each end of the room. Together they are the room width times the rise, and no box calculation contains them at all.
Both of the usual shortcuts are wrong in opposite directions. Measuring the room as a box on the low height understates every material in it. Measuring it on the high height overstates them. The tool works the walls at the low height, adds the gables, takes the ceiling as the sloping surface, and uses the average height for volume, which is the right figure for heating and ventilation.
Measure at the skirting, and measure every wall
Two habits worth more than any calculator. Measure at floor level along the skirting rather than across the middle of the room, because that is where the walls actually are and a tape across open floor sags and wanders.
And measure each wall rather than assuming the room is square, because almost none of them are. An older house can be 50mm out over four metres without anything being wrong with it, and that is more than any standard wastage allowance is meant to absorb. Where opposite walls differ, order against the longer one and fit against the shorter.
Why this is the measure-once tool
Every room tool on this site wants the same three or four numbers. Doing them once, properly, with the openings and any slope included, means the paint, flooring, tiling, wallpaper and radiator tools all work from the same measurement instead of from somebody re-measuring at eleven at night with the tape in one hand.
Everything is given in both metric and imperial for the same reason. Plenty of merchants still quote in square feet and plenty of older plans are in feet and inches. Converting once, here, beats converting three times later and getting one of them wrong.
Common questions
How much wall area do doors and windows actually take off?
Less than the often-quoted 15% in an ordinary room, and more than that in a glazed one. On a 4 by 3 metre room at 2.4 metres, a door and a medium window take off 9%. A door and a large window take off 11%. A door and a patio door take off 16%, and glazing on two sides takes off over a fifth. A calculator that never asks about openings overstates the wall every time, in the direction that sells another tin.
Does an L-shaped room have a shorter perimeter?
No, and this catches people constantly. The floor area drops, so it feels like everything should, but the two faces of the notch replace exactly the wall length the notch removed. A 5 by 4 room notched by 2 by 1.5 loses 3 square metres of floor and keeps all 18 metres of perimeter. Skirting, coving and pipe runs get under-ordered on this every time.
How do I measure a loft room with a sloped ceiling?
Not as a box on either height. The sloping surface is longer than the room is wide: a 3 metre wide room rising from 1.5 to 2.4 metres has a slope 3.13 metres long, so there is about 4% more plasterboard, insulation and paint than the plan suggests. There are also two triangular gable ends that no box calculation contains. Measuring on the low height understates everything and measuring on the high height overstates it.
Should I use the gross or the net wall area?
Net for anything you buy by coverage, so paint and wallpaper. Gross for plastering, because a plasterer works to the reveals around each opening and the openings do not save the labour they look like they should. The tool gives both so you are not choosing blind.
Where exactly should I measure?
At floor level along the skirting, not across the middle of the room, because that is where the walls actually are. Measure each wall rather than assuming the room is square, since almost none of them are. If opposite walls differ, use the longer for ordering and the shorter for fitting: an old house can easily be 50mm out over four metres, which is more than any wastage allowance is meant to absorb.
Why measure here rather than in each tool separately?
Because it is the same three or four numbers every time, and re-measuring at eleven at night is where mistakes get in. Do it once properly, with the openings and any slope included, and the paint, flooring, tiling and radiator tools all work from the same measurement.