Works out the wall area from the size of the room and its ceiling height, then the length of wallpaper and the volume of paint needed. Windows and doors are deducted. Wallpaper is converted to a length at the standard 92 cm width.
The easiest thing to get wrong when repapering or repainting a room is the area. What matters is the area of the walls, not of the floor.
Think of the walls as one band running right around the room. The band is as long as the perimeter and as tall as the ceiling. Windows and doors come off it.
The default input is a room 3.6 m by 2.7 m with a ceiling at 2.4 m and 4 square metres of windows and doors.
The perimeter is , or 12.6 m, which multiplied by the ceiling height gives 30.24 square metres. Subtracting the 4 leaves about 26.24 square metres of wall.
The floor is only 9.72 square metres, so the walls are about 2.7 times the floor area. Estimating wallpaper from the floor area falls hopelessly short.
A ten percent allowance brings it to about 28.86 square metres, which at the standard 92 cm width is about 31.37 m of paper. At five square metres per litre, the paint comes to about 5.77 L.
Plain paper needs about five percent. Patterned paper has to be matched from drop to drop, which wastes the offcuts, so allow ten to fifteen percent. The longer the pattern repeat, the more is lost.
Rooms with many windows or internal corners need more again, because of all the small cuts.
The ceiling is not included. To paper it as well, add the floor area.
How far a litre of paint goes varies enormously with the paint and the surface beneath it. Five square metres is used here as a default, but the figure printed on the tin should replace it. Two coats need close to twice as much as one.
Wallpaper is sometimes sold by the metre and sometimes only in full rolls of 50 m.