How many litres of paint a surface needs, allowing for coats.
Wall area is the room's perimeter times the ceiling height, less windows and doors. A 4 × 5 metre room with 2.6 metre ceilings has about 42 square metres of wall, which in two coats at 10 square metres per litre takes around 8.5 litres.
One coat rarely covers evenly: the substrate shows through and brush and roller marks catch the light from the side. The second coat evens out the film and fixes the colour. Going from a dark shade to a light one usually needs three coats, or a primer.
It is the manufacturer's figure for how many square metres a litre covers in one coat on a smooth, primed surface. On plaster or a rough wall the real usage is 20–30% higher, which is why a margin gets added to the calculation.
No, they are worked out separately, because ceilings normally take a different, flatter paint. The ceiling area is simply the room's length times its width. For a 4 × 5 metre room that is 20 square metres, or about four litres in two coats.