Ceiling area calculator
A ceiling of the same area as a wall drinks noticeably more paint, because you are rolling upward against gravity and the coat goes on thinner and less evenly. Two coats is the practical minimum, and if you are covering old nicotine or a water stain you are looking at a stain-block primer plus two topcoats. Budget roughly 15% more paint per square metre than the equivalent wall.
Ceiling area is simply the room length times its width — the same figure as the floor area. A 4.5 × 3.2 m room has a 14.4 m² ceiling, needing about 2.6 litres of emulsion for two coats and 15.4 m of coving around the edge.
How to work out a ceiling
A sloping or vaulted ceiling is not the floor area. For a single slope, multiply the floor area by the rafter multiplier for that pitch — a 30 degree slope adds 15%. For a room-in-roof with two slopes and a flat section, measure each plane separately. The perimeter figure is worth noting even if you are not coving: it is also the length of shadow-gap bead, the run of a perimeter light track, and the distance you will tape and fill at the wall junction.
Questions
For a flat ceiling, yes. For a sloping one, multiply by the pitch multiplier — a 30 degree slope adds about 15%.