Wall surface is perimeter times height
A rectangular 5 m by 4 m room has an 18 m perimeter. At 2.5 m high, its four walls expose 45 m² before doors and windows. Two finish coats create 90 m² of coating work. Dividing by a labelled 10 m² per litre gives 9 L. Ceiling paint, trim, and a wall that will remain unpainted are different surfaces and do not belong in that total.
Coverage comes from the coating label
The stated spread rate is normally tied to a surface condition, application method, and coat thickness. Fresh porous plaster can drink more paint than a sealed wall; dark colour beneath a pale finish may need a primer or another coat. A litre estimate is a purchasing check, not a promise that one tin covers every substrate at the same rate.
Openings need measured treatment
A 2 m² window can be subtracted from broad wall area, but narrow returns, reveals, doors, skirting, and touch-up work can consume paint that a simple rectangle ignores. Measure substantial unpainted openings, then decide whether edges and repairs justify keeping a reserve rather than chasing a fractional litre.
Wet volume is not final appearance
Paint sheen, roller nap, spray losses, temperature, humidity, drying time, and intercoat preparation affect the finish. The arithmetic cannot identify lead paint, damp, loose plaster, mould, or a coating system that needs specialist preparation. Follow the product data sheet and deal with surface defects before ordering by area.
Use a whole-tin decision
The calculated 9 L is useful for comparing tin combinations and recording a repeatable estimate. Buy sizes that leave a practical allowance for batch matching and future repair, especially where colour continuity matters. This page estimates wall-coating quantity; it does not specify primer, colour matching, labour, or safe preparation.
A paint schedule makes the arithmetic auditable
Mark each wall by orientation and note its measured width, height, coating condition, and colour. A north wall repaired with filler may consume differently from an intact south wall even when both have the same area. Keep the primer calculation separate from finish paint, then compare the resulting tin sizes with the supplier’s stated batch policy. This turns a quick litre estimate into a useful record for a later touch-up rather than a forgotten number.
Inspection note: A paint schedule makes the arithmetic auditable
Before ordering or fitting, revisit this project-specific check: Mark each wall by orientation and note its measured width, height, coating condition, and colour; A north wall repaired with filler may consume differently from an intact south wall even when both have the same area; Keep the primer calculation separate from finish paint, then compare the resulting tin sizes with the supplier’s stated batch policy; This turns a quick litre estimate into a useful record for a later touch-up rather than a forgotten number.