How much paint do I need?
Wall area comes from the room's perimeter × height, minus a standard area for each door and window you enter — most people overbuy paint by forgetting that step. Multiply the paintable area by your number of coats, then divide by how many square metres one litre covers to get the litres you need. Two coats is standard for most colour changes; going from a dark colour to a much lighter one often needs a third coat.
One litre of paint covers roughly 12-14m² per coat — for two coats (the usual finish), that halves to around 6-7m² of wall per litre.
Standard UK deductions: around 1.6m² for a door, 1.5m² for a window — subtracted from the total wall area.
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