Field guide / 03—08
Calculate wall area and openings
Keep gross wall area, doors, windows and still-painted details separate.
Measure each wall
In a simple room, add wall widths and multiply by their shared height. If heights differ, calculate each wall separately. Note any walls using a different colour or product rather than combining every surface into one paint order.
Subtract only what remains unpainted
Subtract a door or window if its area will not receive paint. Frames, reveals and trim that will be painted belong in a separate measurement. An early rough estimate may leave small openings in, but that choice should be explicit.
Area is not litres
Net area is only the first step. The number of coats and the selected product's coverage per litre determine the volume estimate. Surface condition and application method can change actual use.