The calculation
Net area is your wall/ceiling area minus doors, windows, and other openings you won't paint. The default coverage rate (10 m²/litre, or ~400 sq ft/gallon) is a deliberately conservative pick within the commonly cited 350-400 sq ft/gallon range most manufacturers publish — real coverage depends heavily on the paint's formulation and how porous or textured the surface is.
Why the default coverage is conservative
Rough or porous surfaces (bare plaster, textured render, unprimed masonry) soak up noticeably more paint per coat than a smooth, primed wall. Starting from a conservative coverage figure and adjusting it against your specific product's data sheet avoids running short mid-job.
Frequently asked questions
How many coats of paint do I need?
Two coats is standard for most interior and exterior finish paints over a primed surface. A colour change (especially light over dark) or an unprimed, porous surface may need a third coat.
Should I include the ceiling area separately?
Yes — run the calculator once for walls and once for the ceiling if their coverage rate or number of coats differ, since ceilings are often single-coat or a different paint type.
What wastage allowance is normal?
10% is a reasonable default that covers roller loading loss, cutting-in, and minor touch-ups. Complex trim work or multiple colour changes may need more.