Paint coverage is rated by manufacturers in square feet per gallon — typically 350 to 400 sq ft per gallon per coat for interior wall paint, or about 10 square meters per liter. The estimate assumes a smooth, primed surface; thirsty surfaces like bare drywall, fresh plaster, or rough wood drink noticeably more on the first coat, which is why primer exists. Paint is sold in gallons and quarts (a quart covers roughly 90 to 100 sq ft), so the calculator's job is to turn your wall area and coat count into a purchase quantity that leaves you neither short mid-wall nor storing five half-gallons for a decade.
Wall area is computed as perimeter times height, minus openings. A rectangular room's wall area is 2 x (length + width) x ceiling height. Then subtract about 21 sq ft for a standard door (3 x 7 ft) and about 15 sq ft per average window — or measure your actual openings for a tighter figure. Ceilings are length x width, usually one coat of flat ceiling paint. Trim and doors are measured linearly or by small area and typically need a quart, not gallons. Two finish coats is the professional standard for color changes; one coat suffices only when repainting the same or a very similar color over sound paint.
Always round up and buy a little extra: running out mid-project risks a visible color mismatch between batches (even same-color cans vary slightly — pros 'box' paint by mixing cans together). A 10 percent buffer covers touch-ups and future repairs, and keeping a labeled sealed can of each color is the cheapest insurance for scuffs. For the estimate, use the conservative 350 sq ft per gallon figure; if your paint claims 400, treat the difference as your safety margin.