How the paint estimate works
A rectangular room's wall area is the perimeter times the height, minus the openings:
Walls = 2 × (L + W) × H − 21 ft² per door − 15 ft² per window
Multiply by the number of coats, then divide by coverage. Latex paint covers roughly 350 sq ft per gallon (325–400 depending on texture and application); the calculator uses 350. Ceilings are deliberately excluded — add ceiling area (L × W) divided by 350 separately if you are painting overhead too.
Worked example
A 14 × 12 ft room with 8 ft ceilings: perimeter walls = 2 × 26 × 8 = 416 ft². Minus two doors (42) and two windows (30) leaves 344 ft². Two coats doubles the work to 688 ft², which is 1.97 gallons (about 7.4 litres) — buy 2. If you are switching from dark to light (or painting new drywall), plan a third coat of finish over the primer: the calculator's coats field handles it directly.
Where real-world coverage drifts
Rough or heavily textured walls can cut coverage 20%; cheap one-coat promises assume near-white over white; and spraying consumes 10–15% more than rolling. The standard advice holds: buy the calculated amount plus keep the extra quart for touch-ups, because dye lots drift and next year's can will not match this year's wall.