Wall area = 2 × (length + width) × height. The calculator then subtracts 20 sq ft (1.9 m²) for each door and 15 sq ft (1.4 m²) for each window, adds length × width if you tick the ceiling, and multiplies by the number of coats. Paint needed = total area ÷ coverage. A typical gallon covers about 350 sq ft (roughly 8.5 m² per litre), but check the can, because rough or dark surfaces soak up more.
Example: a 14 × 12 ft room with 8 ft walls, one door and two windows has 2 × 26 × 8 = 416 sq ft of wall, minus 20 and 30 = 366 sq ft. Two coats is 732 sq ft, or about 2.1 gallons, so buy 3 gallons (or 2 gallons plus a quart).
With 8 ft walls, 12 × 12 gives 384 sq ft of wall before doors and windows. Two coats is around 2.2 gallons, so plan on 2 to 3 gallons.
Primer helps on bare drywall, stains or big colour changes. Its coverage is similar, so run the calculator again with one coat.
A little. Rounding up covers touch-ups, and buying all the paint for a wall at once avoids slight colour differences between batches.
See also: square footage calculator · area converter · concrete calculator
Tiling instead? Try the tile calculator.