Formula and method
Liters = area × coats ÷ coverage × (1 + allowance/100). Round liters divided by container size up to whole containers.
Worked example
For 50 m², two coats, 10 m²/L coverage and 10% extra, you need 11 L. With 5 L containers, purchase three.
Room-dimension mode
Wall area = 2 × (length + width) × height. Doors and windows are subtracted at their entered sizes; the defaults are a standard 36 × 80 in (0.91 × 2.03 m) interior door and a 3 × 4 ft (0.91 × 1.22 m) window, and you can edit both. Add the ceiling (length × width) if you are painting it too.
Worked example: a bedroom
A 4 m × 3.5 m room with 2.4 m walls has 36 m² of wall. Removing one door (1.85 m²) and one window (1.11 m²) leaves 33.04 m². Two coats at 10 m²/L with 10% extra need 7.27 L, so two 5 L containers.