How this paint calculator works
Multiply the total length of all walls by the ceiling height to get gross wall area, then subtract standard openings — about 21 square feet (2 m²) per door and 15 square feet (1.4 m²) per average window. The result is multiplied by the number of coats and divided by the paint's coverage rate. Most interior paints cover 350–400 square feet per gallon (roughly 9–10 m² per liter) on a smooth, previously painted surface; the label on your can states the exact figure.
One coat or two?
Two coats is the default for a durable, even finish, and it's essential when changing colors. You can sometimes get away with one coat when repainting the same color with a premium paint-and-primer product. Going from a dark color to a light one may need a dedicated primer coat plus two finish coats — enter 3 coats in that case.
Textured and porous surfaces need more
Fresh drywall, bare wood, masonry and heavily textured walls absorb more paint. Coverage can drop to 250–300 square feet per gallon (6–7 m²/L) on these surfaces. Lower the coverage figure in the calculator to match, or prime first — primer is cheaper than paint and seals the surface so your finish coats go further.
Don't forget ceilings and trim
This calculator covers walls. For ceilings, multiply room length by width and divide by the same coverage rate. Trim, doors and baseboards are usually painted from a separate quart or liter of semi-gloss; one gallon (4 L) of trim paint handles the trim in two to three average rooms.