How Much Paint a Room Actually Needs, With a Worked Example
Calculating total wall area, subtracting the area of doors and windows (which don't need paint), and dividing by a paint's coverage rate (commonly around 350-400 square feet per gallon for one coat) gives the number of gallons needed — doubled for two coats, which is standard for most painting projects.
Walking through the full calculation, including the often-overlooked door and window subtraction, shows exactly why a rough "room size" estimate alone tends to overstate the paint actually needed.
A worked example
A room with a wall perimeter of 40 feet and 9-foot ceilings has a total wall area of 40 × 9 = 360 sq ft. Subtracting a standard door (about 20 sq ft) and two windows (about 15 sq ft each, 30 sq ft total) leaves 360 − 20 − 30 = 310 sq ft of actual paintable wall area.
Applying the coverage rate
At a typical coverage rate of 350 sq ft per gallon, 310 sq ft of paintable area needs about 0.89 gallons for one coat — rounding up to a full gallon in practice, since paint is sold in whole containers, not exact fractional amounts.
Doubling for two coats
Since two coats are standard for most painting projects (especially over a different existing color, or for full, even coverage), the practical total needed doubles to roughly 1.78 gallons — in practice, purchasing 2 gallons to have a comfortable margin for touch-ups and any application variance.