Paint coverage calculator
Gallons for a given area and coat count, rounded to what you can actually buy.
Coverage is usually quoted around 300 to 400 square feet per gallon for one coat on a smooth surface, and substrate moves that number more than most people allow for. Rough stucco can drink close to twice what smooth siding does.
Enter paintable area rather than floor area. Walls and ceilings are measured separately because ceilings roll slower and textured ones consume noticeably more.
Wall and ceiling area, not floor area. Length x height for each wall, added up.
Most painters deduct only large openings and leave standard doors and windows in, since cutting in around them costs the time back.
300-400 on smooth surfaces. Rough stucco can be half that.
What to buy
Paint only. Primer, caulk, spackle, sandpaper, tape, masking film, drop cloths, and roller covers are the forgotten half of a materials line, and leaving them out is the commonest gap in an otherwise careful paint estimate. Ceilings are worth running separately, since they roll slower and textured ones drink more over the same measured area.
Coat count changes the total more than almost any other decision. Two finish coats over a primed, previously painted wall in a similar color is the baseline. Going dark over light, light over dark, or anything over bare drywall means primer plus two.
This covers paint only. Primer, caulk, spackle, sandpaper, tape, masking film, drop cloths, and roller covers are the forgotten half of a materials line and are the most common gap in an otherwise careful estimate.
The reasoning behind these numbers is in How to estimate a painting job.